Is it legal to bore holes through roof trusses to install wiring?
Thursday, December 3, 2020
Although the code allows drill holes in roof joists and rafters, with a limit on size and location of the holes, trusses are a different animal. A truss cannot be drilled without authorization of a licensed professional engineer or the truss manufacturer. ”Truss members shall not be cut, notched, drilled, spliced or otherwise altered in any way without the approval of a registered design professional,” according to the International Residential Code (IRC R802.10.4).
The main reason why holes are allowed in joists and rafters, but not trusses, is that joists and rafters have specific areas that are in tension, compression, or neutral. So it’s easy to define the safe places to drill. But trusses are more complicated, with the loads zig-zagging and flipping around as they run through the webs and chords. Plus, some members are more heavily loaded than others.
If you have trusses that have already been drilled and you are concerned about the holes, here’s what the Structural Building Components Association (SBCA), a manufacturer’s group, recommends: "Contact the truss manufacturer if you are concerned about the serviceability of your truss. The manufacturer will be better suited to discuss whether or not a repair is necessary, because they know the loads for which the truss was designed. The truss chord with the drilled holes may have been designed near capacity and any reductions from drilled holes could affect the structural performance of the truss. When you notify the truss manufacturer, they in turn will notify their engineer who will prepare the repair drawings if necessary. Repairs should not be attempted without proper repair detail drawings."
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