Complete Guide to Sizing Deck Footings
Use the charts and tables in this comprehensive guide to choose the right size beams and specify the appropriate concrete footings for your next deck project.
Synopsis: Mike Guertin takes the guesswork out of deck loads in “Sizing Deck Footings.” With plenty of guides for determining beam configurations and footing size, style, load, and frequency, Guertin addresses the central concerns determining safe and accurate support.
Footings transfer the weight of a deck and its occupants to the ground. How many footings you need and how big to make them is specific to each deck. Doing the calculations takes only a few minutes, ensures that I’m following best building practices, and keeps me from digging more than necessary.
The size and spacing of footings tie directly to the maximum spacing between posts of the beam they support. A larger beam can span a greater distance, requiring fewer but larger footings. The American Wood Council publishes the Prescriptive Residential Wood Deck Construction Guide, which is based on the International Residential Code (IRC). Commonly referred to as DCA-6 (awc.org/Publications/DCA/ DCA6/DCA6-09.pdf), it contains tables for sizing deck beams. The DCA-6 assumes a 40-lb.-per-sq.-ft. (psf) live load and a 10-psf dead load. (Live load is the weight of occupants and furniture, while dead load is the weight of the structure.) Although the DCA-6 tables are valid in most jurisdictions, snow loads (found in the IRC) in northern New England or the Western mountains may exceed 40 psf, and you’ll need to substitute that for the live load. Some local building codes may also require designing to a greater live load. In either case, you may then require an engineer’s help.
Knowing the total load in psf, the size of the deck, and the number of footings, I can calculate what each footing has to support. The footing size is based on this load and the bearing capacity of the soil.
Choose a beam
Several factors determine which beam setup to use. Should it overhang the end posts or end flush with them? Is there a backhoe at hand so that digging a few large footings makes sense? Or is this deck on a house with established landscaping that calls for a greater number of smaller-diameter footings that can be dug by hand around obstructions? What is the joist span?
After answering those questions, I choose a beam configuration, such as a double 2×8, from the DCA-6 table and determine the number of footings needed based on the size of the deck. I prefer a double 2x beam because it can rest on notched 6×6 posts. The 2 1⁄2-in.-thick leg on the back of the notch bolts to the beam. Triple 2x beams sometimes make sense, but they require a structural connector to join to the post.
The DCA-6 table allows joist overhangs (cantilevers) past the beam of up to one-quarter of the span between the beam and the ledger. The beams themselves can overhang the end posts by one-quarter of the post spacing. By cantilevering the end of the beam, you often can eliminate one footing.
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