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.

1. Snow types can have an effect on snow load. Each individual snow event and type of snow will impact the design of the system. Ice, denseness and wetness of the snow are major factors to be aware of for the area of install.
2. Uniform roof snow load is based on flat, wide open roofs free from any obstructions. Unfortunately, these roof types are not very common; most roofs have numerous obstructions. Adding solar to a building also creates more obstructions and these need to be accounted for. Rooftop equipment adds obstructions that can collect drifting snow. Unbalanced snow loading occurs when snow accumulates at different locations on the roof. This is often the biggest cause of roof failure from snow loads. Drifting or sliding snow caused by wind can transport snow from one part of the roof to another. This causes more snow to accumulate in some spots and less in others. Because of this, systems should always be designed for the highest calculated snow load.
3. Another factor to consider, is the dynamic force of sliding snow onto a lower roof. The sliding snow can have significant impact force which can overload the roof structure. Depending on how the snow settles on the panels, the mounting system can become highly loaded. A highly loaded system is at risk of failure and could also be dangerous to people on the ground. An accurate snow load is necessary to ensure the system is safe and the building is not compromised.
4. Adding rooftop equipment, such as a solar system adds weight to the roof. It is important to account for the weight of the system when figuring out the snow load amount. You will also need to be aware of the weight of the current roofing materials. Many times, a new roof is put on top of an existing roof, and this should be accounted for in the analysis.

1. Obtain the ground snow load for the project location; consult chapter seven of the ASCE 7-10 manual, the AHJ, or the engineer of record.
2. Determine the importance and occupancy of the building. Is the building a small barn used for storage or a busy hospital with many people coming and going? The rules will be more strict depending on the importance of the building. It is also necessary to consider the thermal conditions of the building. For example, a hospital will be kept warmer than a barn; this could cause the snow to melt quicker on the hospital than the unheated structure.
3. Wind exposure of the roof also affects where snow will collect. You will need to evaluate any possible obstructions to predict where snow will drift and collect.
4. Roof slope and shape are also important factors to consider. Is the roof pitched and snow is likely to slide off or will snow continuously collect because the roof is flat? Will there be snow sliding off potentially landing on a roof surface below? All of these conditions need to be considered to design a safe system.

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.