A roof is made up of a complex assembly of rafters, trusses, roof decking, and roofing material. When designing a solar project, it is vital to determine the allowable loads for the roof. A solar array adds weight to the roof, and in areas which receive snow, you will also need to consider the appropriate amount of snow load the roof can handle.
One of the most common misunderstandings on this topic is the difference between ground snow load and roof snow load. Ground snow load is the weight of snow on the ground surface. In the United States, the International Building Code (IBC) is used to identify ground snow load. Roof snow load is defined as the weight of snow on the roof surface. Ground snow load and roof snow load are rarely the same and typically cannot be interchanged. It is necessary to understand what the ground snow load is for the project before calculating roof snow load.
Fortunately, you can find snow load calculators online, however it is important to understand how they determine the amounts. Every project is different, and designers should be aware of the rules and restrictions. The Everest Solar Systems’ website provides a conversion table to help determine roof snow load based on the ground snow load for residential projects. The Everest online design tool, known as Base, will also assist with these calculations for both residential and commercial projects. Even with these tools available, it is always best practice to consult the engineer of record to understand what the designed roof snow load needs to be for the project.
The ASCE 7-10 manual includes methods of calculating snow loads. In the US, data is collected by the National Weather service to help determine the snow load of a particular location. Maps of ground snow loads in the IBC and ASCE 7-10 codes indicate 92% probability of a load being equal or exceeded in any year. Within the US, there are three different snow load zones. The intensity of the snow loads increases from south to north. Average snow load is between 20-40 lbs./sqft for the continental US.
Designing a solar system that is productive, safe and aesthetically pleasing is an important task facing designers and installers. Every system is different and will pose its own unique challenges. Preventative measures should be taken to avoid failure. Online calculators are a great starting point, but the best thing to do is consult the engineer of record for each project you design.
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