Why Bolting Solar Panels Through Parapets is Bad

It’s never a good idea to bolt (or fastener) directly through a horizontal surface of a low slope roof and in most cases it’s actually against the building code (if not properly covered or overlayed by an approved method).   Technically, for a method of installation of building materials to be approved, it must be specifically written as acceptable by the International Building Code (or as deemed by the AHJ, per locale and / or municipality), or by ICC-ES report, or by the system manufacturer. Specifically in cases where approved by the system manufacturer, the manufacturer must also have it’s products certified by particular organizations to be suitable for their intended purpose. However, DCRA does not inspect roof installations and it is therefore very important to hire a good roof contractor, a specialist in flat roofing (such as Dupont Roofing), a company which not just has some experience but considers the technical aspects of building science and principles of flat roofing to be part of their calling and reason for existence, when repairing, replacing, or working on a roof system, even in cases where installing a solar system. We recommend that our clients consult with us when they’re installing a photovoltaic solar panel system, even if our company is not conducting the panel installation. As a roofing specialist, We have particular training and experience on roofing systems that other contractors such as solar panel installers, HVAC system installers, and other building contractors may not possess themselves.

The picture below shows an example of a photovoltaic solar panel system installed or mounted directly through the horizontal surface of a flat roof parapets.

There are several different types of photovoltaic solar panels system mounting assemblies and configurations, but in this particular example, the solar panels are installed on top of a metal I-beam, used as a purlin.


The most common alternative to mounting purlins or photovoltaic solar panel systems directly on top of a parapet or  roof system with a fastener that penetrates the horizontal membrane would be to install a ballast, a weight or counterweight to hold the photovoltaic solar system in place.  Typical rooftop solar systems are made assembled in large panels and in some way work a little bit like a sail on a boat. In a heavy wind, these panels can actually have a uplift factor that will move and/or displace the panel and in some cases during the heaviest windstorms the solar panels can actually be lifted up in the wind and blown away off the rooftop!   So, if not mounted by fasteners or bolts as shown in this case, it’s extremely important that a ballast or alternative type of system is used to keep the panels in place, but the bolting through the roof is very problematic.

A parapet is the outside wall of a building, where the roof terminates above the remainder of the horizontal roof surface. Not all exterior walls of the building have a parapet, but particularly at the non draining sides of the roof, there’s normally a parapet to separate the building between separate that particular building and the next joined building, generally at the property line.   In most cases, parapets are a continuation of the demising wall between properties. That not only makes a point of separation, but generally also has a special fire rating, normally expressed in terms of time that the building is designed to resist fire at a certain temperature. so essentially, most demising walls are also fire rated.

The terms “flat roof system” and “low slope roof system” are often used interchangeably but technically all flat roofs should actually have some slope or angle or grade which will make water flow from the roof. If a roof is installed perfectly flat it will invariably pond water, that means that the roof will cause water to build-up and stay stagnant. That’s a major problem and not allowed by the building code. So when we use the phrase flat roof, we’re referring to a low slope roof system, but the words are often used interchangeably, mostly as an industry-wide misnomer to distinguish between a low slope and steep slope roof.

Where there are improper installations, curbs, or other elements which break up the linear flow of water, if other supplementary methods, such as crickets, are not installed to continue the flow of water, by gravity alone, ponding will occur.   In a ponding area, the slightest bit of head pressure (the pressure if puddling water) can cause leaks.  Without build-up or ponding, roofs which otherwise allow water to flow may often not even leak even in places where there are breakeven the otherwise continuous membrane.  Concentrically overlaid or lapped layers of materials are an example of this type of configuration.

The demarcation or distinction between a low slope and steep slope roof is generally around 2.5 to 12,  This measurement or angle will sometimes be referred to or expressed in terms of 2.5-12. That degree or angle means that the roof has 2.5 units of vertical incline per 12 units of horizontal length.

The picture below shows a closer view of the bolt which mounts the photovoltaic solar panel system purlin directly to the building structure.  The particular bolt, shown below, is driven directly through the roof membrane and the only material used to seal that hole or penetration through the roof is caulking.   Caulk alone is not sufficient to seal a roof penetration through a horizontal or near horizontal or a low slope area of the roof.

photovoltaic solar panel system purlin directly to the building structure

The building code says that “Parapet walls shall be properly coped with noncombustible, weatherproof materials of a width not less than the thickness of the parapet wall.”   In this particular case, the EPDM roof membrane is installed over top of the parapet, but technically that type of coping may not be acceptable, depending on the requirements of the roof manufacturer.

Requirements of roof manufacturers vary slightly from case to case and manufacturer to manufacture, but in most cases, roof manufacturers require that a metal coping is installed on top of the parapet to terminate their roof system and the building code requires that the manufacturer’s installation instructions are followed.  “Roof coverings shall be applied in accordance with the ….manufacturer’s installation instructions.”


The building code requires that penetrations such as photovoltaic mounting elements are properly flashed and sealed.  “R909.3 Installation:  Rooftop-mounted photovoltaic systems shall be installed in accordance with the manufacturer’s instructions. Roof penetrations shall be flashed and sealed in accordance with this chapter.”

Smart proactive replacement, construction, upkeep and maintenance of low slope roof systems requires an enthusiastic interest and understanding of waterproofing principles and building science.   Here in Washington DC, historic and modern residential and commercial buildings are extremely expensive and the roof and related systems provide the shield that preserves the building.

We encourage all of our clients, and all readers of this article and to our blog in general, to prioritize the value of quality construction and building maintenance, and develop a relationship with our company.  You can learn a lot more on our blog.  Feel free to check it out.  If you have questions about the roof and related systems of your building in Washington DC, fill out the webform below and drop us a line.  We will be in touch if we can help.

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