In past blogs on our website, we talked about the issue of solar panel or PV system damage to low-slope / flat roofs, here and here. This issue comes up often because it’s so common that roofs are damaged by photovoltaic solar panel installers. Often one of the most common methods for mounting solar panels is to install a purlin or beam system that spans from parapet to parapet with the panels suspended and mounted on top of the purlin system. From a structural perspective that described system might work fine, but often installers, particularly here in Washington DC, where low slope flat roofs for row homes run on a relatively limited span, it’s common for the PV installers to drill directly through the purlin base plate system into and through the parapet coping. That type of installation is destructive to the coping.
A photo of one of the many example follows, just for reference. This particular installation leaked and caused damage to the houses on both sides of the parapet. The solar panel installer came back to ostensibly fix the problem but just put caulk directly on top of the area where the metal plates met the substrate coping but forgot to prep and chemically clean the substrate coping prior to the application of the caulking. This is an insufficient form of remediation or repair.
In most cases, modern flat roof systems use a sheet metal at the coping area. That type of metal, when applied in a parapet coping, cannot be pierced with a bolt and then sealed with nothing more than a gun applied sealants. Sealant like that, in itself, is not an acceptable method approved by the building code for mounting solar panels to a rooftop. In a high slope roof, out in the county, where the roof is covered with asphalt shingles or something like that, different rules apply and that type of installation might be sufficient. Here in the city though on a historic flat roof, that type of installation is insufficient and problematic. It’s not approved or acceptable to the local building code as it applies to a low slope roof system. It’s important to understand this, and it’s not just because the building code says it’s not acceptable that it shouldn’t be done, there’s a much more important reason and that is simply because that type of installation is highly prone to leakage.
Another alternative PV system that we looked at in the past on our blog was one that was installed with nothing more than flexible mats that rolled onto the rooftop surface. This type of system may have required a bit more preparation of the rooftop membrane, but altogether was one that avoided damage to the roof and didn’t require bolt mounting at all. That type of system will probabluly put the solar panel installers out of business in time, but it will be terrific for buildings because it will avoid almost the leaks caused by bolting through parapets.
Recently, we had a chance to look at another PV solar panel system that used a ballast instead of using bolts that penetrated through the rooftop membrane. One important distinction and clarification is that, it is accepted to bolt through a low-slope membrane or parapet coping, but these mounting locations must be treated with an approved seal, not just caulk. Here, in Washington DC, solar panel system installers do not seem to be aware of this distinction. Boots, collars, and Pitch Pockets can be installed at penetration or mounting locations to properly seal bolts or mounting devices that drill or penetrate through the Rooftop membrane. By and large though, here in Washington dc, when solar panel installers install their systems, they do not use and approved method, instead they just put Caulk at some of the spots around their penetrations and that is not sufficient and leads to roof leakage.
This alternative solar panel system uses a ballast.
A ballast is a weight that counterbalances or resist against the forces which would otherwise move or destabilize a structural physical element of a building. For example, in the case of these panels shown in the photo below, they are supported using cementitious blocks placed on the feet of the panels.
We should mention, systems like this or any type of mechanical or non-roof system placed upon a roof membrane should be separated with at least a slip sheet but in this case they have omitted or forgotten to install the slip sheets before setting the panels on the rooftop membrane. That, of course, is a mistake, but overall this is still better than putting holes directly through the rooftop membrane without sealing them properly.
At a glance, just from an intuitive perspective it appears that this type of system has a relatively low ballast weight requirement per respective surface area of panel. And from a simply intuitive perspective it also looks like they weight of this ballast would likely not require any specific or particular additional provisions from a structural support perspective for the substrate roof system below the panels. In Washington DC, the typical row home is built with a structural demising wall at the side in a side-to-side row home configuration. Therefore, the parapets are generally part of a structural wall and are built directly above the structural load path, one of the main load paths of the building. This system appears to be at an intermediate part of the load path where the roof frame spans between bearing points below, yet still not a weight or at a level of mass that requires additional support.
Overall though, this low slope system needs upkeep and maintenance. At a glance, you can see that it’s a granulated modified bitumen membrane, but the granules have gotten so dirty that it almost looks like a faded black roof. For proper reflectivity, the roof must be at least gently cleaned from time to time. It doesn’t have to be done often, but every few years it just needs a simple cleaning. Also, you can see that there are debris from construction that are strewn about here and there and these debris can blow around and cause problems, some of the metals may have sharp edges and every roof, particularly commercial low slope or flat roofs and residential homes need to have a contractor take a look and check things out every once in awhile.
We recommend every building owner in DC who values the longevity of their roof (and their investments) and building use a contractor who values the simple and important principles of proper roof construction like Dupont Roofing DC. Learn more about our company and the proper techniques of working with roofing on historic buildings in Washington DC here on our blog at DupontRoofingDC.com, and you can call us at (202) 840-8698 and email us at dupontroofingdc@gmail.com.
