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When a couple from Fallbrook, Calif., approached me last year and asked if I would design and build their Eastern Sierra vacation home, I remember thinking I was in for a hard start. Their 10,000-square-foot lot consisted largely of a terraced granite cliff, with grades as steep as 50 degrees. Getting a house to stay put on that rock hillside was going to be a challenge. On flatter lots, rock outcroppings are often blasted away to accommodate the floor elevations. Since this lot dropped away from the road access, the floor levels finished above the outcroppings, and we were able to form to undisturbed ledge. The method used to fasten a foundation to ledge depends on the type of rock and its shear strength, and it’s important to bring in an engineer early in such a project. We were fortunate that the ledge material we were working with was hard, stable granite, which allowed us to pin directly to the rock without having to blast away any loose or shattered material. Rough Start From the beginning, forming to the ledge presented challenges; even the first step of staking the corners required a rotary hammer. To erect batter boards, we drilled 3-inch-deep holes in the ledge, inserted 4-foot-long steel form stakes, and attached horizontal 1x4 wooden form stakes with drywall screws, screwing through the predrilled holes in the steel stakes (see Figure 1).
Figure 1. When working on ledge, even the batter board stakes must be supported in drilled holes.