What are the most common problems with wood roof trusses?
Friday, July 20, 2018
Prior to the mid-1960s, a roof structure was framed at the homesite using dimensional lumber, cut diagonally at the ends to size, notched at the bearing points, and then carefully secured along a ridge board or ridge beam at the center of the span. Minimum size lumber was 2x6. Longer spans meant using even larger lumber. The work required a skilled and experienced carpenter, and was labor-intensive.
When manufactured trusses became widely available it radically changed roof framing. Trusses can be made with smaller 2x4 lumber, achieve longer clear spans, and the entire roof is easily placed in one day using less-skilled labor. But trusses are somewhat fragile until installed upright and braced, and require proper handling and protection at the job site.
Here’s a list of our top 5 most common truss defects found during a home inspection:
1) Cut or removed web members - Once the trusses are installed, the web members in the center of the truss cannot be cut or removed. Each one is designed to be alternately in tension or compression, while transferring loads placed on the roof through a continuous path between them and the top and bottom chords to the bearing points at the end of the truss. Any cut or removed piece interrupts the elegantly designed load path and compromises the strength of the truss.
We regularly get asked by homeowners how much of the center of a truss can be removed so they can create a clear area in their attic for more storage. The answer, unfortunately, is none. Web members that have been removed after the home was built are our #1 most common truss defect in a home inspection. Sometimes a contractor installing an air handler or water heater in the attic will want to cut out a couple of web members to install the equipment. Don’t let them convince you that just removing one or two web members is alright. It’s not.
And, because a truss is an engineered product, once a web member has been cut away or otherwise damaged, a licensed, professional engineer has to specify the repair. Scabbing on a couple of pieces of lumber is not acceptable.
2) Unrepaired or poorly repaired damage from a roof impact or leakage - If the top chord has fractured or has a rotted section from leakage, even a small must be repaired. Sometimes the damage appears to be due to rough handling during installation.
3) Defective lumber - Sometimes a knot or check in the lumber of a truss is intact and appears structurally sound when the roof is installed, but pops open later. Resin dripping from the wood, as in the photo below, is ugly but not a structural defect.
Damaged or missing gusset plate - It must be intact and free of corrosion.
Not enough nails at the anchor strap or clip - A minimum of three nails is required at each side of the connector from the wall to truss.
Also, see our blog posts Are roof trusses better than roof rafters (stick framing)? and How much of a roof truss can I cut out to make a storage platform in the attic? and Can I remove the interior walls under a roof truss? and What are the hazards to avoid when going into an attic?
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To learn more about roofs and attics, see these other blog posts:
How To Look At A House
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