Sealing the holes required in most rooftop solar installations is one of the most critical steps in ensuring a waterproof installation. After drilling pilot holes into roof rafters, contractors will fill the holes with sealants such as MS polymer, silicone or butyl to prevent leaks throughout the array’s lifespan.
There isn’t a one-size-fits-all sealant in the solar industry, and selecting the proper waterproofing method depends on the AHJ, temperature and moisture/precipitation levels in the environment where an array is being installed.
“You’re poking a hole in a perfectly good, sealed roof, and a solar installer does not want to be responsible for any leaks,” said Rocky Stroud, national sales manager at rooftop sealant manufacturer Chem Link. “[Sealants have] been a learning curve for solar people because they’re not roofers, and this is very invasive to the roofing material.”
Sealants proven to meet specific environmental needs have long been available from solar and roofing product distributors like Chem Link. But how those sealants are applied is changing to leave even less room for leaks.
Changed in a flash
Flashing used in general roofing projects is a metal sheet that creates an additional barrier for waterproofing. It covers roof penetrations and is bonded to the roof using an adhesive or sealant.
“At the moment, the sealant is actually a ‘belt and suspenders’ approach to waterproofing,” said Sue Stark, senior manager of training at solar mounting manufacturer IronRidge. “We know that our mounts are waterproof without sealant, and only require it in the pilot hole, because the testing is done that way. However, most people will use sealant, and that’s more of a roofing manufacturer’s installation instruction than it is ours.”
Solar systems installed on comp shingle roofs often use a similar metal flashing that slides underneath a layer of shingles. A “U” of sealant is applied to the bottom of the flashing to keep the metal sheet in place, diverting rainfall, snow and other moisture away from roof penetrations.
“The great debate is metal flashing vs. non-metal flashing, and there is no wrong answer,” said Johan Alfsen, senior director of training at solar mounting manufacturer K2 Systems. “Pick your flavor and do what you want, but at least these products are tested. Back in the day, they were just putting sealant all over the L-foot. I call that ‘goop and a prayer.’”
Recently, many solar racking manufacturers have developed top-mounted attachments that go through the shingle and have a much smaller profile and fewer pieces than their metal flashing counterparts. Each top-mount has a different method for applying sealants, and some refer to it as chemical flashing, meaning the sealant is the primary waterproofing method on the attachment.
One example of chemical flashing is Unirac’s FlashLoc line of top-mounts, which is flashed by injecting sealant into a built-in reservoir that waterproofs the roof penetration it’s installed over. QuickBOLT’s flagship mount QuickBOLT2 uses proprietary Microflashing — a stainless steel compression washer paired with EPDM rubber — to seal penetrations, and it can be installed with or without sealants. Then there are Roof Tech and K2 Systems, which use pre-installed butyl pads on the bottom of their over-the-shingle mounts that seal penetrations as bolts pass through the sealant.
“The brackets keep getting smaller, or at least engineered in a manner in which they’re lighter and less invasive,” Stroud said. “Personally, if it’s my roof, the fewer holes the better. But I also want it to stay there, and I really don’t want it to leak, so I think the sealant is as important if not the most important thing from a longevity standpoint, because it’s the only thing keeping water from coming into your house.”
Selecting the sealant for the job
Many racking manufacturers pair sealants with their mounts. For those that don’t, there are a range of roofing sealants with different temperature, precipitation, movement and UV ratings. The simple way to select the right sealant for a specific environment is to read the product label and determine if it meets the environmental conditions of a jobsite.
Each roofing sealant is rated to perform within a temperature range. Some sealant temperature ratings can go as high as 300°F, which is ideal for arid regions like the Southwest, where temperatures on a roof can reach and exceed 150°F. Despite the challenges of installing in those high-temperature conditions, sealant application tends to be more difficult in colder climates.
“Anything below freezing is not recommended. Our rule is 32° and rising is fine. Thirty-two degrees and falling, you’re probably not going to want to use a sealant,” Stroud said. “Chem Link’s a little unique where you can actually apply our sealants in sub-zero weather — they just won’t cure until it comes above 32°.”
Curing is the chemical reaction of turning a liquid into a solid material. One method is moisture curing, which means once the sealant is exposed, it will pull moisture from the air and harden. While some sealants can bond in moist conditions, it’s not ideal to install them during rainfall. Alternatively, there are sealants that will not bond when exposed to moisture.
While a solar array is secured to a rooftop, there is still minor movement happening in the system. Matter exposed to the elements — whether that’s below freezing or higher temperatures — experiences thermal expansion and contraction, as well as other natural forces like wind that cause movement. That’s why it’s necessary to select a sealant that allows for movement without breaking. Sealant movement rating is determined by percentage, and the recommended range is between 35 and 50%, according to Stroud.
“Everything is on the move,” he said. “You don’t want to use Liquid Nails or anything that’s glue, because it just means that whatever it’s stuck to, something is going to give, but it’ll be something that you don’t want to.”
There are sealants that are incompatible with specific roof materials. Chem Link recommends avoiding clear adhesives on asphalt roofing because when sunlight passes through a clear sealant, asphalt can leach into it and break down the adhesive over time. Opaque or solid-colored sealants are the safer choice.
UV ratings disclose how effectively a sealant will perform in direct sunlight. For a system that is built to be exposed to the sun, it’s necessary to invest in adhesives that can withstand those conditions.
“You should be looking at sealants that are made for professional roofing,” Stroud said. “You don’t want to use something that you’d use around your siding. There’s a big difference between caulking and sealants. It’s not just a word, but legitimately there’s just a big difference.”
Choosing mounting products that optimize sealant can give rooftop installers assurance and keep homeowners happy throughout the lifetime of their solar systems.