Loads from wind, snow, and the combined weight of the system components impact rail span and spacing and rail gauge, as well as roof design limits. If the mounting system isn’t sized accordingly, strong winds can put additional uplift strain on your modules and mounts, causing damage to the system and roof structure. Similarly, the additional weight of snow on top of the modules can compromise a roof’s integrity. If you’re retrofitting a system to an existing roof, you’ll need to consult with an engineer to make sure your roof is beefy enough to withstand the added array weight (the roof’s “dead” load) and wind load. Similarly, if you’re building a new house, you’ll need to design the roof accordingly to accept the increased loads of the system.
Wind load (uplift and downlift pressure). Three factors are used to find down-force and uplift from wind pressure:
- Basic wind speed—found by consulting local wind maps or building code officials
- Effective wind area—the total continuous area of modules
- Roof zone—which section of a roof is utilized, whether interior, end, or corner.
These factors come together in an engineering formula or table that specifies down-force and uplift pressure in pounds per square foot according to the input factors. This pressure will then need to be adjusted for other factors, including topography (whether the building is in an urban area or windswept plain), height exposure category (based on building height), and importance factor (such as whether the building is a residence or hospital, for instance, and whether the building is in a hurricane region).
Snow load is the weight of the heaviest snow likely to occur in an area, calculated in pounds per square foot. Data is typically available from local building officials and code books.
Dead load is the additive weight of modules, and rail and racking components, calculated in pounds per square foot.