Most of our basement walls are poured concrete buried up against the hill, but part of the south and west walls are exposed with a walkout, and we are making them out of slip-form stone masonry.
Underhill House is a being built as a kind of laboratory to try and demonstrate a number of alternative building techniques and materials, and slip-form stone masonry is something that our construction manager, Bryan Dalstrom, has been very eager to explore.
We’re going to use slip-form wall construction to make the bottom 18-inch base around the entire upper floor as well to keep the straw bales up off the ground.
Slip-form stone masonry is billed as a fast, easy way that allows even novices to build stone walls that are strong and straight. Luckily we didn’t have to wing it because Tom Spicer is an experienced slip-form stone mason, and he is leading this part of our project.
As with a traditionally-poured wall, you start with a plywood-faced form that defines the front and back of your stone wall and keeps everything plumb and level as you build upwards in a sturdy, balanced way.
Tom learned his technique from a book by Helen and Scott Nearing, who moved from New York City to rural Vermont in the 1930s and wrote the classic back-to-the-earth handbook, Living the Good Life. We went over to his farm and looked at some of the walls he has made. We loved their old barn foundation appearance. It seems a very honest look.
We had briefly considered the kind of stone that you can paste onto a wall. Everyone knows this look well. You see in a lot of new construction, but we just couldn’t bring ourselves to pay a premium to glue rock to our house, squint and pretend it was solid stone.
Of course, a slip form wall is not solid stone either because the back of the form is filled with concrete and scrap rock, but it is made of real stones that have real heft. I can attribute to their mass with my aching arms.
Once we had agreed to work with Tom, step 2 was to find the stone. Bryan found some interesting stone in a quarry about 10 miles from our building site. Doug and I went over to check it out. Our part of the state is dotted with these limestone quarries. Because they are often deep, hollowed out structures, most of them cannot be seen from the road. But just on the other side of a little berm may be a huge hole in the ground, and this quarry was one of them.
We went to Swiggum Quarry and were bemused to be there. When you drive in, it feels like another world. We liked the look of the stone. It had a warm color, and met the “local” qualification. (We have been able to get all the timbers from our land, but we only had a small amount of soft and crumbly sandstone on site.)
When it was delivered, Tom looked at it, and he wasn’t thrilled. As we started to work with it, I understood his reservations. The best stone for this type of project would be stones shaped more like dominos. Ours is shaped more like dice, and some pretty wabi-sabi dice to boot.
The idea is that you build your wooden frame, then put in a shallow layer of concrete. The concrete is actually a very coarse mix of gravel, sand and Portland cement. On top of that first layer, you start setting your stone, choosing the face you want to look at and placing that against the outside edge of the frame. You set the stones about an inch apart, and when you have a row of stone to your liking, you shovel in the concrete and cement the rocks into a cohesive network. The inside edge is actually made of concrete studded with smaller chunks of irregular stone that Tom called “ugglies”, because the idea is that you’ll never see these jagged rocks again as you bury them in the concrete. But they do serve the purpose of minimizing the amount of environmentally un-friendly Portland cement needed to make a wall.
Tom said that because of the locally-sourced stone in the wall, he only used 3 bags of Portland cement instead of the 12 bags it would have taken to make this 2-cubic yard section of wall out of a solid “six-bag” per yard mix.
It’s win win. The wall is both more beautiful and uses less of that very un-green Portland cement. Portland cement is manufactured by heating limestone or chalk with clay in a rotary kiln to a high temperature (about 1450°C) to produce hard nodules of clinker that are then ground with a little gypsum in a ball mill. The firing process consumes significant quantities of fuel, usually coal. So the less cement we can use, the better. The Nearings recommended keeping the forms to no more than 18 inches high so individual stones could be placed at the bottom without having to reach down too far. After making the first layer, you keep adding layers of stone held in place by a bit of concrete. The concrete does not ooze through and cover the front of the stones, because it’s so chunky with gravel . In fact, the first day that I worked with Tom, my main job was to jab the concrete mix down between the stones with a piece of steel rebar.
After missing two work days last week because of rain, completing our first section of slip-form wall became job number one last Thursday, and Doug and I worked with Tom and the rest of the Whole Trees crew, fanning out along the slip-form frame as we all searched for the right stones, and working in the concrete. I’m very curious to see what it is going to look like when the forms are removed.
I’ll post part two of this slip-form wall construction on Friday, after the forms are off and the outside surface gets tuck pointed. The final step will be to grout between all the stones and make a good solid surface on the outside.