Credit: Adapted from “Prescriptive Residential Wood Deck Construction Guide”
Most decks can be supported by wood posts resting on concrete footings, piers, or a combination of the two.
Size. Assuming a minimum soil bearing capacity of 1,500 psf, 8-inch-diameter concrete piers bearing on square footings measuring 2 feet on a side and 9 to 11 inches thick are adequate for most single-story decks where beams are spaced 14 feet apart or less and joist spans are 14 feet or less (see Footing Size table).
Depth. Keep deck footings at least 5 feet from a house foundation. Deck footings closer than 5 feet should be set at the same depth as the house footing. Otherwise, place footings below the frostline or at least 12 inches below grade.
Reinforcement. To prevent cracking of the footing due to point-load forces, add steel reinforcing. Make sure an unreinforced footing is not too wide. It should extend at least 2 inches beyond the pier but no more than the thickness of the footing. In all cases, center piers or posts on footings.
Engineered alternatives. For problem soils, steep slopes, or sites with difficult access, or where environmental or other conditions preclude excavation, several engineered alternatives to conventional concrete footings and piers are available. Hydraulically driven helical piles (photo) are typically installed by subcontractors who use torque readings to achieve the required bearing capacity in a variety of soils. Precast pin footings (photo) bear on steel pipes driven using a hammer drill through sleeves in the piers to a depth determined by the frostline.
Call 811 before you dig. Whether you plan to dig holes for footings or use an engineered pier system, take the precaution of calling ahead to determine the existence and location of any underground services in the project area. In most of the U.S., simply call 811 at least three days before digging (in Canada, each province has a separate phone number for information).
Michael Chotiner is a contributing editor to JLC.