Deck footing calculator
Enter the beam span (the post spacing) and the joist span. You get the footing size and thickness from the American Wood Council's deck guide, the tallest 6x6 post allowed, and the concrete for all the footings.
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Result
Deck footing and post height table
DCA 6 Table 4: maximum 6x6 post height for the species selected above, and footing size for any post, by beam span and joist span. NP means not permitted.
| Beam span | Joist span | Max 6x6 height | Round footing | Square footing | Thickness |
|---|
Values are from the American Wood Council's Prescriptive Residential Wood Deck Construction Guide (DCA 6-15, based on the 2015 IRC). They assume a 40 psf live load and 10 psf dead load, No. 2 grade lumber and wet service. Hot tubs, snow loads above 40 psf, roofs on the deck or a local code amendment all change the answer.
How the footing is sized
Each post carries the deck area halfway to its neighbours in every direction, so the load grows with both the beam span and the joist span. DCA 6 Table 4 turns that into a footing big enough to spread the load over soil that can take 1,500 psf, and thick enough for 2,500 psi concrete not to crack. Corner posts carry less, and the guide allows 90% of the size there.
An 8 ft beam span with 12 ft joists falls in the 8 ft by ≤ 14 ft row: a 24 in round or 21 in square footing, 10 in thick. That is about 2.6 cu ft of concrete each, or five 80 lb bags.
Depth
All footings bear on undisturbed soil at least 12 in below the undisturbed ground, or below the frost line, whichever is deeper. Footings within 5 ft of the house's foundation wall must bear at the same depth as the house footing. Ask your building department for the local frost depth and whether they want a soils check.
Post height
The height limit is for 6x6 posts, measured from grade or the top of the footing, whichever is higher, to the underside of the beam. 8x8 posts may be used anywhere in the table up to 14 ft. Corner posts over 2 ft tall need diagonal bracing, and DCA 6 requires all deck posts to be at least 6x6.
Questions
How big should deck footings be?
It depends on the beam span and joist span. In DCA 6 Table 4 they range from 18 in round, 7 in thick, for 6 ft beams and 10 ft joists, to 42 in round, 19 in thick, for 18 ft beams and 18 ft joists. An 8 ft beam span with 12 ft joists needs 24 in round, 10 in thick.
How deep do deck footings need to be?
At least 12 in below undisturbed ground, or below the frost line if that is deeper, and at the house footing's depth if within 5 ft of the foundation.
How tall can a 6x6 deck post be?
Up to 14 ft for short spans. The limit drops as the spans grow and varies by species; at 12 ft beam span and 14 ft joists it is 13 ft in Southern Pine but 9 ft in Hem-Fir and 5 ft in Ponderosa Pine, Red Pine or SPF.
How much concrete for a deck footing?
A 24 in round footing 10 in thick is 2.6 cu ft, about five 80 lb bags. The calculator totals it for all your posts.
Can I use 4x4 posts for a deck?
Not under DCA 6, which requires 6x6 or larger posts. 4x4 posts appear in the guide only for supporting stair stringers.
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