Internal Wood Blocking for Roof-Mounted PV
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For residential pitched-roof applications, array mounts are typically secured to the roofing system by one of two approaches. The first method utilizes lag screws to attach the array racking to the roof system’s trusses or rafters. The second method is to fasten the mounts directly to the roof ’s sheathing. From a structural loading standpoint, a third method should also be considered: attaching array mounts to internal blocking that is secured to the trusses or rafters.
Here I discuss engineering obstacles that installers may face when common attachment methods are specified, and I recommend materials and techniques for use with internal wood blocking. If you frequently use internal blocking, it may be prudent to have your engineer develop standard drawings that include tabulated tables based on fastener and bracket data to minimize ongoing costs.
Common Attachment Issues
Integrators may be confronted by a variety of issues related to direct attachment to structural roofing members such as trusses or rafters. Paragraph R802.10.4 of the 2009 International Residential Code states, “Alterations resulting in the addition of load that exceeds the design load for the truss shall not be permitted without verification that the truss is capable of supporting such additional loading.” Some AHJs interpret this to mean that attaching a PV array to trusses is an alteration. They rightly conclude that structural members with mounts attached support a greater load. This is because the mounting system changes the way the load is distributed on the roof. For example, in the case of snow loading, PV arrays change what was formerly an even load distributed over the entire roof system to a significantly larger load concentrated at the array racking attachment points.
Potential loading issues are compounded when trusses or rafters are skipped in an effort to minimize roof penetrations. In addition, you forfeit the ability to use the “15% repetitive member factor” that is normally allowed in structural member calculations. This allowance is applicable when three parallel structural members share the load and they are spaced 24 inches or less apart from each other. The factor allows additional calculated capacity, primarily because there is a low probability that all three members will be the weakest strength for the type of wood present, and that all will have defects in the same location. As illustrated in the photo below, maximizing rail spans to minimize penetration and attachment points creates more significant point loading and can complicate engineering requirements.
Paragraph R802.10.4 of the 2009 International Residential Code also states, “Truss members shall not be cut, notched, drilled, spliced or altered in any way without the approval of a registered design professional.” This requirement is problematic. Unfortunately, trusses are now typically designed with proprietary software controlled by a small number of truss manufacturing firms. In fact, ANSI/ TPI-1, “National Design Standard for Metal Plate Connected Wood Truss Construction,” no longer provides the equations required to accurately calculate truss loads. Consequently, without a ready calculation method to support them, structural engineers occasionally refuse to allow attachment to the trusses, leaving PV system installers with limited options.
Installation methods that rely solely on attaching racking to plywood or other roof sheathing types present their own set of potential issues and can be questionable practices. This is especially true in locations with strong wind loads, or if the array design specifies widely spaced mounting feet or long mounting rails. This is not to say that this method cannot be used, but you should proceed with caution. Plywood and other forms of engineered wood sheathing are made up of highly variable composite materials. The nails that secure the sheathing to the structural members are already under shear loading in high wind conditions. Shear failure often results in nail pullout, and additional loads associated with PV arrays should be mitigated. For example, a PV array mount attached to the corner of a plywood sheet could easily overload the nails.
You must also consider thermal expansion and contraction. Thermal cycling of rail-mount systems stresses the fastener joint due to daily and yearly swings in temperature. Plywood is especially susceptible to the effects of thermal expansion, as the fastener rotates in a relatively shallow hole. I have done tests showing that elongated holes and reduced pullout strength are possible, if not common. If you must mount a PV system directly to wood sheathing, it is best to use mounts that allow for multiple fasteners to reduce possible rotation, use plenty of racking attachment points and, most importantly, keep the rail lengths short to minimize the impact of thermal cycling on the attachments.