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Building a Code-Compliant Deck

Construction How-To, Decks, Decks, Outdoor Living July 13, 2015 Sonia


Rule of thumb for joist span: Take the “depth of the framing member” and add half that number to determine a rough footage estimate for the deck’s joist span limit. For example, on a 2x10 take 10 and add half of that (5) to equal 15. The rule of thumb says that 15 feet is your maximum allowable span for generic 2x10 joist (16” on center.)

Rule of thumb for joist span: Take the “depth of the framing member” and add half that number to determine a rough footage estimate for the deck’s joist span limit. For example, on a 2×10 take 10 and add half of that (5) to equal 15. The rule of thumb says that 15 feet is your maximum allowable span for generic 2×10 joist (16” on center.)

The frost line depth is not consistent around the nation because there is such a dramatic difference in climate from state to state. The frost line in southern Florida is less than a foot, while in northern Minnesota it approaches

6 feet. The average frost line in Massachusetts is between 30 and 35 inches. My inspector wants to see deck footings 48 inches deep. The best source of information regarding your frost-line depth is your local building department.

Footing Diameter

The diameter of the holes for your footings especially large ones, create significant concentrated loads within the soil. If you calculate the weight of the entire deck, the furniture placed on it and the combined weight of a large group of people who might be standing on it, you will be shocked at the number. Even a small deck might weigh in at 10,000 or more pounds.

Use metal joist hangers to connect all joist ends.

Use metal joist hangers to connect all joist ends.

If your deck has wood support posts and a ledger board that is bolted to the house, the deck posts holds up approximately 50 percent of the total weight. In this situation, the soil under each deck footing may be called upon to support 3,000 plus pounds.

Small sections of joist material can be installed between the joists as mid-span blocking to help prevent deck bounce and strengthen through-the-deck rail post attachments.

Small sections of joist material can be installed between the joists as mid-span blocking to help prevent deck bounce and strengthen through-the-deck rail post attachments.

Depending on the porch load, the diameter of the hole will probably range between 8 and 24 inches. Larger diameter footings spread the weight out over more soil area. We used 12-in. diameter footings on this deck.

Footing tubes come in different lengths from 4 to 10 feet. The tubes are cardboard and are easy to cut with a hand saw.

Fill them with concrete, using a shovel to work out air pockets. Apply the J-bolt and allow the concrete to cure for one week.  Once the footing is cured we install support posts. Depending on where we are on the project, I might install the joists before pouring the footing or vice versa. Either way, the temporary supports will hold the deck in place until we install and connect the deck posts to the footings.