Stone Foundations
Links to the Ask the Experts page

Jose Garcia has been a landscape contractor for 24 years and has gravitated to doing a lot of rock work. He has built innumerable retaining walls of timbers, boulders, drystacked and mortared stone. He has built foundations out of stone and mortar and put rock veneer around the base of a straw bale building to raise the level of waterproofing. He lays about 20 tons of flagstone a year in patios and walkways. Over the last couple of years he has built a half dozen mortared flagstone staircases. In Colorado we are blessed with a wonderful red flagstone with great tensile strength that he uses to make benches. He tries to work with the stone's shape as it comes, and can generally lay out a patio with a minimal amount of cutting or chipping, and the benches are free form and distinctly shaped. Mostly he's out rolling boulders and flipping flagstone on a daily basis.

Q: I am building a strawbale/cob/post and beam, oval home near Paonia, CO. The 3 foot wide rubble trench with cement bond beam is in place. Am about to start on the rock stem wall. The footprint sits on a hill with an elevation change of about 5 feet. Thus, on the low side, the rock wall will be 3 feet (tapering up to 2 feet) thick by 6.5 feet tall. Can you recommend the best/strongest mortar mix and offer any other advice to a novice owner/builder?

A: The mortar part of the question is easy. I use a masonry cement with three parts clean sand. You can buy pre-mixed masonry cement but the cost can be prohibitive and I've had mixed results with it. Add just enough water to mix well but try and keep the mix stiff enough to hold itself up. From this point it gets more complex. It would be hard for you to goof up terribly but there are too many subtleties to good masonry work to relate here.

Q: I live in WV. Wanting to build a house out of rock. I can't find any information on the size footer to use (width and thickness); the house will be 40w 60L and 2 story with a full basement, all rock, and the walls 2 foot thick. Any information would be great.

A: I don't have any specs on footer width but a two story rock house will put tremendous pressure on the ground that it sits on. The foundation should look like an elephant's foot, wide and thick at the base and tapering as it goes up. I would sure try and make the second story as thin as you can get away with, I doubt a mortared rock wall can be much thinner than 18" depending on the stone that you are using. The footer should undoubtedly be at least 3 foot thick if not 4 with the foundation wall tapering to the 2 foot thick width as it comes out of the ground. Quite the undertaking, I wish you success.

Q: Hello from Tennessee. Kelly, do have a resource for construction of foundations on solid rock?

A: (Kelly) I can't think of a specific resource to direct you toward, but I would say that building on solid rock would likely produce the most solid foundation possible. There is no possibility of frost upheaval or settling or shifting. It depends on what you want to put on the foundation, but most conventional building requires a continuous concrete foundation. I would suggest digging down to the rock where you want the foundation to be and drilling into the rock to embed some vertical rebar at perhaps 2-3 foot intervals, form for the foundation above the rock, and pour the concrete. You might want to check with any building authorities in your area to see what they would advise.

Q: To keep a long story short, we are planning to build a dome in Clear Creek County, CO. and we are having trouble convincing them that it will work if we build the basement walls out of native rock. The blueprints call for cement blocks filled with concrete, and rebar in every other cell, all set on a poured concrete footing. We do not have any problem with keeping the footing, and expect to expand it to carry the extra weight. Most of that extra weight is due to the wall going from 12" blocks to 16+" of native stone. While the engineer had to warm up to the idea, the county is in need of more convincing about this "unconventional" method of building. Never mind the innumerable stone buildings in Europe, etc. which have been around centuries longer than this country has. Any suggestions, references, etc., you might be able to pass along to help convince these "highly educated" people would be greatly appreciated.

A: (Kelly) Ah, for the fine imagination of our building officials! I would say that your idea of using a stone foundation for a dome is an excellent one, especially since the circular shape will, in itself, provide considerable strength to the foundation. One of my books on stone masonry suggests that most single story stone buildings do not really require reinforcement, unless they are in an active seismic zone, and then some use of rebar in a central core may be appropriate. You may be forced to do the cinder block, or poured cement foundation, and then face it with stones.

Q: I am building a 6 foot brick wall. I have dug the hole for the foundation (width = 22' X depth 18'). My question is: should I use rebar or another support (or rebar plus another type of support material) and how do I configure the internal support for the foundation (rebar?) ?

A: (Kelly) If the wall is straight, it will definitely need to be reinforced and connected to the foundation. A curved wall might stand a chance of staying in place without such reinforcement. I would suggest using the kind of bricks that have a hollow space that would allow rebar to be threaded through and into the foundation, then the void should be grouted solid with mortar to secure the wall.

Q: I am planning to build an addition to my 150 year old home. The home has a full length basement about 8 ft deep and 30 ft long constructed of stacked stone. I would like to build a basement under the addition which will be the same length as the existing house and basement. How close can I dig to the old foundation and pour a new foundation wall?

A: (Kelly) Since the original stone foundation is just stacked, and not mortared, it may not be as stable as if it were cemented together. You might be able to dig right up to the old wall, but I would proceed very cautiously and be prepared to stabilize the wall with braces if necessary. Once the new foundation wall is poured, it should help stabilize the old one, especially if you make some rebar attachments through both.

Q: We have plans to build a brick house with a full basement. Last week we dug down 6 feet and hit solid rock. We tried different places on the piece of property. We have had friends tell us that having rock under the basement can and probably will cause the house to shift after few years. Is there anything we can do, or is it better to just site it on a crawlspace?

A: (Kelly) I suggest getting the opinion of a building engineer who is familiar with the rock strata in your area. It seems unlikely to me that placing a basement foundation on solid rock would lead to eventual shifting of that foundation; quite to the contrary, bedrock is what bridge and dam builders seek to secure these massive structures!

Q: I was referred to this site with a question about using rock in a shower wall, and lo and behold...!What type of material did you use as a foundation and what type of mortar did you use to waterproof the wall?

A: (Kelly) The foundation for my shower is a poured concrete pad, over the drain plumbing, about 6-8 inches thick. I used a standard mortar mix, which is very rich in Portland cement, for the rock wall. This is virtually waterproof as it is, but I also coated the wall with some linseed oil after it cured, partly to bring out the colors in the rocks.

Q: We have extremely highly reactive clay soil which will need very sturdy foundations in which we will be seeing an engineer. We can obtain rocks from the land around which would not cost anything to build with. Would building with rock be beneficial with clay soil. Can the reactive clay be used as mortar if it has much more than 30% clay, or can something else be added to break the clay down. It is understood that the mortar could crack, but wouldn't this be reduced with the stable foundations.

A: (Kelly)With a substantial foundation built to below frost level, you would be able to build with rocks just fine. Using a mud mortar is possible, and would like hold up OK with your solid foundation, but I would go with cement mortar myself, since it would hold up better over time and really is not going to use very much cement. If you do use the clay soil, you can add enough sand to it to make the clay % no more than 30%.

Q: I'm thinking of building a beach wall in front of our house. I have enough rock, but don't quite know what kind of foundation to have and how to make a rock wall strong enough that the waves don't break it down after a month or so. Mostly everyone down here just uses beach sand and water to mix their own cement by hand--what would you suggest that would be the most cost efficient and worthwhile?

A: I've lived on the ocean enough to know that any wall you may build will come down over time. Most with the first storm. Can you get a foundation below the shifting sand? I would definitely wash the salt out of the sand before using it as the salinity will weaken the mortar. My feeling is that you will need as deep and wide a foundation as possible keeping in mind that you are building a temporary structure at best.

Q: I'm going to be building a granite rock house on top of an outcropping of solid granite. I want to sink some rebar but can't find out how to drill holes in the granite without using expensive drilling equipment. Any ideas?

A: You should be able to drill holes in granite with a hammer drill and a masonry bit. Rent one for a day and they should supply the bit. The trick way to do the same thing without rebar is to carve out flat spots and depressions in the bedrock to key your foundation into the stone. Depending on the outcropping this may be easier or harder than the rebar work. You may only need to put rebar on sloped sections where the rock and foundation join. Clear the outcropping and wash it where your mortar will tie in then ask permission and advise of the rock. Don't be surprised if the rock asks something of you in return.

Q: I live in New Hampshire on a 10 acre farm I cleared. More rocks than anyone in the county. I would like a to build a stone barn approx 30x40, 2-3 stories. It will be built into a bank for access to 2nd floor. Now for the questions:
1. Am I insane, or is this feasible?
2. I guess one would have the footer dug and poured concrete? (I have massive boulders that could be place with an excavator...I do not know the "code"on this.
3.I have a library of stone books, none however are very detailed for a project of this magnitude any recommendations?
4. I presume the safest wall is mortared vs dry stone (my area the foundations for houses and barns are dry stone to the sill
5. An unrelated question, after reading Q&A on the site I was going to tile my entry way but sounds like I could use flat granite stone, what type of base would be put on the subfloor, and ? concrete for grout?

A: 1. Yeah you are probably crazy to take on such a big building as your first masonry project, but yes it is feasible. By the end of your construction you will no doubt be an accomplished stone mason. You may also be an old man. Keep in mind that your experienced work will be resting on a novice's foundation.
2. No need to pour a concrete footer, the mortared stone will be just as strong. The immense weight of a multi-story stone building will require a large foot to spread the weight over the ground. Think of an elephants foot.
3. I don't have a book to recommend, but consider that the stones have great knowledge that they are willing to share. Be open to suggestion, not every thought that pops in your head is self generated. Rocks can sing you endless songs of gravity and friction and lecture for days on sense of place.
4. You will need a capillary break under a stone floor. A few inches of gravel with a few inches of course sand or crushed rock over it will keep ground water from surfacing in the building. If you can lay the stone with tight joints skip the mortar altogether. If not, then a concrete mortar slightly recessed will work fine.

Q: I am building a natural stone retaining wall approximately 6 ft at its tallest spot. It gradually gets shorter in height as it tapers into the side of the hill. I am trying to tie this natural stone into manufactured block (on a portion of the wall). I live in Seattle, WA, where it rarely freezes, if at all. My question is: must I pour a cement footing (foundation) for this wall, or can I use crush compacted stone? Also this wall will be mortared and battered toward the slope of the hill, will that be enough to to retain the soil. Will the wall be strong? What advice could you give to make the wall stronger if need be in addition to what I have described above?

A: You have an ambitious project ahead of you. A six foot tall retaining wall has a lot of pressure on it. You shouldn't need a concrete footer, the compacted gravel will do fine. Make sure to start the mortared stone 8 inches to a foot below grade to hold the toe of the wall in place. A little batter in the wall reduces the pressure on it and a curve in the wall will strengthen it as well if you can put one in. Water pressure on a retaining wall is another consideration and I would recommend a gravel drain of 3 to 6 inches behind the wall with weep holes through the wall or a perforated pipe to daylight to drain the water off. Your other big consideration to make sure that the wall withstands the pressure that an earthen bank puts on it is the width of the wall itself. You are relying on gravity as well as the mortar and friction between rocks to hold the wall in place. At the six foot end of the wall you may need to be as wide as 30 inches and as the wall gets shorter a thinner wall will do. I will often set some larger stones back into the bank to act like a buttress. This puts an irregularity into the wall that is not visible but breaks up the pressure on the wall. Good luck.

Q: I'm planning a house out of entirely natural materials (no concrete, osb, polyurethane, etc) and would like to have a rock foundation. Both flagstone and river rock are available. What kind of mortar can I use to hold the rock together below and above ground that is entirely natural. ps. The area gets about 15 inches of rain a year.

A:Your river rock should work fine for the foundation walls. Use a masonry cement mixed with three parts sand. If no masonry cement is available a portland cement with about 1/3 lime can be substituted. Cement is a natural product but has a lot of embodied energy in it. Here in New Mexico there are some old adobes whose foundation walls are held together with mud, a little lime would help it hold up longer. I think you would need a pretty angular stone to use mud as a mortar and spend a little time each spring on maintenance. You won't get any toxic offgassing from the cement so I would go with that if it is available to you.

Q: We live on 80 acres complete with tons of wonderful stone. I feel a need to build a structure with this free gift. I want to build a 10 X 10 building. I have my mind set on this and I would not be happy with building just a wall or something sensible. I have been looking into all that will be required. I am aware that although the stone is free the project is not going to be cheep. I have been thinking only what will be required above ground. Now a few curves have been thrown my way since I have been talking about this project with other people. We live in Wisconsin and I am finding that we will have go go down 48" for frost footings. My question is this: What would be the best method for building a foundation? This project is becoming very complex and I would like to use the simplest method. I think I'm losing my husband on this one.

A: I am pretty sure that it was Frank Lloyd Wright who developed the technique of using a rubble trench foundation. Dig a trench as if you were going to pour the 4 feet of foundation and fill it with gravel or the small rubble of collected rocks from your site. Be sure to compact well in 6 inch to 1 foot lifts. Start your mortared rock about 8 in. to a foot below ground level and go up from there. I just finished a similar project myself to house my photovoltaics; I went with a round structure for added strength, straight walls and corners are weak spots. Good luck with your project.

Q: Muchas gracias for your answer. I had no idea I could use gravel or rubble for footings and foundation. That's awesome! I may have gotten my husband's enthusiasm back. I do have a couple more questions if you don't mind:
1) Are you saying I can use gravel alone without mixing it with mortar?
2) When we start our first row of stone, how do we anchor it if there is no mortar below?
3) Could that first row be cement block rather than stone seeing that it is below ground level?
4) How could we incorporate a cement slab for the floor? How would that connect or tie in to the walls?

A: First, all you need in the trench is compacted gravel, no mortar. If ground water is a concern put a 3" perforated pipe in the bottom of the trench that runs to daylight or a sump. There is no need to anchor the bottom of the wall to cement, just keep the first foot or so underground which will anchor it well. I wouldn't recommend cement block as the weight of the walls may be too much for the block. The bottom of the wall should be the strongest and thickest, like an elephants foot. Taper the walls as you go up, so a 16" wall below ground may be 14" when it gets out of the ground and only 10" by the time you get to the roof. Let the slab float separate from the walls of the house. Tying them together will cause problems. Most building codes spec an expansion joint between slab and wall. There are many more questions that will come up for you so get a book or two and learn as you go. Don't be afraid to make mistakes. Everyone who has written a how-to book has made more mistakes than you will on this project.

Q: I have always wanted to build a stone wall but have not had the opportunity in my 69 years. However, when our town dug up an old cobblestone street and offered the cobblestones to any resident who wanted them, I decided that now is my chance. So I am planning, with the help of my good and patient husband, to build a semicircular wall that rises from 10" to 2 1/2' high around part of the perimeter of a little pond. Our frost line requires a footer of 2'5". We are thoroughly confused about what will make a sufficient footer. One handyman friend says just sand under mortared cinder block, another says sand under 4" to 6" of concrete then mortared cinder block, and the books that I bought at Home Depot don't address a short wall's footer requirements. What do you suggest?

A: Boy I guess that I would recommend skipping the cinder block altogether. A rubble trench foundation should work just fine for a garden wall. Dig your trench and fill it with coarse gravel, inch and a half river rock if you can get it. Pack it down tight as you go till you get within about six inches of the existing grade. Start your mortared cobblestone wall
on the gravel and build it up out of the dirt. This type of wall has worked well for me. I would recommend a couple of additions also. Put a gravel curtain drain behind the wall that daylights around the ends of the wall or through weep holes. This will reduce the amount of water pressure on your wall and absorb some of the soil movement. Since this is your first wall you might want to pour a grade beam also. This is the 4 to 6 inches of concrete that was recommended at the bottom of the wall to help tie it all together. You can set your first stones in the wet cement of this grade beam. So about 20" of packed gravel, a 6" grade
beam, and mortared rock from there on up. With a gravel drain behind the wall you will have a first class wall.

Q: I am in East Tennessee. I built a small cabin with a pole foundation on the side of a hill. On the uphill side the cabin is about 18" above grade. On the downhill side the cabin is about 9' above grade. The poles are inset 1 foot from the perimeter of the cabin. My plan is to build a 10" thick rebar reinforced slipform concrete and rock foundation up under the rim of the cabin outside of the poles. Taking over the support function from the original pole foundation. I have several questions. How deep should I dig the foundation to get below the frost? If the wall is 10" thick can I just go ahead and dig a 10" wide trench around the perimeter and fill it full of concrete and rebar for the footer? Or does the footer need to be wider than 10"? I plan to gather rock here and there, and or have some delivered. Is there any type of rock I should steer clear of? Is sackcrete ok to use? How should I handle stepping the footer up the hill? As I dig the footer trench I will run into one or two big boulders. They are close to the surface and are too big to move. What do I do here? Is it OK to pour the footing right on top of the mostly buried boulder? And finally as the foundation rises up and gets close to the building, how do I tie the existing structure to the new foundation, and should I try to actually transfer the weight onto the new stone wall?

A: The depth of your footer trench should be below frost line for your area. Your regional building dept. will give you the info. The footer spreads the load of the building to the ground and should be wider at the base than the 10" walls. I like a rubble trench foundation
myself. You should step the foundation as you go up the slope towards the shallow end so the wall has a flat surface to bear on rather than a sloped one. As for tying in the foundation to the existing house, I am at a loss without seeing your project but I am sure that you will find a suitable fastener. I would be inclined to leave the support poles in place if they are not in the way.

Q: I have a home with a cedar siding, including the fireplace on the exterior of the home. I would like to cover the fireplace only with 4-6" diameter river stones. The property is in the mountains of Colorado. Temperature ranges of -15 to 90 degrees Fahrenheit.

My questions are:
1) Do I need to install footings, or can a gravel based be adequate? If I need to put in footing, would it need to go down 4 feet to be below the frost line?
2) Can I cover the existing siding with tar paper and then metal lathing? Is there a better way to do this?
3) Since the chimney will be approximately 20 feet tall should I place metal tabs on the lathing hold the mortared rock to the house?
4) What would be the best mortar mix for this type of project?

A: You can use a rubble trench foundation to get below frost line and spread the weight of the chimney to the ground but you will want a solid base of cement starting about a foot below grade that you can tie into the house foundation. You will want the house and chimney to act as one. Tar paper and expanded metal lath should work well especially if you use metal ties to hold them together. Use Portland to pour the grade beam and type N or S masonry cement for the rock work. Type N will crack less if the chimney is supported well from the house but if the chimney carries it's own weight the type S may be better.

Q: I specialize in custom additions, decks and docks. I have just received an order for a whole house to be covered in flagstone, which I am confident I can accomplish. My problem lies in whether or not the foundation, which is a slab, will be compromised because of the 18 tons of stone that will be attached. The house is at least 20 years old with an un-cracked slab. Please reply if you can advise or comment just for my peace of mind.

A: What a job, with a lot of complications. Will you be able to set the stone on the slab? I don't think you can count on the siding to hold all that weight even if the slab can. You would have to start the stone on a solid base. I really have too many questions to even comment on a tricky job like this one.

Q: I am building a "cabin" 22' x 36' in Ashfork, AZ. The elevation is 5700 feet and I'm on the Coconino rock shelf. While digging the trench for the coconino rock footers, I consistently dig to solid white rock. I took out the smaller ones (400lbs or less) and left the ones I couldn’t remove with the mini-backhoe. Some of these "large" rocks approach the surface. Seems like I'm safe to use them as bases to my rock foundation? Also it sounds like you do not recommend lime in type S mortar - is this correct?

A: You should be fine building your foundation over the big rocks. I feel confident that the bottom of the
boulders are below the frost line with no threat of heaving. The type s masonry cement has lime in it and is formulated for use in stone masonry that needs a structural characteristic so no additives are needed.

Q: I am in the planning stages of building a heavy timber construction cottage 24'x24' and want to build it on a river stone foundation. I want to set my support beams in the rock foundation so I can have strength and an ability to start my floor on the recessed beams.My first question is should there be a slight amount of mortar between the stones or just enough to fill in the open spaces?Also do I need to somehow tie the stones together periodically, like something as in rebar running length ways down the wall?

A: Sounds like a good project to me. I am probably biased as I am doing roughly the same thing in northern New Mexico. I am using the native sandstone and will have an earthen floor. I am setting some heavy timber posts out of rock foundation. The rounded shape of river rock lends itself to bigger mortar joints. This is not a problem but the stone will always look better than the mortar so try and minimize the gap by touching one stone to another and recessing the mortar from the face of the wall. You shouldn't have to use any rebar in your foundation if you keep the wall about two foot thick and stagger the stone well. No rows or columns and use long rocks to tie the width of the wall together now and again. I always try and put tar paper or something similar between cement and wood to prevent rotting the wood out.

Q: I live in Maine in an area of heavy clay. I’m thinking of building a stone foundation for a stick built home 30’ by 30’. I plan on making the walls 3’ thick and 5’ high. I do not want to use mortar, but just fit the stones. I would prefer to use field stones as these are readily available, but can use angular stone. Does this sound reasonable or will the lack of mortar make this not possible?

A: It's completely possible to build a tight, strong wall of the size you mention without mortar but I wouldn't advise it for your foundation. Bugs, rodents and snow melt will all find their way into your house. Use the fieldstone on a rubble trench foundation. You can probably get away with the minimal use of mortar with tight joints and lay up the outside to give the appearance of dry stack.

Q: I am building a 24 x 40 foot log cabin home. I have priced out a solid traditional concrete foundation for the project with 10' side walls in the basement There are a lot of natural flat limestone rocks on the property. The property is mostly solid rock after the first 6" of soil. I am planning on having a hole dug 6 feet into the rock for the foundation. Instead of using the standard poured concrete foundation, I am contemplating using the flat limestone from the property to form a natural basement floor and mortaring the rock together to form the floor. I want to then use cinder block for the side walls and come back when done and put on a limestone rock facing by mortaring the rock to the cinder block on the outside of the foundation for appearance. I am on a large mountain top with great drainage and no water problems. What are your thoughts on doing this for a decent sized 2,000 square foot log cabin?

A: What you describe sounds like it would probably work fine. I haven't worked with limestone personally, but I know that a lot of people have used it in construction. I hope it is not too hard to dig that far into the natural stone. Another concern would be to make sure that water doesn't surface inside the building from some fissures in the natural stone, but it sounds like you are confident that this is unlikely. As far as supporting the log home, this foundation should work, as long as the cinder blocks are well reinforced.

Q: For the last ten years I've been building (inching away) my dream retirement home by the beach in Baja. I joke saying that it's a rich man's house built with proletarian wages ha! well I saved a little bit of dough and I'm ready to take off for a couple of months to build the main entrance to the house, which happens to be on a slope. I have plenty of flagstone and volcanic rock on premises which I intend to use for the foundation of the main entrance (foyer, great room-cantina) with a measurement of 18'x 27'. I intend to build this room with flagstone with a wall height of 12', 2.5' in width for the first floor based on a foundation of 3' in width and 5' in height on the deepest part of the slope. This section of the house will be two stories high and will have a terrace on the roof top. What suggestions do you have for steel reinforcements for the foundations? We are in a seismic zone, not to mention the constant pounding of the ocean waves on the cliffs,

A: (Kelly) Trying to include rebar in a stone foundation is a very difficult proposition, and is rarely done; well done stonework does not require it, especially if you place a reinforced concrete bond beam on the top of the stone foundation.

At what distance should I spread the steps of the foundation considering the measurements I gave you.

I think you will need to work this out on paper with the exact dimensions of the slope and home design in mind.

You can't even sink the tip of a nose shovel since I'm building on a plate of solid volcanic mantle.

This is good news, and will make the foundation even stronger. Be sure to start the rock foundation directly on top of this solid mantle, rather than on any fill.

Q: We are planning to build a house with stone walls, insulation sandwiched in-between. My question is whether a rubble trench foundation could be used for this amount of weight. It will be a 30' x 40' house with 12' walls.

A: (Kelly) With a stone wall you can use a rubble trench foundation, but I would suggest that you begin assembling the stone well below grade (perhaps 12") so that the wall is firmly embedded into the site.

Q: We are going to construct a new house. For that I would like to know the suitable stone for the foundation. Somebody told me that the black stone is better than red stone. Please clarify this for me.

A: (Kelly) I cannot comment on the specifics your your stone because these vary so much around the world, but in general I would say that practically any dense, well-consolidated stone that doesn't chip or flake easily should serve your purpose. I suggest that check around your locality and find out what kind of stone has been used historically and go with this.

Q: My husband and I just bought a new home that has an exposed concrete foundation. We have tons of river rock in our yard, which has no landscaping. How do we begin a masonry river rock wall to cover the foundation?

A: (Kelly) The simplest thing to do would be to just start dry stacking the stones up against the existing foundation, since this is really more of an aesthetic project than a practical one. I you want to get fancier, you could create a small rubble trench (about 1 foot by 1 foot trench filled with gravel) next to the existing foundation, and then carefully lay and mortar the stones into place. It depends on what you want the finished foundation to look like.

Q: My wife and I just purchased an older home. A 12  foot section of the stone foundation blew in. The total run of the wall is 20 feet. The corners are holding the weight along with several floor jacks. Several friends and a few contractors all suggest building a new block wall, which would be fine except, having just bought the home, we are very limited on budget. Would it not be possible to use the fallen stone to rebuild the section just temporarily until we can raise funds to build new block walls for the area...maybe one year at most. Also, could it be rebuilt from the inside? We have been able to dig out the stone and dirt that have fallen and dig back behind the foundation a few feet. I understand this is not the right way to do this, but in theory it could be done as a temporary fix, right?

A: (Kelly) Rebuilding the foundation with the existing stones is certainly possible, but I would not do this as a temporary measure. Stone masonry is much more time consuming than laying blocks, but the result is just as strong and better aesthetically, especially since you will be matching an existing foundation. As long as the house is safely supported for working under it, then the foundation work could be done from either side.

Q: I have a question about footings. I live in north carolina; the frost line is 12". I have a 28 by 70 double wide. It has underpinning. I want to take it out and put a concrete block wall around it with rock veneer my question is how deep do I need the to dig the footing and does it need to be all concrete or can I use rock in the bottom and concrete on top and do I need rebar? The ground is level.

A: (Kelly) If your frost depth is only about a foot, I would recommend that you dig that deep for the footing for your wall. You could begin with a rubble trench of drain rock, and then pour about 4" of concrete on top of this, with perhaps two parallel rebars running through it. If this project needs permits, you might check with the authorities about this before you do it. Also, if you are removing any interior supports for the home, this may not be advisable...

Q: I am going to build a one story, with large loft, stone and timber frame house (32x40) in northern Michigan. I would like to use all stone for my foundation and around my pilasters that will hold the trusses. Will I be able to run my hose for the radiant heat under the stone foundation or will I need to run it a coarse below the top of my floor? Or could I pour a 6-8 inch concrete slab put down the hose and maybe conduit then stone? I don't want my foundation to sink or crack do to the radiant heat and frost. There is a lake near by so the water table is only a foot or two below where bottom of the foundation will be.

A: (Kelly) I think that it is best to keep the radiant tubes out of the foundation; they work much better under the floor. Be sure to insulate the radiant floor from the soil to keep from losing all of that heat.

Q: I am planning an addition adjacent to my 200 year old home. The existing home has a stone and mortar foundation with a 2 foot brick top layer. We only have 6 feet of headroom in the basement now. The addition we are planning will have a full basement that will be a garage. We are hoping to achieve a head height of 8 feet in the addition. Would it be a better choice to sister a foundation next to the old one and take the chance of undermining the existing or would it be better to knock the fourth foundation wall out and reconstruct a newer deeper fourth foundation wall that will hold the existing and the addition?

A: (Kelly) If I were to take on this project, I would probably leave the existing foundation as is, and try to "sister" your new one next to it. Most soils are stable enough to remain sufficiently intact while you're excavating for the new foundation. You will know when you dig down to that level whether the soils are of a caving type or not. If you have a very sandy soil that will not stay put while working on the new foundation, then you may need to pound steel retainers in place, but I bet this won't be necessary.

Q: We have a relatively small one car 'garage' which is currently jacked up about 4 ' off the ground, and the plan is to continue to lift it to just under 2 story height. It is probable that the upstairs would be finished as a no-utilities studio. There is a very old slab and the original plan was to use concrete filled cinder block reinforced with rebar as footings. The idea has surfaced to build an approximately 4' high native limestone rock foundation under this building, and rest the building on it. I am skeptical. Is this a practical undertaking for one with limited experience with rock walls? What kind of footings and/or trenching would be required/recommended and must they go below the frost line? Should the rock wall be reinforced in some way to hold the weight of a 2 story building? Should there be other structural support for the building besides the wall? What is the minimum and ideal width at the base for the wall? What should the interface be between the rock and the building?

A: You should definitely get the foundation below frost line. Sometimes a slab will have a  sufficient foundation incorporated into it, but probably not. The bigger the building the more frost  will heave the structure in different ways and put varying stresses on the walls. The depth of the foundation depends on where you live and the depth of the frost line. In most instances you can use a rubble trench foundation with a grade beam starting a foot or so below grade. A concrete filled cinder block stem wall with rebar reinforcement is a simple solution for the beginner but a rock wall can be just as strong or stronger if done well. I would hate to have relied on the first stone wall I built but an experienced mason can give you a stronger wall than cinder block. At the top of either wall you should pour a bond beam of solid reinforced concrete with J-bolts embedded in the masonry to attach greenboard or redwood. Your new framing will attach to that board.

Q: I would like to build an earth sheltered home and have lots of native sandstone on site. I also have access to river gravel and another type of harder, more angular rock (I'm not certain of the type.) We have a red clay soil and the frost line is probably about one inch. The soil temp is 58 degrees, and the environment has hot, humid summers and mild winters with the occasional ice storm. Could I. . . 1. build a rubble trench foundation 2. use stone for the walls 3. then a waterproof and insulation layer 4. then berm with the clay soil? I'm sure there will have to be French drains all around. Would a round design be better than angular? All I have read about uses rebar and concrete for earth bermed construction.

A: It sounds like you have a similar building site to mine, although my frost line in northern New Mexico is a couple of feet deep. I am building an earth bermed house out of native sandstone in red clay soil. I used a rubble trench foundation and I am sure that you can do the same. I would suggest that you make it a couple of feet deep even if you don't have a frost problem so that ground water doesn't cause the clay soil to heave under your walls. I am really partial to the sandstone for at least the stem walls of the house, it is relatively easy to chip away at so you can get a tighter fit. I set my house into a hill on the north side about 7 foot deep. The rock wall starts out three feet thick at the base and tapers to two feet where it comes out of the ground. I put a curtain drain behind this wall that starts about six inches underground and runs down the wall and into the rubble trench. The rubble trench foundation has a 4" perforated pipe in it that runs to daylight. I skipped the insulation and waterproofing, the curtain drain has served the waterproofing needs. My house is oval shaped and any rounded structure is stronger than one with straight walls and corners. You will also get more interior square footage for every foot of exterior wall in a rounded structure.  I think you are on the right path using the material that you have at hand and tying into that cool ground temperature will sure help during those hot humid summers. I would say that you have a long intimate conversation with that sandstone ahead of you. Good luck with your project.

Are you using mortar? If so, what kind? Could you direct me to information about "curtain drains?" The main complaint about underground homes in this area is that the walls sweat -- cold ground, warm interior. This is why I had in mind to insulate on the outside of the wall. Do you have any thoughts or suggestions?

I did mortar my rock wall with local sand and masonry cement. A curtain drain is a "french drain" that does not get to the surface. I used 2" river rock about 6" thick against the back of my rock wall. I don't want to have surface water against the wall, but channel any ground water away from the masonry. I haven't had any problems with the walls sweating. I won't discourage you from using insulation. It will help to keep your house warm irregardless of the sweating issue. I skipped that process more for philosophical reasons. My house is completely above ground on the south side for solar gain.

Q: I'm about to start working on a foundation. The two methods I've been considering for the house are earthbag and cordwood. I'm in upstate New York. The soil is clay rich. There are countless stones on the 73 acres. I have no money to speak of and am endeavoring to make the foundation with what's available. I know little about where to start. I have lot's of energy and am thoroughly fed up with all of the other ways of living I've tried. What is necessary besides shovels, rocks, come-alongs, ropes and wheelbarrows?

A: (Kelly) Your foundation needs are somewhat different with earthbags and cordwood, but in either case you can use your local stones. For earthbags, you can dig a trench at least as wide as the finished wall will be, and at least as deep as your local frost depth. Then you can fill this with fist-sized stones for a rubble trench foundation. Use smaller size gravel at the top of this trench and fill the first couple of courses of bags with gravel...and that it your foundation!

For cordwood construction you need a raised solid masonry base to support the cordwood masonry wall. This can be created by starting with the same rubble trench, which is then finished above grade with stone masonry to make a flat, level foundation for the cordwood.

Q: I have a stone and mortar basement; we did extensive excavating to seal the outside of the foundation and also put in drains around the house at the base of the foundation, topping it off with gravel. Additionally, we installing a sump pump on the outside of the house on one corner, near where an underground natural stream is, in order to further eliminate any water build-up. I would like to finish the basement now but before we start, what type of wall treatment do you suggest? Should we merely use Kilz and then paint over the stone and mortar? My initial thought is to leave the basement open for a loft-like appearance, sheet rocking only the utility room, bathroom and laundry room.

A: You will have to use a better product than Kilz. Now that you have the excavation done it would be to your advantage to touch up any loose mortar or even put a layer of mortar over the old foundation then paint the outside with a waterproof coating. It is commonly sold and is usually a black rubbery substance. The curtain drain is the important part of the solution to your problem. I always put a perforated pipe in the bottom of the drain that runs to daylight if possible or at least to a sump. Now that the excavation is done you may as well do it up right.

Q: I am planning a 12'x20' stone cabin on my property in the mountains of western Maryland. The property is FULL of relatively flat sandstone that seems perfect for building (and free). My question is what to do for the foundation? The frostline is 3 feet. I have researched Rubble Trench foundations but will probably not be able to get the amount of gravel/1/2-2" rocks necessary to fill the entire trench (about 216 sq ft). This is because of the location of the property on top of a large mountain and the lack of my own truck to beat up bringing it up there. Could I use one layer of small rocks/gravel at the bottom and then larger rocks above that-creating a pseudo-rubble trench? I have no problem begriming a mortared wall 6-12" below grade. The soil has pretty good drainage and is 25% scattered rock. Also the planned building site is on a slight decline to facilitate drainage. I am trying to avoid making a mortared stone foundation.

A: You can definitely use the rubble rock that you have on hand. I am sure the original rubble trench foundations didn't consist of graded 2" stone. Throw the large stone in the bottom of the 3' trench and use any small stone you have to fill in the voids as you go up. Since you have some grade to the site it would be nice to put a perforated pipe in the bottom of the trench that runs to daylight. When you run out of rubble pour your self a grade beam and start mortaring your sandstone. Sounds like a fun project. My back gets sore just thinking about it.

Q: I am building a stone house using 12"-24" angular granite. This is going to be non load bearing with 4x6 timber frame on the interior. The stone will obviously wrap the outer exterior. What are my options for footings or other recommendations?

A: You will have to build a normal foundation I would imagine, as the weight of the stone will be enough to warrant it even without the roof load. I really like a rubble trench foundation myself. Take the trench down below frost line and fill with a good draining rock of 1 1/2" to 2". Compact the rubble with a tamper. Start your mortared stone 8" to a foot below grade and go up from there. This will save a lot of work and materials over a solid foundation and give you the same protection against frost heave.

Q: My husband and I have dreamed of building a small "real" Michigan barn and had wanted a fieldstone foundation. That is not possible due to building codes. OK, fine. What is your opinion about cut stone applied to the cement block for the stone foundation look? How do you actually "cut" stone and is it reasonably possible to do? Many of the 100+ year old barns we have looked at here in Michigan have cut fieldstone - if they did it 100+ years ago it should be even easier to do today, right?

A: (Kelly) Yes, it is possible to cut stone with a diamond tipped blade, but it tends to be a lot of work, and would probably not be worth it. You could also build the foundation with the concrete block and then face it with uncut field stone right next to it. If you do this, make sure that the footing for the foundation is wide enough to accommodate the stones.

Q: I want to know, what will be the thickness of foundation of stone masonry wall of 40mm and depth for hilly region having clayey silt soil?

A: At 40 mm the stone you speak of would be good for aggregate in a cement wall. A stone masonry wall should use rocks from 160mm to 600mm or bigger. The thickness would depend on a lot of other factors like load and the ability to withstand earthquakes. Still all things considered you should make the wall at least a half meter thick and if you are holding back the hillside up to a meter thick. Depth once again depends on your circumstance. If you have no frost issues you might go as little as 2 feet deep. I like a rubble trench foundation in a clay soil.

Q: When planning to build a basement, would it help to think of the walls as retaining walls? Could I safely use drystone retaining walls with proper drainage as basement / cellar walls? Is there any reason that such walls could not be used as footings for the ground floor walls?

A: There are a lot of reasons to mortar a basement wall. Water is the foremost. An outstanding drainage system could keep your basement dry and then you would worry about rodents. Lots of burrowers would love to call your basement home, nice and dry with nesting material handy and maybe a food supply. Insects are of course a concern. Spiders in particular love the deep recesses of a drystack wall. Consider also that a drystack wall will almost certainly require more stone and meticulous fitting to hold back 8 feet of dirt. A mortared basement wall is no small undertaking but to stack it dry and keep the basement habitable would be a Herculean task.

Q: I'm involved in the construction of a two-story 50'x50' clinic for a very poor Guatemalan village. We're trying to build as naturally as possible. Our stumbling point is the foundations; we'd like to build a rock and lime mortar foundation (with a French drain and plastic sheet to keep it dry) but the site is on an old coffee grove with excellent agricultural soil that has a high organic content and is often very damp. It's also sloping about 8 feet over the length of the building. We want to do a cut and fill but we don't know how big to build the foundation. What is the optimal size for this building? Bear in mind that Guatemala is also an earthquake zone!

A: Quite a challenge you have gotten yourself into. It sounds manageable enough though. I would consider a rubble trench foundation that your stone masonry stem walls can float on. I can't give you an exact analysis of your needs but I would start with a trench a couple of feet deep and a couple of feet wide. Put a perforated pipe or tiles in the bottom and trench to daylight. Fill with rubble rock or some drain material that will pack tight and hold the weight of the building. Stay at least 6" below grade with the rubble to give your self a toe. Lay a grade beam of cement with rebar, maybe a lot of rebar in an earthquake zone. I put my house 7' into the slope of a hill and where the back wall had so much pressure on it I put the largest stone and made the wall 3' thick at the base and tapered to 2 foot at the top. I also put a 6" curtain drain behind the stone wall that drained to the perforated pipe in the bottom of the trench. No need for plastic sheeting.The cement and sand your use is going to be crucial. If you can get your hands on a type S Masonry cement you will be miles ahead. My guess is that there is a local mason that can point you in the right direction with the materials at hand.

Q: I live on the big island of Hawaii and want to build a stone foundation for a 900 sq. ft. home out of the lava rock on the land. No building permits. I am capable of learning stonework and would gladly pick up any books you may recommend on the subject of stone foundations. Can you give me some advise on building a stone foundation for this size of a home using as little concrete as possible, alternatives to concrete, etc. I want the stone foundation to also be the ground floor of the home which will be covered with an earth floor.

A: Before you learn stone masonry you should study architecture or at least what it will take in your location to design a solid foundation. The best resource for this is your local building codes. I am not an advocate for permits but I do recommend building so the house would pass code in your area. I take it you are more concerned about earthquakes than frost. These considerations will affect what you do before the first stone is placed. My guess is your grade beam will be will be critical to the whole process...lots of rebar. Then the fun begins. Mortaring stone in a foundation wall is a serious challenge that I highly recommend you take on. The cement in the wall is essential to keep bugs and water out of the house. The tighter you lay the stone the less mortar you will have to use. If you can utilize a rubble trench you might be surprised at how little cement you actually need. Around here in New Mexico the original homesteaders had no concrete and laid stone up with clay. This served them well in the short term but those houses are melting back into the land now.

My main question is if it can be done, given the lay of the land. The land is not flat, it is sloping downward, gradually. And it is full of lava rock, very uneven, some huge mounts of solid lava rock here and there. I was hoping I could just build up with stone, filling all 900 sq. ft. until I could reach a height where the whole 900 sq. ft. could be flat, and then this would serve as the floor of the home as well. Can that be done? So that it is not just a wall, on top of a rubble trench, but an entire, solid 900 sq. feet of rock, level at the top to function as the floor. So there are the issues of the land sloping downward and the unevenness with so many lava rock formations. Is that possible? I know it will be a ton of work, but besides that, can it be sound, safe, doable?

I can picture what you want to do but it doesn't sound plausible. It is important to anchor the house into the slope. Your foundation should tie your house together and lock you to one spot on the land. Building up with stone sounds like putting your house on stilts made out of big marbles. If the ground shakes the whole building is going down hill. It would be much better to find a spot where you can dig some rocks out to make a level spot. At least cut and fill to get your level footprint. The uphill side of the foundation will be your anchor but still put the downhill side underground some for a toe. If you cement the footer to existing rocks that you stepped over instead of digging out, all the better. I can picture a home site that might be too rocky to dig out enough for a level footprint. If this is the scenario at your place there are other options, the mud floor might get lost but you can get the same feel with light clay and earthen plaster walls.

Q: I want to construct a house, but in the land there are 2 to 3 big rocks. If the rocks are very huge, can a rock foundation be done, or is it impossible to build foundation if rocks are there?

A: You can sure use the large rocks that are within the footprint of your house. They will probably mess with your floor plan if they stick up above the floor, but if they are below grade just build your foundation around them. Actually using them. I like to integrate a large stone in the foundation by chipping a groove in the stone to give it a tooth for the cement to bind to the stone. I really like drilling a hole in the rock and pounding a short rebar that extends into the cement. Make sure that the stone is clean of any lichen or loose pieces where the cement will touch and wet the stone before pouring the cement over it. I have done this many times; you should have no problem.

Q: I am planning a little greenhouse with a river rock and mortar foundation, about 18" above grade. I will try to get the last top layer as flat as possible. I plan to build the remainder of the wall with stick framing. My question is how/what do I do between the rock and the stick frame? Does the mortar help level the top layer and the stick frame wood just sits on top?

A: Basically that is the way to do it. Pull a string line to your wall height and mortar your stone on the face roughly to the string. Behind the face bring your mortar and backing stone to the right height. Set some J bolts in the wet mortar to hold down the framing. I like to set the base plate in the wet cement and tap it down to the exact height. Wait a couple of days to tighten the J bolts down to the base plate. The base plate should be cedar with tar paper or equivalent between it and the cement. Once your base plates are in place you can screw or nail the rest of your framing to them.

Q: How easy or hard it is to do a stone basement/foundation for an earth 2 floors house? Is it true that it is easier to do it in concrete? We asked all constructors in the region and they all went for concrete as safer option.

A: It is harder to build a stone foundation only in that it takes skills that most modern builders don't have. Their grandfathers built this way with no problems but few in these times take the time to learn the ways of stone. A stone foundation will use less concrete and have way more aesthetic appeal. It will probably have to be a bit thicker than concrete and rebar but can be just as strong and way more appealing. Anyone with the right formula can build with concrete but a stone foundation takes knowledge of old ways that are readily learned but not often taught by man. I got pointers from a mason or two and learned the rest from the rock itself. Practice on a small project and seek advice from a craftsman and not the builder. Build with stone and your grandchildren will admire you in a way that builders with concrete would envy.

Disclaimer Of Liability And Warranty
I specifically disclaim any warranty, either expressed or implied, concerning the information on these pages. Neither I nor any of the advisor/consultants associated with this site will have liability for loss, damage, or injury, resulting from the use of any information found on this, or any other page at this site. Kelly Hart, Hartworks, Inc.


Home       Site Map        STORE

For Email contact go to About Us
Established in 2001, GreenHomeBuilding.com is primarily a labor of love. Kelly, and the GreenHomeBuilding team of experts, have answered thousands of questions for readers over the years, and we continue to publish up-to-date information about increasingly important sustainable architecture. If you feel moved to assist us in this work, your kind donation would be much appreciated; this can be easily done through our PayPal account:
Custom Search


  [Solar Car]      [Earthbag Building]     [Dream Green Homes]
See Your Ad
in This Space!

Click Here
for More Information