Q In Bobby Parks' recent article about the design and construction of arbors ("Tips for Building Arbors," May/June 2013), his guidelines for sizing beams and rafters seem to be based mostly on aesthetics. But what does the building code have to say? After all, some plants in full leaf can be quite heavy, and in some parts of the country, snow loads could be an issue.
A Contributing editor Glenn Mathewson, a building inspector in Westminster, Colo., responds: There are no prescriptive structural standards for shade structures, so local jurisdictions are left to fend for themselves or require engineering, especially for elaborate and large designs. Here is how my office handles it, though keep in mind that the article's author lives in Georgia, where there are no snow loads to deal with.
When a deck structure has slats on top that are closer together than 3 inches, we require it to be designed for the full 30-psf snow load in our region. This is based on the International Residential Code (IRC) plumbing requirement of a minimum 3-inch diameter for a rooftop vent that's subject to snow closure; if snow can bridge over a small pipe, it can also bridge an opening between closely-spaced slats.
When the slats are 3 inches or more apart, the structure must be designed for a 20-pound live load. This is the IRC live load requirement for "attics with limited storage," a fair comparison to the load on a pergola supporting vegetation, snow accumulation, a hanging swing, hanging potted plants, and the like. Depending on the design (some structures have minimal material overhead), the live load requirement may be reduced to 10 psf, the IRC requirement for "attics without storage."
As for dead loads, it's also fair to use less than the standard 10 psf provided in most span tables (the American Wood Council has a nice online span calculator at awc.org that provides for many different species of lumber, live and dead loads, and spans). It's hard to find pre-engineered references for sizing beams, however. While the AWC's prescriptive deck-construction guide DCA 6 (also at awc.org) provides beam spans for various timbers and species, those spans are based on 40-psf live loads.
Ultimately, few pergolas are engineered, and most building departments are relaxed about the subject, relying on experience and comparisons to similar structures to determine sizing. More often than not, shade structures are designed with oversized framing to begin with; when the members are overspanned, it's likely going to lead only to excessive deflection, not catastrophic failure.