A C-46 contractor is designing a commercial rooftop PV system on a building with a flat, asphalt-covered roof. The structural engineer has specified a maximum distributed load limit of 5 pounds per square foot (psf) for the PV array. Which of the following racking systems would be MOST appropriate to stay within this structural limit while minimizing roof penetrations?
- AFully penetrating flush-mount system
- BMechanically attached elevated racking system
- CNon-penetrating ballasted system
- DAttached rail system with concrete ballasts
Show answer & explanationAnswer & explanation
Correct answer: C. Non-penetrating ballasted system
A non-penetrating ballasted system uses weights (ballasts) to secure the array without penetrating the roof membrane, which is ideal for minimizing penetrations. These systems are designed to distribute the load broadly across the roof, making them suitable for roofs with distributed load limits like 5 psf, provided the ballast weight calculations are done correctly. Other systems involve penetrations or might concentrate loads differently.
Why the other options are wrong
- A. Fully penetrating flush-mount systems require many roof penetrations, which the question seeks to minimize.
- B. Mechanically attached elevated systems require numerous penetrations into the roof structure, which is not ideal for minimizing penetrations.
- D. This option is contradictory; attached rail systems typically penetrate the roof, and concrete ballasts are for non-penetrating systems. This option is ill-formed.
Non-Penetrating Ballasted PV System
A non-penetrating ballasted PV system anchors solar arrays on flat roofs using weights (ballasts) rather than mechanical fasteners that penetrate the roof membrane, thereby minimizing roof damage and potential leaks.
- Avoids roof penetrations, reducing leak risk.
- Relies on ballast weight to resist wind uplift.
- Requires structural assessment for distributed load capacity.
- Commonly used on large commercial flat roofs.
Memory trick: Penetrations Prevented? Pick a Ballasted, Broadly-Loaded Base.