Mortar for Stonework

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.

Questions and Answers

Q: When building tall walls of stone for a house, (two story), besides having a good SOLID foundation on which to build on, what kind of mortar/cement do you use for such a structure and how do you mix it, (ratio of sand to cement), do you use lime also? This home will be located in Woodstock, VA. and they have a very humid climate in the summer time and it can get very cold in the winter months, but not for extended lengths of time. Please be very specific, as I have read different things about mixing the components and am a little confused about mortar and cement. Are there certain brands that are better than others......are there some to stay completely away from?

A: Use a type S masonry cement mixed with three parts clean sand. I have heard of people using a type N masonry with a shovel of portland for every two of masonry. The type S gives a more structural capability necessary for a tall wall. The N is more flexible and appropriate for a veneer or brick work. Try a small project first and the rock will teach you more than I can with these few notes.

Q: I am putting flagstone on my foundation. What do you use? Mason S? And how do you use Mason S? And how do you get them to stick on the wall? It is between 3/4 to 1 1/2 inch thick.

A: The type N masonry might be better served for your purpose. It is a little more flexible and you won't need the structural capabilities of the S. I would try and stick right to the concrete foundation with a little masonry glue in the mortar and paint some on the wall too. You shouldn't need an expanded metal lath if the wall isn't too high.

Q: I'm wanting to cover a cement block wall with stacked flagstone to give it the look as if it were made of flagstone. I would like it to be held together with mortar, but I don't want to see the mortar, I would like it to look dry stacked. The wall is only about 30 inches tall. I've already attached aluminum straps to the block wall to help hold the flagstone in place, but I'm not sure exactly where to go from her. I want to make sure the mortar I bought is the right type (Portland mortar). I also like to make sure my mix is right. I was planning on a 3 sand to 1 mortar mix. Is there anything else that is important to this type of wall or is there a better way I should be going?

A: Portland cement is not the way to go for such an application. You will need a masonry cement, type N for this particular application. A 3 to 1 mix is appropriate. If you can't find the masonry cement or feel that you must go with the portland it can be mixed with lime to give it the flexibility that is needed. I would try a test mix to get the best results but 2 Portland,1 lime, 9 sand is a good starting point.

Q: I'm wanting to use small rocks that I picked up out of a creek for a backsplash for my kitchen and in my dining room on a wall; also I'd like to use them for trim around my windows. What do I use to place the stones and seal them?

A: (Kelly) Using little stones in such a decorative way can be lovely, but is often difficult to achieve. One problem is that they don't have a solid base to sit on while the mortar cures. Another problem is that they pretty much rely on the mortar to hold them in place over time. To overcome this, I suggest that you use a very sticky mortar that has great adhesion. You might experiment with some of the tile setting or grout formulas for this purpose.

Q: I need help with what bonding material for the cap stone. I built a dry laid, 60 foot circular wall (for a garden bed) about 15-24 inches tall (sloped land). I dug a trench and put in 4 inches of tamped 3/8 minus, then cinder block with the holes filed with 3/8 minus. I carefully stepped back the thin colonial gray stone and saved some nice flat pieces for the caps. There is 4 inches of 3/8 minus behind all the stone and then landscape fabric before the composted great soil. I'm in New England and need protection from freezing temps. Do I use cement or mortar to fasten the cap stones? I've seen both.

A: Sounds like you have a solid design for a wall. Use a type N masonry cement for the cap stone. The mortar should be keyed into the block an inch or so and up between the cap stone. Since the cap stone doesn't have much of a tooth to bind to you may have to do some repair over time but that should be easy.

Q: I am interested in using river rocks that I bought at the craft store to put on my backsplash in my kitchen. Would regular mortar you use for tile hold it and then would I use regular grout and since it is not flat how would I get the grout in there?

A: (Kelly) Getting small, smooth, rounded stones to stick to a vertical surface is not an easy task. You need a very sticky mortar with good adherence for this job. Probably a good tile mortar is about your best bet, especially one that is mixed with latex to make it stick better and be moisture resistant. I would suggest applying the mortar thick enough that the stones are seated in it without the need to grout between the stones later, since applying grout in an ordinary way with a squeegee is not possible.

Q: I am going to lay a shower floor, as one of my projects. I have some small glass tiles (3/4 " sq or a little less) that I am going to set in a scattery pattern (no pattern). Was thinking I could set them right in regular cement. I think standard concrete sites might recommend against this...or suggest latex additives. What do you think?

A: (Kelly) Yes you can do this if you are careful. I would suggest pouring the concrete first and jiggling the surface enough to get the larger aggregate to settle some, exposing the finer sand and cement on top. Then you can carefully press the tiles onto the surface so that they are as close to level as possible. Don't worry about cleaning stray cement off the tiles until it has set up enough that touching them won't dislodge them. Once the cement has set up enough you can even use a hose or sprayer to wash them clean, like is often done with exposed aggregate concrete. Adding latex to the cement is probably a good idea because it will help with the adhesion to the tiles. In Mexico the use of such an additive is very common, and you can buy a few liters of what the call "sellador" (sealer) at many hardware stores.

Q: I would like to know what would be the best type of mortar to use to apply some rock to the side of my house. I am placing it over 1/2 '' chip board if that will do. Would like any and all information to do the job right.

A: Put tar paper over the chip board and then expanded metal lath. Use a type N masonry cement. You may have to lay a narrow footer to support the rock depending on the weight and thickness of the stone. You might also want to use a masonry glue in the mortar to compensate for lack of experience at getting the mud right. Use long screws to help hold individual stones in place while the mortar sets. Beyond that you will learn as you go.

Q: I recently had a house built with a dry stack rock on the front (mortar not showing). The concern is there are many gaps in mortar around the rock, some small and some the size of 50 cent piece. It's not noticeable but wondering about long term if I should get this repaired, water damage and bugs? Is this something a novice could do and what steps are best practice? I live in San Antonio, Texas with hot summers and mild winters.

A: You can sure fill in the gaps yourself pretty easily. Get a bag of type N masonry cement if you can, type S will do if you can't find N. Mix one part cement with three parts masonry sand, pretty much any sand that will go through an eighth inch screen. Make the mix as stiff as you can, it will moisten up when you put the trowel to it and will be easier to clean up. Keep the stone moist ahead of yourself and use a thin trowel to pack in the mud. The real trick is in the clean up. Brush off any cement on the rock the same day. I like to let the mortar set up an hour or two and strike back the mortar joints with a trowel to reveal the sides of the stone. I then go over each stone with a toothbrush and a fine mist of water. It doesn't take that long but what a difference.

Q: I'm dry stacking a 16" stone wall and have been filling small holes with mortar (mostly for "critter control") as I have been laying. I continued laying while it was too cold for mixing mortar. So here's my fairly dumb question. Would it do any good to pour dry mortar from the top and add water as holes below start to fill in? I wouldn't expect any structural support from the mix, but I'm wondering what the result would be.

A: I am sure you realize that pouring dry mortar and adding water later will not give you a consistent set. It would probably be just as effective to put small gravel in these voids. Both would keep mice out, though neither application would do much against flies or ants. Your best bet might be to use pea gravel in the voids and mortar the inside face of the wall to keep critters out. Go over stone, gaps and everything so you have a smooth surface even the little crawlies can't get past.


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