Reinforcing a roof for solar panels: needed or not?
Solar panels don't actually weigh that much, but that weight can still be too much for some roofs and require structural support. Here's what to know.
Most home and small commercial solar installations are mounted on a rooftop. This makes a lot of sense, because a rooftop is a readily available surface for mounting panels.
Solar panels weigh less than you might think. Even so, sometimes a rooftop isn’t suitable because of the additional load that a solar array would place on a structure. In those cases, a homeowner might be able to upgrade their roof structure to support the additional load. However, that additional cost could easily mean that it doesn’t make financial sense to install solar panels.
The structural requirements for rooftop solar panels are determined by local building codes, so this article can’t tell you whether or not your roof can support a solar array. However, I will try to outline some of the considerations, explain the lingo, and help you have a more knowledgable conversation with your solar installer.
Glossary of roofing terms
Before we get into it, there’s some terminology that will be useful to know.
- psf: pounds per square foot
- psi: pounds per square inch
- dead load: the weight of the roof structure itself and anything permanently attached to it. This includes the rafters, sheathing, and shingles or tiles.
- live load: the weight of any temporary loads placed on the structure. Even though solar panels are attached to the roof, they are counted as a live load. Other live loads include snow and wind.
- rafter: these are the beams that provide the main structural support for your roof. They look like this:
An example of a structural engineering report for a roof
When I had my solar panels installed back in 2013, one of the steps in the process was to have a professional engineer who is licensed by the State of New York inspect my house. He was hired by the solar installer, so my only task was to be home to open the door for him.
He crawled up into my attic, shined a flashlight around, took some measurements, and left. He later delivered a report to my solar installer that read:
Per your request, I have visited the above referenced residence and reviewed the roof framing. The roof has an 6-1⁄2 on 12 slope. The live load for this slope is 38 psf. The live load for the slippery slope reduces to 26 psf. The solar panels weigh less than 3 psf.
With a total load of 4 psf (26 L +15 DL +3 panels), the 2x6 rafters at 20" on center are stressed to 1,061 psi. The allowable stress for these 2 x 6s is 1,504 psi. The framing is adequate to support the solar panel installation.
As you can see, the live load from my solar panels is quite small: less than 3 psf. According to the engineer, this is well within the allowable limit for my roof.
Your local building codes will have different requirements. An inspection may not even be required. Nonetheless, any rooftop solar installation project should ensure that the roof can safely support the load.
How much do solar panels weigh?
Solar panels for residential projects are roughly 42 inches wide by 65 inches long, give or take a few inches, and generally weigh 50 pounds or less.
In addition to the panels, the racking and attachment points add additional weight. These are generally made of aluminum and are light for their strength. In total, you can generally expect the entire system to weigh less than 3 pounds per square foot (psf).
If you have a flat roof, you can have an attached racking system, which will also weigh in the range of 3 psf or less. However, another option is a ballasted rack. This type of racking system isn’t attached to the roof, but is held in place by heavy weights - often concrete blocks. The advantage of this is that bolts into the roof aren’t required, with the tradeoff of a higher live load.
While eliminating holes in your roof is a desirable aspect of ballasted racks, the higher live load might exceed the capacity of the roof.
How much load can your roof support?
While most rooftops that are in good condition should easily support the additional load from a solar array, local codes can add significant requirements that reduce the live load capacity of your roof. These can include:
- snow loads
- wind loads
- seismic loads
Heavy snow, hurricane force winds, and earthquakes are events that your home might have to deal with. Even if a solar array might not place much additional stress on your roof on a normal day, if you are located where any of these hazards can happen, a solar array could push your roof over the allowable live load.
For example, solar panels can act like a sail in high winds and place significant uplift force on your roof. This is true even though rooftop panels sit only a few inches off the roof surface. That’s still enough of a gap for high winds to get under the panels and place a lot of load on the roof.
Whether snow, wind, seismic, or other considerations apply to you will depend on your local building codes, so check with a roofer, solar installer, or your local municipality.
Sistering the rafters can increase the strength of your roof
If an inspection determines that your roof is unable to support the additional load of solar panels, that doesn’t mean you’re out of luck. One option that your roofer or solar installer might recommend is to sister the rafters.
Sistering means to join multiple pieces lumber to increase the strength. A sistered joist could involve using two pieces of lumber in place of a single - essentially, doubling the thickness of the resulting beam.
Sistering could even involve multiple additional beams, as depicted in the illustration above.
This technique will add significant strength to a roof, allowing it to support the load of a solar array.
Reinforcing your roof for solar panels: is it worth it?
Doing techniques like sistering can add significant expense to a solar installation project, so you’ll have to review the numbers with your solar installer to determine whether it will be financially worth it.
Keep in mind that in the majority of cases, a roof in good condition won’t need reinforcement to support the weight of a solar array. If the structural engineer or solar installer says that you can’t proceed with a solar installation without additional work, be sure to dig into the reasons why. It may be the case that your roof happens to be in poor condition and needs repairs or replacement.
If that’s the case, it might be best to consider more extensive repairs, such as a tear-off replacement, before going ahead with solar. It may be that you will need this work done anyway. It’s far better to have a roof in excellent condition before installing solar panels, because you can expect a solar array to function for 25 years or more. If you have to replace or repair your roof during that time, removing and reinstalling your solar panels will add significant expense.
