Deck and dock calculators that show their basis

A span number is only as useful as the cell it came from. Every span and size on these pages names one — the table, the footnote and the page, in a single dated edition you can download for free and read yourself. That is what makes an answer quotable at a permit desk. It is also what lets you catch us if we are wrong, which is the point. The one non-structural deck page here, the material calculator, counts parts with the in-app takeoff engine — on framing sized from these same tables.

These read transcribed tables from a single, identified document: AWC DCA 6-2015, Prescriptive Residential Wood Deck Construction Guide (based on the 2015 IRC), © 2018 American Wood Council. The 2015 edition matters — it carries the reduced Southern Pine design values published after 2013, and a calculator built on the 2012 edition returns longer Southern Pine spans that are not conservative.

The load basis on the span-table pages — joists, beams, posts and footings — is the same: 40 psf live load + 10 psf dead load, No. 2 grade lumber, wet service conditions, L/360 deflection. The ledger page states its spacings at 40 psf deck live load plus 10 psf deck dead load, the basis Table 5 itself prints; the stair page screens geometry against IRC R311.7 and carries no psf basis at all. And when an input lands outside those tables — a joist past 18 feet, a beam carrying more than the table covers — these say so instead of extrapolating. That refusal is the most useful output a code calculator has. An interior beam line carries joists from both sides, so for equal spans its beam, posts and footings are read at twice the joist span, which the guide’s Commentary says “is acceptable” (DCA 6-2015 Commentary, p.C5). Unequal spans either side of a beam are outside that procedure: the designer sizes such a line conservatively, at twice the longer span, and flags it for a design professional to confirm.