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
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
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. When working on ledge, even the batter board
stakes must be supported in drilled holes.
We found that a 3/4-inch-diameter bit produced the
proper-size hole to support the form stakes.
We used the same method to position and hold the first few
courses of 2x12 form boards. We ran strings, then used a plumb
bob and spray paint to lay out and mark the stake holes for the
outside form. After drilling and placing the outer stakes 4
feet on-center, we measured in 11 inches and drilled holes for
the inside form stakes. This dimension took into account the
8-inch-thick stem walls, plus two 11/2-inch-thick form
Scribing to Ledge
To begin assembling the outside wall form, we first scribed the
bottom 2x12 form board to the rock outcroppings (Figure 2).
2. Carefully scribing form boards to within
3/4 inch of the irregular rock ensured a neat