The quality of a solar installation has many parameters. One frequently underevaluated aspect is a compliant roof attachment. Load calculations that determine the number and spacing of attachment points prescribe not only the length of lag screws that must enter structural members but also their edge distance or the effective centering of the lag screw in the rafter or truss.
The National Design Standards of the American Wood Council defines edge distance as the distance between the center of the lag screw and the edge of the structural member. For most solar applications, the minimum edge distance is 1.5 times the lag screw diameter. Placed in a common residential PV setting, the center line of a 5/16-in. lag screw entering a 1-1/2-in.-wide rafter must not exit the middle 9/16 in. (37.5 percent) of that rafter. In other words, 9/32 in. (just over 1/4 in.) is the maximum allowed deviation from center. Engineered trusses, or TJIs, have their own attachment specifications available from the manufacturers.
Edge distance compliance factors
First is the accuracy of the installer’s determination of center from the rooftop. The most common locating method, drilling enough probing holes to estimate one or both edges — and from that inferring center — is used where rafters are hidden. Rafters that are exposed (in a vaulted ceiling or porch roof, for example) increase inaccuracies because more complicated methods are often used to avoid cosmetic damage. Second, if a pilot hole is used, an error in placement or angle can set the lag off center. The tip of a 3-in. lag screw angled 2.5 degrees off of perpendicular will end up 1/8 in. off target.
Factors outside the installer’s control and often awareness include warped or skewed rafters that may be 5 degrees or more out of perpendicular, sending the point another 1/4 in. or more off target. In a typical installation with 30 to 50 standoffs, each subject to a combination of these factors, “spinners” are common, and they can be challenging to remedy. Other noncompliant lags go undetected, and some of these will fail under loads they are required to meet.
Noncompliant roof attachments can be minimized by attention to their primary causes. Installers should be equipped with the necessary tools and trained in a variety of methods for rafter center location as well as missed-target remediation. Site-specific parameters will point to preferred approaches. Where possible, visual inspection of rafters before lagging can inform a compliant trajectory in a skewed rafter. Post-attachment inspection may help identify noncompliant lags.
Duane Ediger is an installer with Technicians for Sustainability in Tucson, Ariz., and the founder of RafterEye LLC.
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