Insight on engineering and codes
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, www.afandpa.org (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.
As mentioned previously, the IRC specifically allows design that's in accordance with the Wood Frame Construction Manual (R301.1.1). In that book, the cantilever allowance for joists is L/4 — 1/4 the total span — which is identical to the allowance in the Deck ConstructionGuide (Figure 3). Using this design provision shouldn't be a problem, as it's essentially in the IRC.
Joist Span €“ Free Standing Deck
On the other hand, the allowable cantilever for beams (also L/4) provided in the Deck Construction Guide is not in the IRC (Figure 4). While beam cantilevers may be scrutinized by the building official, the acceptance of the guide's provisions would certainly add design flexibility.
Deck Beam Spans
The Deck Construction Guide specifies a minimum 6x6 post and cites section R407 of the IRC. Strangely, this IRC section requires a minimum 4x4 post. The larger cross section required by the AF&PA may be a result of diagonal bracing provisions in the Deck Construction Guide that are not in the IRC. This bracing places forces perpendicular to the posts, which increases bending stresses on them. The load path to the supporting soil requires all the components of the structural system to work together, so the system must be designed as a whole. By specifying braces for lateral support, the post size must be reconsidered. With these thoughts in mind, a jurisdiction might require the Deck Construction Guide to be used in its entirety, as a system design.
Not all forces on a structure act vertically. Lateral forces, often imposed on decks by the movement of people, act in a horizontal direction. Live loads account for the weight of people, but not the horizontal forces generated by their movement. Decks are notoriously places of high occupant load and high occupant movement. Couple that with a lack of lateral bracing and the party on the deck may end early.
The concern is the deck pulling away from the house. The difficulty that may arise from trying to use Deck Construction Guide methods for lateral bracing of decks is that many building departments also consult the newer supplemental codes for guidance in approving alternatives. The IRC requires some sort of lateral bracing and the 2007 supplement to the IRC provides one possible method (Figure 5). I think it's conservative — and impractical for existing structures.
Deck Attachment for Lateral Loads
While this particular connection is not required by the IRC, it does establish a minimum standard. By setting the bar for lateral restraint so high, the 2007 supplement to the IRC makes the Deck Construction Guide methods a questionable "equivalency."
The ledger fastening table in the Deck Construction Guide is nearly the same as that in the 2007 IRC supplement. However, the two differ significantly in one way. While the Deck Construction Guide provides a detail for hanging a girder from a ledger (Figure 6), the 2007 IRC supplement states: "Girders supporting deck joists shall not be supported on deck ledgers or band joists."
What this means is the standard practice of hanging a double beam from the ledger to carry other joists will no longer be allowed.
This fact may make a building official wary of approving an almost identical table that then allows beams to bear at the ledger.
Framing Around a Chimney or Bay Window
To my knowledge the Deck Construction Guide is the first "reputable" document to provide structural details for typical deck stairs. While this is great, I also think that as written, it's both conservative and misleading. On the conservative side, notched stair stringers are limited to a horizontal span of 7 feet with southern pine, and 6 feet for other species. This may be a shock to deck professionals who are accustomed to longer spans. I assume that the use of a beam, posts, and footings set at midspan of the stairs would be an acceptable alternative that would allow longer flights with notched stringers.
What I find misleading about the section on stairs is that it allows stringers to be spaced at 36 inches and allows either a 2-by or 5/4 board to span this distance (Figure 7). Stringer spacing should be based on the maximum span of the tread material: Some synthetic decking materials can span only 8 inches when used as stair treads. I think the stair portion of the Deck Construction Guide should be used and approved with caution.
The Deck Construction Guide is not, nor is it intended to be, an all-inclusive code document. While it provides many useful specifications that aren't in the code, it also leaves some out. For example, opening limitations and minimum heights for stair guards are not mentioned, nor is the use of type II graspable handrails (those with a perimeter greater than 61/4 inches). But the value in the Deck Construction Guide is that it fills some holes in the IRC and provides a good framework for generally acceptable deck design.
Glenn Mathewson is a building inspector in Westminster, Colo., and a former deck builder.