The American Wood Council (AWC) has an online calculator that can be used to calculate the size and grade of joist needed for a given joist span, on-center spacing, live load and dead load.

Most building officials recognize the American Wood Council as a source for prescriptive designs and standards, and will probably accept the calculations provided by the AWC’s calculators.

I like the calculator because it offers more options than standard deck joist span tables in the DCA-6 and the 2015 IRC. On the input side you can select Dead Load choices of 5, 7, 10, 15 and 20 psf. That’s useful when installing heavier decking materials like stone, tile and pavers that would exceed the standard code and print table limit of 10 psf. You can also choose from live loads between 30 psf and 100 psf in ten pound increments. This gives deck builders in high ground snow load areas (the Upper Midwest, the Mountain West, and Northern New England and New York) a way to size joists.

Calculator results

The calculator result is a small table listing lumber grade (select structural, #1, #2) and outputs the joist size (2×6, 2×8, 2×10, 2×12) needed to meet the joist span and other inputs. In this example video, I input a 12 ft-4 in joist span and the results offered a 2×8 #1 grade or a 2×10 #2 grade for joist options. If my local lumberyard stocks both #1 and #2 treated lumber I can cost out which option would be more economical.

For deck builders who are inclined to increase lumber size as a ‘safety factor,’ safety factors are already included so there’s no real benefit to upsizing joists beyond what the calculator offers.

The calculator is also available as a smart-phone app for the iPhone and Android OS.

(c) Mike Guertin 2017

More on Decks:

2015 IRC deck joist span table  

DCA-6 Prescriptive Deck Construction Guide