Using the Prescriptive Residential Deck Construction Guide

Related article to coincide with Nicks post on the 2007 IRC supplement.

by Glenn Mathewson

When it comes to structural provisions for decks, the International Residential Code falls rather short. According the IRC, much of what we see in deck construction is an “alternative.” Alternative designs and methods are those that are not prescribed in the code — they fall outside of the cookie-cutter construction recipe. It is within the authority of the building official to approve alternative materials, designs, and methods, as long as they are at least equivalent to what’s provided in the code and are based on accepted engineering practice.

The local jurisdiction may also approve alternative provisions that are published by government agencies and reputable organizations. One such organization, the American Forest & Paper Association (AF&PA), has recently published the Prescriptive Residential Deck Construction Guide, which is available as a free download from its Web site, (PDB, January/February 2008, page 22).

The AF&PA is also the publisher of the National Design Specification (the cornerstone for wood-frame engineering) and the Wood Frame Construction Manual, a standard that is specifically referenced by the IRC. Considering the AF&PA’s involvement in code standards, it would be reasonable to submit deck plans for local approval incorporating its provisions.

The Prescriptive Residential Deck Construction Guide is not a building code in itself, however. It’s intended to provide one alternative method to satisfy the code, but not represent the code specifically. In many ways it goes beyond code and in other ways it may be questionable. In general, it’s very conservative. Following are some highlights of the differences between the AF&PA deck guide and the IRC.

Joists and Beams
The IRC contains span tables that are often used for sizing deck joists, but these tables include only a few wood species, and they don’t account for wet-service conditions or incised materials, both of which may slightly reduce the materials’ structural capacity.

The Deck Construction Guide provides a simple joist span table that accounts for all these reduction factors and includes values for redwood and western cedars — species absent from the IRC’s span tables (Figure 1). Span tables are also provided for multi-ply beams, from doubled 2x6s to tripled 2x12s (Figure 2). These tables are incredibly useful, and I imagine most jurisdictions would approve the spans with little question.