Wind Loads From Solar Panels
Question:
I am looking for information on point loading trusses. We manufacture mounting structures for solar panels. Typically, 10 to 15 sq. ft. of solar panel is supported by one standoff. Under extreme conditions – 50 lbs. per sq. ft. of wind load - we can transfer 500 to 750 lbs. of force onto one point of one truss. Are there any standards on this issue?
Answer:
Trusses can be designed to support a wide variety of loads. The trick is to specify the loads at the design stage so that the lumber and metal plates can be properly designed, sized and assembled to support these loads. Very few trusses will be able to support a great amount of extra loading if they are not designed with that loading in mind. The trusses will not necessarily fail, but they will be overstressed under full loading conditions which could lead to structural performance problems.
The truss manufacturer generally will be able to tell you, based on local building codes, what the standard loads are in your area, such as environmental loads due to snow and dead loads from standard construction materials (ex. asphalt shingles vs. concrete tile roofing). Any loads beyond that are the customer's responsibility to specify.
If you don’t yet have the trusses and you know the weight of the solar panels, tell the truss manufacturer. If you know the wind load forces and where they are located, tell the truss manufacturer. Give the truss manufacturer as much information as possible up front so the designer can account for those loads.
If you are talking about trusses that are already designed and installed, you obviously don't have that option. Even so, the loads you are proposing are not that large, especially in areas where trusses are already typically designed for high snow loads. Also, wind loads are generally of short duration, which helps lumber's load carrying capacity.
To accommodate the loads in this case, you might want to use 22.5 inch 2x4 blocking between the top chords of adjacent trusses in that area and plenty of nailing to the roof sheathing. This will help develop some roof diaphragm action and help spread the load out. For more information on roof diaphragms, you can contact APA – The Engineered Wood Association.