The Art of Slipforming

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As noted in the section on footings, we start with vertical lengths of 1/2-inch rebar spaced about every two feet along the wall. We bend these up around the window frames as we come to them, so there is comparatively more reinforcement at these points.

We lay a length of rebar horizontally about every foot up the walls. This is especially important at the corners, and the rebar should be bent at these points, extending for three or more feet in each direction.

Reinforcing bar is remarkably cheap, costing only about three dollars for a 20-foot length of 1/2-inch diameter rod, but it still adds up when you buy a lot of it. Fortunately you can use about any reasonably clean, skinny piece of steel for rebar. We tied together all the walls in our house above the door and window frames with 40-foot lengths of one-inch diameter steel cables we found. Our walls and footings are full of all sorts of other cables, steel fence posts, and barbed wire. You may find that masonry work can be a constructive way to clean up your neighborhood!

Rocks

Rocks can be purchased at almost any brickyard, but it is much better to get your own if you have a source. Brickyard rocks are expensive, partly because they are usually transported hundreds of miles from a quarry, but also because the rocks are selected for freehand masonry, with similar thicknesses and flat, bricklike platforms all the way around. Slipform masonry is much more forgiving than freehand work, and you can easily save $100 to $200 per hour of effort by driving around the countryside picking up almost any rocks you can find. Just be careful not to overload your vehicle. As a loose rule of thumb, a load of rocks a foot deep in a pickup bed is a cubic yard, weighing approximately 2,000 pounds!

The rocks should be solid and not fractured or crumbly. There should be at least one good, flat face to place against the form. Avoid those tempting thin stones that are only an inch or two thick These may ultimately pop off the wall, leaving an ugly patch of concrete exposed. The rocks should have an average thickness about two-thirds the width of the wall you are forming, but can vary from 3 1/2 to 7 inches in a wall 9 1/4 inches wide. Do not use rocks that are as thick as the walls; the concrete backing is an essential part of the slipforming system.

Farms are often good sources of stone. Farmers pick rocks out of the fields and place them conveniently in a pile, ready to load. Usually they will let you have the piles as they have no use for them. We picked most of our rocks in the mountains within a mile of our house, but we also gathered river rocks for around the hot tub, and we brought back a few special rocks from picnics. The house in these photos (25 by 40 feet outside dimensions) was built with rocks we gathered at an abandoned quarry. There are 16 truckloads in it, and probably twice that in our own home.

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