Deck Footing Size and Joist Span: How to Read the IRC Tables

In the 2021 International Residential Code (IRC), Table R507.6 gives deck joist spans and Table R507.3.1 gives footing sizes. Reading them is mostly arithmetic once you know your post's tributary area. We show single cells and link the tables.

This page explains how to read the tables. It doesn't design your deck. Your building department decides which edition applies, and some decks need an engineer.

The two numbers you need

• Joist span: the distance joists cover between supports, such as ledger to beam. A cantilever, the part past the beam, is counted separately.
• Tributary area: the patch of deck, in square feet, that one post and its footing hold up.

Working out tributary area

The American Wood Council's DCA 6 deck guide gives these formulas in its appendix:

Corner post = (joist span ÷ 2 + joist cantilever) × (beam span ÷ 2 + beam overhang)

Center post = (joist span ÷ 2 + joist cantilever) × beam span

The beam carries half the joist span plus any cantilever. A corner post carries half the beam span beside it, and a center post half of each span on both sides.

DCA 6 measures beam span post center to post center, uses the larger of two unequal spans, and multiplies a center post's area by 1.25 where the beam runs continuous over it.

Reading Table R507.6: joist spans

Find your species, joist size and spacing. The cell is the longest span allowed. These cells are at a 40 psf live load, from 2021 IRC Chapter 5:

The table writes "14-0" for 14 feet 0 inches. Your joist span must be no longer than the cell. Closer spacing buys span: a Southern pine 2x10 goes from 14 ft 0 in at 16 inches to 16 ft 2 in at 12 inches, with more joists to buy. The deck materials calculator counts them.

Reading Table R507.3.1: footing size

You need the tributary area and your soil's bearing capacity, in pounds per square foot (psf). These cells are at a 40 psf live load:

Doubling the soil's capacity shrinks the 80 sq ft footing from 20 to 14 inches square, so don't guess it. DCA 6 assumes 1,500 psf, and where weaker soil is likely, a soils investigation sets the figure.

Footnote a to the table says interpolation is permitted and extrapolation is not. You can work between two rows, but a post beyond the last row needs a design.

Worked example

A 16-foot-wide deck has Southern pine 2x10 joists at 16 inches, spanning 10 feet from ledger to beam with a 2-foot cantilever. The beam sits on three posts 8 feet apart, with no overhang. Soil is taken as 1,500 psf.

1. Joists: 10 feet is under the table's 14 ft 0 in cell. DCA 6 also limits a cantilever to a quarter of the span: 10 ÷ 4 = 2.5 feet, so 2 feet is within it.
2. Corner posts: (10 ÷ 2 + 2) × (8 ÷ 2 + 0) = 7 × 4 = 28 sq ft
3. Center post: (10 ÷ 2 + 2) × 8 = 7 × 8 = 56 sq ft
4. Corner footings: 28 sq ft reads up to the 40 row: 14 inches square or 16 inches round, 6 inches thick.
5. Center footing: 56 sq ft reads up to the 80 row: 20 inches square or 22 inches round, 7 inches thick.

Reading up to the next row never gives a smaller footing than interpolating would. For the concrete each footing takes, use the Sonotube calculator or the footing calculator.

How DCA 6 compares

DCA 6 is based on the 2015 IRC. Its Table 2 gives the same spans for the joist cells above, but its footings run a little larger. These cells are at 1,500 psf soil, from its appendix Table B3:

DCA 6's main footing table assumes full cantilevers on both joists and beam, so it can call for bigger footings than your deck needs. Its appendix works from your actual layout instead.

In the example, a beam running continuous over the center post raises its area to 56 × 1.25 = 70 sq ft under DCA 6, and Table B3's 70 sq ft row is 19 inches square or 22 round, 9 inches thick. Ask your building department which document it accepts.

Frost depth and how deep to dig

DCA 6 says footings bear on undisturbed soil at least 12 inches below the ground surface or below the frost line, whichever is deeper, and to ask your local authority for the frost line. There's no national figure, as the footing calculator explains.

DCA 6 adds that deck footings closer than 5 feet to the house's foundation wall must bear at the same level as the house's footing, and none should go over septic systems, utility lines or enclosed meters. Contact 811 before you dig. Stair footings follow the same depth rule in DCA 6; the stair stringer calculator covers the stairs.

Common mistakes

• Using the corner formula for every post. A center post carries about twice the area.
• Counting the cantilever in the joist span. It belongs only in the tributary area.
• Assuming 1,500 psf soil. It's DCA 6's assumption, not a fact about your yard.
• Extending the table. Footnote a doesn't allow extrapolation past the last row.
• Mixing IRC and DCA 6 figures. Use one document throughout.

Frequently asked questions

Sources

We use primary sources where we can: building codes, federal rules and manufacturer documents. Read our editorial standards.

• 2021 International Residential Code, Chapter 5, Section R507
• American Wood Council: DCA 6 prescriptive residential wood deck construction guide
• 811: What is 811?