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Roof-Mounted Solar: Identifying Low-Slope Roofing Types
Commercial and industrial (C&I) roof-mounted solar photovoltaic (PV) systems come with a unique set of engineering challenges and technical risks. While certain factors like structural analysis and roof loading plans apply to all low-slope roofs, others—such as waterproofing details or slipsheet requirements—are specific to the type of roofing system in place. Understanding these differences is essential for successful PV installations.
In this two-part article, I'll walk through the key engineering and risk management considerations for C&I roof-mounted PV systems, focusing on the most common types of commercial roofing. Proper identification of the roof system is the first step toward ensuring a safe and effective installation. This article will guide you through identifying different roofing materials and evaluating the condition of the roof before proceeding with solar deployment.
### Roofing System Identification
The type and condition of the roof are critical to the success of any PV project. Roofs are typically categorized based on their outermost weatherproof covering, but other elements such as the roof deck, insulation, and vapor barrier also play a role when installing solar panels. The most common low-slope roofing systems in the U.S. include single-ply membranes, built-up and modified-bitumen roofing, and metal-panel roofs.
#### Single-Ply Membranes
Single-ply membranes are made from various polymers and elastomers. The three main types are:
- **TPO (Thermoplastic Polyolefin)**: The most commonly used membrane in the U.S., TPO is known for its cost-effectiveness and durability. It's often white, which makes it ideal for bifacial solar modules due to its high albedo. TPO is less elastic than EPDM and typically has a 15–20 year warranty.
- **EPDM (Ethylene Propylene Diene Monomer)**: A rubber-based membrane that’s highly elastic and resistant to temperature extremes and UV exposure. EPDM is usually black and can last up to 50 years with a 30-year warranty. White EPDM is available for cool roof applications.
- **PVC (Polyvinyl Chloride)**: Known for its flexibility and tear resistance, PVC is often used in cool roof applications. It can be heat-welded or mechanically fastened and comes in custom colors. PVC typically has a 20-year warranty.
#### Built-Up and Modified-Bitumen Roofing
These systems have been widely used for over a century. Built-up roofing consists of multiple layers of tar and gravel, while modified-bitumen uses asphalt-based rolls. These systems add weight over time, which can be a concern when retrofitting solar panels. Assessing the number of layers is crucial for load calculations, as older buildings may not support additional weight.
#### Metal-Panel Roofs
Metal panels, especially standing-seam types, are becoming more popular for low-slope roofs. They offer long service life and are well-suited for solar mounting using compression clamps. S-5! offers a range of clamps for different profiles, and solar installers should carry samples to ensure compatibility.
### Roof Condition and Warranty Status
Beyond identifying the roof type, assessing its condition is equally important. Signs of aging, such as alligatoring or poor drainage, can signal that the roof needs replacement. Installing a 25–30 year PV system on a roof with only a few years left in its lifespan is a risky move.
Also, maintaining the original roof warranty is key. Any new penetrations or modifications should be handled by the original contractor to avoid disputes and preserve the warranty. Always plan carefully to protect both the roof and the solar investment.
If you're looking for expert guidance on de-risking your C&I solar project, contact Pure Power Engineering. Our value-engineered design and construction drawing services help optimize performance and ensure long-term reliability.