Replies

1. 
   AndyEngel | Apr 17, 2014 09:46am | #1

   It's not specifically spelled out

   But it's fairly simple. You need to figure the tributary load each footing will bear. To do that, figure out how much of the deck and roof each footing supports. Typically, that's halfway back to the house, halfway to the next footing, plus any cantilever. The standard deck load in the IRC is 50 lbs per square foot (40lbs live load and 10lbs dead load). Roof loads are a little trickier because you need to use your local snow load plus, typically, a 10psf dead load. The IRC has snow load maps, or your building department can tell you. The tributary area is figured the same way, and as if the roof is flat. Once you have the tributary areas, multiply each by their respective design loads to find the tributary load. Add the roof and deck tributary loads together for each footing.

   Once you've got those numbers, you need your soil bearing capacity. The default number in the IRC is 1500psf. Yours may be higher, but you need a soil engineer to tell you that, and it's probably not worth the money to save a little concrete. Do the math, dividing the tributary load by 1500, and you'll get the minimum footing size. There's an article about this in this year's Decks and Outdoor Projects issue, which you can find on newsstands now.

2. 
   Mike_Mahan | Apr 17, 2014 10:05am | #2

   Handouts

   Many building departments have handouts that cover residential decks with standard construction including the footing size. Attached is one from the city of San Diego. Check with your local juridiction or one near by.

   
3. 
   AndyEngel | Apr 17, 2014 10:18am | #3

   Also,

   Google DCA6-09. It's a deck specific alternative code based on the IRC. It's got great details, and your building department might let you use it. In fact, the deck details in the 2015 IRC come from it.