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.