Deck Footings in Rocky Ground
When it's not possible to dig frost-depth footings, a freestanding deck may be the best option.
I’m building a deck where we have a 4-ft.-deep frost line. I might have more rocks than dirt—I’ve tried digging in several spots and using some specialty footing products without success. I’m curious what you do if you are unable to get below the frost line? I’m about to throw in the towel.
—Chas Threlkeld via email
Editorial advisor Mike Guertin replies: My solution is to build a freestanding deck. There is an exemption in the IRC from the requirement for frost-depth footings when a deck is not connected to the house. Free-standing decks only need footings that are 12 in. deep. Design the deck with a beam a couple feet out from the house and a beam along the outside edge, or run the beams perpendicular to the house and the joists parallel to it. You’ll no doubt encounter a few immovable rocks even digging shallow footings. I figure if there’s a rock big enough that it can’t be dug out of a 12-in.-deep hole, the rock itself is deeper than 12 in. Just drill a couple 4-in.-deep holes in the rock, insert short pieces of rebar, then form and pour a concrete footing over the rock and rebar.
Leave a 2-in. space between the deck rim joist along the house side and the house itself, and don’t connect the deck to the house. The footings may still heave during the winter when the ground beneath them freezes, but there’s no harm when the deck rises and falls a few inches seasonally, as long as any step from inside the house onto the deck doesn’t exceed the maximum rise (7-3⁄4 in. per the International Residential Code) when the frost melts.
From Fine Homebuilding #294