The Art of Slipforming

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For the next generation in slipform technology we plan to utilize 6-inch-thick sheets of white polystyrene headboard insulation with 1 x 2 furring strips embedded in one face for attaching plasterboard (E). We will cut out all the doors and windows and stand up the 4-by-8-foot sheets, boxing in the whole house with the insulation. This insulation will serve as the form inside the house, thereby eliminating the need for half of the slipforms and half of the form-setting work. This will also insure straight, plumb walls from the beginning.

Footings

The footings for stone walls are much like those in conventional construction, just bigger. For most stonework I recommend framing the footings with 2 x 10s. Make the footings wide as the walls, plus enough extra width to set the forms for the first layer of slipforming. Most importantly, use lots of steel reinforcing—at least three runs of 1/2-inch rebar in a footing 18 inches wide, preferably more. Footings of this size will be sufficient for stone walls up to about 12 feet tall. For higher walls you should consult an engineer. You should also consult an engineer if you are building on soils that seem perennially damp, contain significant amounts of sand or soft clay, or otherwise seem like there may be a potential for shifting.

The "experts" say the footings should be placed below frost line (4 to 5 feet deep in Montana), but contractors are building conventional homes here with footings only 2 1/2 feet down, for what reason I still don't know. In any case, there are several ways to minimize your costs, so you are not building and burying in the ground four feet of perfectly good wall. One method is to design a house with a half basement, so you are utilizing the full height of the foundation wall. Another method is to build a shallow footing and insulate the ground outward from the house to raise the frost line above the footing. A third method is to pour the footings as part of a "floating slab" on a thick pad of gravel, so the house "floats" above any frost heaves.

Specific information on designing and building footings is beyond the scope of this article but is available in many ordinary construction books. However, the final step to prepare for a stone wall is to place anchors in the fresh concrete to tie the footings and future walls together. An easy way to do this is to jab lengths of rebar down through the footing every two feet along the walls, except in the doorways. This way the vertical rebar work is completely done for the walls, and you will not forget it later.

Slipforms

Slipforms are easy to make. Simply screw 2-by-8-foot pieces of 1/2-inch plywood onto 2 x 4 frames. The plywood should be painted with used motor oil or a commercial form oil to keep the concrete from adhering to the wood. Slipforms are a multipurpose tool. We use them for scaffolding material, and one time we bolted all of them together to make a temporary storage shed for the winter.

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