One of the considerations when deciding whether or not to invest in a solar system is how it is going to affect your roof. Since every solar system installed must go through a full permitting process by the local jurisdiction, it will receive a full review.

During this engineering and permit stage of the process, which takes about two months to complete, we submit all of the design criteria to the structural engineer for review. He is going to look at the live loads, the dead loads and the seismic effects of the proposed solar panels on your roof structure.

These solar panels, mounted on aluminum racking, weigh about 2 pounds per square foot. Every building built in the last 50 years has had a live load of 10 pounds per square foot designed into it. This is to account for the weight of people walking around on top of it. In areas where there is snow, this live load is increased to compensate for the snow that may accumulate on the roof. Since you cannot walk on top of the solar panels, it has been agreed by the permitting agencies to include the weight of the solar panels in this live load. With 10 pounds per square foot as a minimum amount accounted for, and 2 pounds per square foot being the weight of the solar panels, you can see that every roof would qualify easily.

However, many companies prefer to use ballasted solar systems. This reduces the cost but causes other financial metrics to suffer such as system output due to less tilt of the panels. This raises two issues; the weight of the solar system on your roof is greatly increased by all the ballast used to hold the solar system down in case of high winds, and your building may not be able to hold all of that additional weight swaying in an earthquake. The ballast can add from 1 to 2 pounds per square foot, depending on the potential of high winds in your area. This will still come out of the live load, but it will have an actual effect on your roof and will cause some sagging and puddling of water in the future.

Another requirement for the structural engineer is the seismic calculation. This is much more complex than a simple weight per square foot analysis, and is beyond the scope of this article. With an attached system at 2 pounds per square foot, the shifting that can occur with an earthquake is controlled by the attachments at each corner of each solar panel. With a ballasted system, attachments must be made to hold the solar panels in their position on the roof. Typically about half as many roof attachments are made with a ballasted system than with a completely attached system. While every building will pass the new seismic requirement after calculating the additional weight of an attached system, not all buildings will pass these same calculations with the added weight of the ballast.

So in conclusion, if your building passes code today, it will pass code after an attached solar system is added to your roof. This may not be the case with a ballasted system. In order to reduce the amount of the ballast in order to pass code, additional attachments can be added, again passing code.