Q. Plans for a freestanding deck call for new piers about 2 feet from the house foundation. Is it necessary to dig down to the bottom of the house’s footing to reach undisturbed soil, or would the dirt around the foundation of a 60-year-old house be compacted enough to be considered undisturbed — in which case we could simply dig to below the frostline (per code)?
A. Dave Crosby, an excavation contractor in Santa Fe, N.M., responds: Because undisturbed soil is usually more reliable than backfill, I typically make the extra effort to dig down to it, especially if the backfill contains soft soil (anything I can easily dig without a pick or bar). However, if the undisturbed soil is a long way below the frostline or has a lot of silt and clay — and I’m confident that I have good, compacted fill — I may dig only to the frostline. In either case, whether the soil is disturbed or undisturbed, what I’m looking for is adequate load-bearing capacity.
Bearing strength is a function of the soil’s composition and density. Dense (because it’s either undisturbed or has been compacted), well-graded soil that is properly drained and has little or no expansive potential should easily support a deck with properly sized footings. The age of the house and the surrounding soil is irrelevant, because no matter how long soil sits there, it won’t compact itself. So be sure to address the soil’s composition and density in the design of your footings and piers, and test the soil whenever you’re in doubt.
You also have to be careful about lateral loads caused by placing your deck footings too near the foundation. Loads spread out through the soil underneath a footing at about a 45-degree angle, so that at a depth of 2 feet under a 2-foot-by-2-foot footing, the zone of influence is about 4 feet by 4 feet, or 16 square feet. Depending on the elevation of the bottom of your piers, deck loads could create lateral pressure on the foundation wall.
Side loading isn’t a problem in my area (seismic zone 2), because we’re required to build strong foundations. If your foundation wall is reinforced cast concrete and the floor joists run perpendicular to the deck footings, it shouldn’t be an issue for you, either. But if the foundation is fieldstone set in lime mortar, you’ll need to be careful about those lateral loads.
