How to Build a Stone Foundation

Reader Contribution by Robert Maxwell
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When I decided to build a forest cabin back in 2009, I knew almost nothing about building. Over the next three years I learned a lot about construction, including how to create attractive, long lasting stone foundations for almost no money. If you’re putting up a building and think you’d like to set it on something more beautiful than cinderblocks or Sonotubes, this blog might just provide the inspiration you need.

Choose and Prepare a Location

Before you start, you’ll need to pick a spot. For my cabin, I was lucky enough to find an area where flat, smooth bedrock was just a few inches below the soil. After some digging,  I had a perfect surface to start laying stones and mortar. But easily accessible bedrock is 

pretty rare, and you’ll probably be building somewhere with several feet of sand, soil, or clay between you and the hard stuff. If this is the case, you’ll need to hire some heavy equipment to dig down below the frostline, then have a cement truck pour a reinforced concrete pad slightly larger than the dimensions of your building.

Build and Position Plywood Forms

Building a solid, unbroken stone foundation is fairly daunting. That’s why I chose to build eight separate stone pillars instead. Rather than working completely by eye, I bought some plywood and made eight separate rectangular forms, which I positioned around the inner edges of my cabin’s footprint. I put one pillar at each corner, and two more on each long side of the building. I’d already cleared the soil in these areas down to bedrock. Once I was sure the forms were in the right places and square to each other, I filled in the dirt around them, anchoring them in place. When you make your forms, be sure to make one more than you need for your foundation. I’ll explain why later. 

Do the Below-Ground Stonework

Filling up the bottom of your plywood forms with stones and mortar is the first real step. This work doesn’t have to be pretty, since in the end it won’t be seen. Gather a large supply of fist-sized field stones from wherever you can, then pile them next to your plywood forms. Next, mix up a large batch of mortar. I found that a 50/50 mix of masonry and Portland cement worked best. Dump the mortar into the forms until it’s about as high as the surrounding soil. While it’s still wet, add as many field stones as you can without overwhelming the mortar. Repeat this process for each plywood form. When you’re finished, the field stone and mortar mixture should be a couple inches higher than the surrounding soil. Wait for it to harden, then use a laser level to mark a line of consistent height on the outside of each form. This will become the top of your finished pillars. Once you’ve marked each form, use a circular saw to carefully cut them down to size. 

Do the Above-Ground Stonework

You’ll have plenty to keep you busy for the week or so it’ll take for your field stone and mortar mixture to harden sufficiently. The next step is to gather as much building stone as you can. You’ll probably use at least twice as much as you think. If there’s a limestone quarry in your area, feel free to have a load or two of stone slabs delivered to your place. If you’re more of a purist, you can try gathering your stone the old fashioned way– by looking for natural outcroppings of sedimentary rock and cutting what you need from the earth. Next comes the most challenging part — working the stone. Remember that extra plywood form you built? Now is when it comes in handy. You’ll also need a saw with a stone-cutting blade, a stonemason’s hammer, and some chisels. Use the saw to cut blocks of stone to the approximate shape you want. Then shape the stone’s faces with the hammer and chisel, giving them a slightly rounded profile. When you think you’re done, place the stone into your spare plywood form and see how it fits. Fill the form with one layer of stone at a time, choosing pieces that fit well together. Aim for about an inch of space between each block. The exact height of stonework you’ll need depends on what height you cut your forms to when you positioned them at your building site. Make sure each new layer of stone doesn’t have joints that overlap those in the previous layer. This will keep your stonework strong and crack-free. Once you’ve shaped enough stones to fill a form, bring your stones, some mortar, and a trowel to your building site. Slather about an inch of mortar onto your now hard field stone mixture. Then lay your first layer of limestone blocks inside the form, nestling them firmly into the wet mortar. Fill the space between them with as much mortar as you can pack in. Don’t worry about being neat at this stage. Once you’re done, cover the top of your limestone blocks in another inch of mortar, and repeat the process. Try to guide the stonework so that when you’re done, the top layer is just a fraction of an inch below the top of your form. 

Finishing Touches

Once your stonework’s had a chance to harden, fill in the remaining space in the form with mortar. Use a board to screed off the excess, leaving a perfectly flat, level surface. Once this mortar has cured, you can remove the plywood forms. Chances are your stonework will look pretty messy, with loose, crumbly mortar hanging out of the joints. Fear not. This is where pointing comes in. Use a hammer and chisel (or an air hammer if you have one) to knock the crumbly stuff loose. Then mix a bucket of mortar and grab a trowel. Carefully fill each joint with mortar, then smooth it with the trowel, until your stonework looks clean and attractive 

Building a stone foundation certainly isn’t for everyone. It involves a lot of sweat, hard work, and stone dust, and many people don’t consider the aesthetic benefits worth so much trouble. But if you’re anything like me, and long-lasting, beautiful work matters to you, Maybe you should consider breaking out the hammer, chisels, and cement mixer for your next project.