This installation tip was provided by Jeff Spies, senior director of policy, and Sue Stark, PV director of technical sales for Quick Mount PV
If a PV system is mounted on a roof without staggered attachments, the entire load of the PV system, and the resulting wind and snow loads, is concentrated on a few rafters or trusses. Not staggering the attachments may necessitate structural reinforcement of the roof, which can be costly.
AHJs, code officials and solar professionals are re-evaluating the best methods for connecting a PV system to a roof. Unfortunately, structural codes and standards are a murky bowl of acronyms. The IBC and IRC use ASCE-7 and the American Wood Council’s NDS to establish PSF capacities, permissible point loads, fastener size and position, attachment capacity and appropriate safety factors. For the average solar installer, making sense of these confusing requirements is a challenge and determining the best structural configuration can be complex.
Certain states with snow load considerations like New Jersey and other Northeastern states have started mandating staggered attachments on all steep-slope PV systems. Even California’s streamlined permitting gives preferential consideration to solar systems that use staggered attachments.
By staggering attachments, the load is more equally distributed across more of the rafters or trusses, minimizing the load that is concentrated on a single structural member.
While staggering does require a few additional roof attachments, it’s much less costly than reinforcing the roof. Staggering attachments is a good design practice and puts less strain on the roof.
Quick Mount PV’s new QRail system can be easily designed using staggered attachments with the QDesign software tool available at quickmountpv.com/qdesign.