Some homes built during the overheated construction market of the century’s early years are now, less than a decade later, revealing they weren’t built to the most exacting of standards.
One common weakness seems to be sloppy detailing of engineered lumber. The symptoms—wavy roofs, bouncy floors, walls that rattle in the wind—are seldom catastrophic, but they haven’t helped the industry’s reputation.
To help minimize such problems in the future, representatives from APA - The Engineered Wood Association have been speaking to builder groups about the most oft-seen framing errors, based on hundreds of field inspections. The list of mistakes covered depends partly on which field rep is talking, but some items seem widespread enough to make everyone’s list. They include:
Inadequate panel supports. While it’s not unusual to see 16- or 24-inch wide strips of plywood or OSB supported by just two joists or rafters (one on each edge), that’s not enough to keep them from buckling. Every panel needs at least three supports: one on each edge and another somewhere in the middle. With 16-inch on-center framing, that means using panels at least 32 inches wide.
Inattention to grain. The rule here is the same as for solid wood. That is, even a multi-ply panel is strongest in the direction of the grain, which is to say in the long direction of the top ply. Some framers seem not to understand this. So go with the grain.
Unspaced panels. One of the most prevalent errors, according to APA’s inspectors, is panels butted tight against one another. If they swell, they can buckle at the edges and telegraph waves through roofs and floors. The framer needs to leave a 1/8-inch gap, which is easily done with self-spacing H-clips.
Overzealous nailing. Nails should be driven so the heads lie flush with the surface, not deep below it. With plywood or OSB, driving nails deeper reduces the panel’s effective thickness. This is most problematic on shear walls: Nails heads driven 1/8 inch into a 1/2-inch panel will lower its shear resistance to that of a 3/8-inch panel.
Skimping on glue. Some builders don’t put enough adhesive between the subfloor and the joist. The result is a floor that squeaks when walked on. It’s an annoyance rather than a structural defect, but it’s a good bet that those squeaks have customers cursing their builder’s name. Don’t skimp. A few extra tubes of glue are a lot less expensive than a callback.
Flipped glulams. Some glulam beams are manufactured with a slight camber, and will only perform to their span rating when that camber faces upward. Unfortunately, a lot of beams have been installed upside down. Avoiding this mistake isn’t difficult, since one edge of the beam is labeled with the word TOP. Guess where it goes.
Inconsistent joist spacing. When using I-joists, some builders have framed one part of the house with 16-inch spacing and another part with 24-inch spacing. The problem is that floors with 16-inch spacing are more than twice as stiff, so floors laid over the wider spaced joists feel squishy by comparison. Everything will feel fine if builders use consistent spacing throughout.
Improper notching and cutting. Some inspectors say that they find this code violation more than any other in homes with I-joists. Although the greatest shear forces on the web are found near the end of the joist, some workers insist on cutting holes for utilities in this area. I-joist manufacturers provide clear hole-cutting guidelines with their products; the builder just needs to make sure they’re followed.
Wood is Wood
The above defects are symptoms of two basic misunderstandings. One is an assumption that engineered products are fundamentally different than solid wood. They’re not. An engineered beam or panel still swells with changes in moisture, and its strength characteristics are still affected by grain orientation. The other is a seeming ignorance of how building assemblies handle loads, which is why some framers see no problem hacking through the ends of I-joists.
Inattention to load transfer also causes problems with conventional wood framing. “I see a lot of mistakes with dimensional lumber,” says Chris DeBlois, an Atlanta-based structural engineer who specializes in wood-frame buildings. Among the most frequent he runs into are things like missing joist hangars or blocking, undersized headers, and splices made between supports rather than over them.
Builders are getting the message, and the best ones are working hard to make sure things get done right. For instance, consultant Gary Zajicek, who was vice president of construction at Veridian Homes in Madison, Wis., from 1992 to 2011, helped create quality specs to identify and solve problems, and started regular meetings with framing subs to make sure they got corrected. He says the program reduced framing errors by at least 75%. “We went from one framing defect per home to four homes without any defects.”
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