Is there a section in the IRC to determine the proper diameter of a footing to be used with a deck? If not, how would you determine the correct diameter?
Thanks
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You have to know the bearing capacity of the soil and the load of the deck, live and dead.
Here's what the code is for NC. Not sure where you are.
http://www.durhamnc.gov/departments/inspections/decks.cfm
Good link, I used to have a Charlotte link with a bochure with the same info plus some beam/joist tables. Could not get it to load last time someone had a question. The tabular stuff is direct lift from NC Res code but looks nicer.
Ask your building department. It's their call.
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Be careful about asking your building inspector this question - they may decide that you don't know what you are doing and insist that you have your plans stamped by an engineer.
The deck designer specifies the correct number and size of footings - based on the load on the deck. If you don't understand the process, you should consider hiring an engineer.
Soil loading, expected weight on the deck, height above ground, and code all factor into this.
Having said all that, most decks are significantly underdesigned.
As a rule of thumb, assuming you have competent soil and won't have parties on your deck (i.e. max 4 people at a time on the deck) and the deck is at ground level, you should be OK with 24" diameter footers in a 6' grid. But you have to do the calculations for your situation to be certain.
Your answer looked pretty good until the last paragraph. Where did that size footer and layout come from? Seems a little large on diameter and close spacing. That said overkill is a good thing. With that layout he should be able to have the party bigger than 4 people (:-).
Where did that size footer and layout come from? Seems a little large on diameter and close spacing.
Since we know nothing about his soil conditions, I used a pessimistic soil rating and was conservative on the design. For much of the US, the soil rating will be significantly higher and the number of piers could be safely reduced.
Without knowing his specifics, I can only post a design I can guarantee will work anywhere in the US.
Please also note that these are 24" diameter footers - 12" diameter piers should be fine. The footer needs to be larger to help prevent frost heaving (assuming the OP is in an area that freezes in the winter).