Assuming a minimum soil bearing capacity of 1,500 psf, 8-inch-diameter concrete piers bearing on square footings measuring 2 feet on a side and 9 to 11 inches thick are adequate for most single-story decks where beams are spaced 14 feet apart or less and joist spans are 14 feet or less (see table below).
| Beam Span |
Joist Span |
Round (diameter) |
Square | Footing Thickness |
|---|---|---|---|---|
| 8′ | <10′ | 20" | 18"x18" | 8" |
| <14′ | 24" | 21"x21" | 10" | |
| <18′ | 27" | 24"x24" | 11" | |
| 10′ | <10′ | 23" | 20"x20" | 9" |
| <14′ | 27" | 24"x24" | 11" | |
| <18′ | 31" | 27"x27" | 13" | |
| 12′ | <10′ | 25" | 22"x22" | 10" |
| <14′ | 30" | 26"x26" | 13" | |
| <18′ | 34" | 30"x30" | 15" | |
| 14′ | <10′ | 27" | 24"x24" | 11" |
| <14′ | 32" | 29"x29" | 14" | |
| <18′ | 37" | 33"x33" | 16" | |
| 16" | <10′ | 29" | 26"x26" | 12" |
| <14′ | 35" | 31"x31" | 15" | |
| <18′ | 40" | 35"x35" | 18" |
Adapted from “Prescriptive Residential Wood Deck Construction Guide” (DCA6)
For more about designing and building decks, visit the JLC Field Guide.