How to Calculate Deck Footing Size

The American Wood Council’s DCA 6 ‘Prescriptive Residential Wood Deck Construction Guide – Based on the 2012 IRC’ has a new appendix (B) that guides users how to calculate footing tributary area and a new Table B3 ‘Footing Sizes Based on Tributary Area for Various Soil Capacities.’ A section on Tributary Area Calculations and example figure go into deeper detail of the calculation than what I covered in the July 2014 article ‘Sizing Deck Footings.‘ The detailed calculation description and illustrations show precise points where dimensions for tributary loads are measured from and the math.

Table B3 found on page A6 of the guide has tributary area rows in 10 sqft increments from 10 to 250 and columns for soil bearing capacities of 1500 psf, 2000 psf, 2500 psf and 3000 psf. The table assumes and is limited to a total deck load of 50 psf (40 live and 10 dead).

A comparison of the footing sizes I calculated for the example deck resting on 1500 psf soil in the Sizing Deck Footings article and DCA 6 Table B3 shows an almost identical result. My calculations came up with 15 3/4 in. diameter center footing size and Table B3 calls for a 16 in. footing. Footing size based on the original Table 4 in the main body of DCA 6 for the same sized deck calls for a 24 in. diameter footing.

It will take a few minutes to do the tributary area calculations and then match them up to Table B3 but the time will be well worth the effort given the labor savings when digging the footings.

Design for Code Acceptance (DCA) 6 is available as a free download from the American Wood Council and a great guide for designing and constructing code-compliant decks.

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