Q: My building department says that parts of my house plans—such as the walls next to the garage-door openings—don't meet wall-bracing rules. How can I address these requirements?
A: House frames don't just have to handle the downward load created by gravity. They also have to resist lateral (sideways) loads introduced by wind pressure or by earthquakes. In high wind-speed zones near the coast, and in the stricter seismic design categories, you may need an engineer to verify that the structure can handle those loads. Typical houses outside the highest-risk areas aren't required to have engineering approval, but even those structures need some wall bracing against lateral loads. Perhaps the simplest way to provide that bracing is with structural panel sheathing—oriented strand board (OSB) or plywood.
Wall-bracing requirements are outlined in Section R602.10 of the International Residential Code (IRC), and the code allows some flexibility in how a builder complies. Still, depending on the size and location of window and door openings, the bracing rules may be tricky to meet. For example, in some cases the code will call for a 4-foot sheathed section on both sides of a garage-door opening, or at the house corner, or flanking an entry door—interfering with the designer's window or door placement or forcing an unwanted adjustment in room dimensions.
But engineers at the APA - The Engineered Wood Association have been working on ways to provide builders with more design flexibility while staying within the IRC's simple prescriptive requirements. Zeno Martin, P.E., is the APA's point man on the project. Martin explains: “Historically, for the last 30 years, there have been eight different code-approved methods of bracing, using different materials—gypsum board, fiberboard, hardboard, OSB, or plywood, etc.” For a wall of a given length, a builder must provide a certain amount of bracing at specified locations. “But the code has always required a 4-foot-wide segment to count as bracing,” says Martin. “So you need a 4-foot piece of OSB, plywood, fiber-board, or whatever. The problem is that 4 feet is too wide for most people, especially for garages. No one wants to put 4 feet of solid wall on either side of a garage door.”
The APA's solution was to devise a narrower wall section that uses familiar materials—plywood or OSB sheathing nailed to studs—but with fastening and construction details that provide racking resistance equivalent to that of a conventionally installed 4-foot sheet of the material. Says Martin, “We just said, ‘OK, let's measure the performance of this 4-foot segment, and let's introduce something that is skinnier and get the same performance.' We tested permitted bracing segments, and then we found something else that will perform just as well.” The APA narrow wall bracing method is explained in a newly released APA pamphlet, “Narrow Walls That Work,” available as a free download from its Web site (www.apawood.org). The method was also recognized in the 2004 Supplement to the International Residential Code.
A CLOSER LOOK The IRC identifies two categories of homes for purposes of determining wall-bracing requirements: partially sheathed and fully sheathed. A partially sheathed house has plywood, OSB, or other structural sheathing just in some areas, while a fully sheathed house has structural sheathing over the entire exterior. Both types of house need a specified amount of bracing to handle lateral loads, but the way you calculate the amount of bracing provided differs between the two types.
“In a partially sheathed house, where you are using isolated sheathed segments to meet your total bracing requirements, you have to use 4-foot segments—if it's not 4 feet, it counts as zero. It doesn't even count,” explains Martin. “But if the house is fully sheathed, you can use segments as narrow as 24 inches. When you fully sheathe, you get a lot of flexibility.”
The APA's narrow wall bracing method goes a step further: It allows the builder to construct a 16-inch wall segment that provides as much bracing as a typical 4-foot wall section. But to use the method, the rest of the house must also be fully sheathed.