15 - Stone Work

The exterior walls of the basement and towers will be faced with stone. The rest of the house will be smooth stucco. Most of the stone masons around here like to use 4"-thick stone, because it's easy to stack and goes up pretty quickly. But at 25 cents a pound, stone that thick was not an option for me. By using 1-inch-thick stone instead, the cost per square foot of wall facing would only be 1/4 as much.

But I still wanted to give the impression that the rock was thick, so I studied the look of a thick-stone wall. I found that I could mimic the look fairly well by laying the stone mostly horizonally, and when not possible, at least have the grain running horizontal. I was able to find thin rock at the local stoneyards, and it even had a "grain", but it took a lot of hand-picking to get suitable ones.
The only problem was the corners, where the exposed edges were a dead give-away of the true thickness. So, how to hide the corners? In European architecture, it has been common for centuries to have columns of thick, smooth stones at each corner of the structure, then fill in between them with smaller, rougher rocks. These cornerstones are usually staggered for strength. Perfect - that look would fit in nicely with my design. But what to make the cornerstones out of?

There are some Engineered Stone products (Robertson Stone, for example) which offer pre-cut corners of real stone, but the cost would be prohibitive for me, and they only make 90-degree angles, not the 45-degrees which all my corners would require. So I decided to cast them myself from concrete. This turned out to be fairly easy. I built 5 forms out of plywood, added some rebar and a metal strap (to help secure the finished corner to the wall), and poured quick-set concrete into them. It was a fairly hot day, so within two hours there were 5 finished corner pieces.

The molds were held together with screws, so it was easy to take them apart, release the finished cornerstones, then screw the forms back together.  I sprayed a light coat of Pam cooking spray on the forms each time, to act as a mold-release.

My daughter and her boyfriend helped with making the corner pieces, and in a couple of weeks we had a stockpile of about 150 of them. This will probably be about half of what I'll need, but it's easy to start up the production line again when I've used up what we've made.

Attaching the cornerstones to the walls is time-consuming, but not very hard. First, I coated the ICF block with a waterproof membrane (I use Blue Max, but there are many available), and put on lath (chickenwire). Then I just screwed the stones to the wall, using the tabs I had cast into the stones. I used Tapcon screws which were long enough to go through the 2" of styrofoam and into the concrete core of the wall.

The next step has been a bit controversial. I injected a spray-foam adhesive behind the stones, filling in all the gaps. This is a black outdoor version of the popular yellow foam, and is typically used to hold rocks together in landscape elements, like waterfalls. When I hired a stone mason to do some later walls, he insisted on using the traditional thin-set morter construction. It will be interesting to see which method holds up best in 50 years (when I'm 112).

Now, how to deal with the round-top basement windows in the stone walls? At first I was going to cut the stones around those arches, but then I thought that a concrete trim would look nicer, and match the corners. I asked a stone mason about this, and he thought commercial trim for those windows would be about $100 each.

Wait a minute...if I could cast the cornerstones myself, wouldn't it be just as easy to make window trim? Yes, it turned to be quite easy. I used masonite to make curved forms, which were tacked down to plywood. Straight sections for the window sills were even easier. I added two pieces of rebar in each section, and the result was total success -- great-looking trim for about $4 per window!

This is as much of the stonework as I have done working alone. I tend to over-think each piece, which makes the work extremely slow-going. I hired a stonemason to do several more walls, but he was only about twice as fast as I was, and the labor costs were a little too high. I'll tackle this again in the spring, perhaps laying out the stones for each wall on the ground first.