If you've ever looked out from a high-rise window and noticed rocks or gravel scattered across a neighboring rooftop, you might have wondered what it's doing there. Why do they put gravel on roofs? While it may seem strange, gravel serves a specific purpose in certain commercial roofing systems.
Gravel on roofs is sometimes used as the final surface layer in built-up roofing (BUR) systems, a time-tested solution with more than 100 years of proven performance behind it. In these systems, gravel serves as a protective barrier, helping to shield the underlying layers from ultraviolet (UV) rays, extreme weather, and mechanical service foot-traffic damage. As a commercial contractor, understanding built-up gravel roofs can help you deliver a durable, long-lasting roofing system to your client's building's needs.
What Is a Built-Up Roofing System?
A built-up roofing system consists of alternating layers of reinforcing ply fabrics and hot asphalt, which is typically hot-applied. These layers are installed directly over the roof deck, insulation, and sometimes a cover board, creating a durable, multi-layered barrier that offers built-in redundancy that can help reduce the risk of leaks and system failures.
BUR systems are ideal for commercial buildings with low-slope roofs. They're also often considered a more cost-effective alternative to single-ply membrane systems, like TPO or PVC, while still delivering durability and long-term performance.
Why Do They Put Gravel on Roofs?
Gravel is a critical finishing component of BUR roofing systems, serving several important functions that can enhance the roof's performance, protection, and service life. Here's a closer look at the key benefits.
UV Protection
Built-up roofs with gravel provide exceptional protection against the sun's UV rays, helping to reduce surface degradation such as cracking and blistering. This can help extend the service life of the roof.
Ballast
Larger ballast stones are used to anchor single-ply roofing membrane systems in place, while smaller pea-sized gravel is typical for BUR systems. Both types provide weight to keep the roofing membrane securely in place.
Wind Uplift Resistance
The multiple layers of BUR systems are adhered to one another, forming a monolithic membrane that doesn't rely on fasteners. No fasteners means fewer stress points on the roof, which is beneficial during high-wind events. The added gravel weight helps increase resistance to wind uplift and enhances stability during storms.
Physical Protection
Gravel can help shield the roofing system's waterproofing layers from potential physical damage caused by hail, foot traffic, falling branches, and other types of debris. This added protection helps prevent punctures and other damage that can lead to costly repairs incurred by owners (most manufacturers do not cover punctures unless special coverage is purchased).
Fire Resistance
Gravel is a significant contributor to BUR systems' fire-resistant qualities, which often meet or even exceed the industry standards for fire resistance. Select systems are classified by UL in accordance with ANSI/UL 790, including as a component of Class A fire resistance-rated roofing assemblies. We recommend contacting the manufacturer's technical team and refer to UL Product iQ for specific approved assemblies using this surfacing as an option in the building roof solution.
Drainage and Erosion Control
In landscaping, gravel has long been known as a solution to help control water flow and promote evaporation. Similarly, gravel can help prevent water from flowing across the roof's surface, potentially preventing leaves and other debris from washing into and clogging roof drains. Clogged drains are a leading cause of water intrusion on flat roofs, and gravel plays an important role in reducing that risk. That said, proper roof maintenance is still required, even on roofs with gravel.
What Type of Gravel Is Used on Roofs?
Many types of gravel can be used on roofs. However, it's important that the gravel you select for a project meets ASTM International standards for roofing applications to ensure proper performance and durability. For BUR systems, factory-applied crushed stone aggregate is the most common choice when compared to loose gravel installed on the jobsite. A factory-manufactured cap sheet is typically much lighter in weight and available in a variety of colors, including shades of tan, black, white, and bright white.
For ballasted single-ply roofing systems, weight is important. These systems often rely on larger river-bottom stones to provide the necessary downward pressure to hold the roofing membrane in place without the need for adhesives or fasteners.
When Does a Gravel Built-Up Roof Make Sense?
Built-up gravel roofs are an excellent choice for many commercial buildings with low-slope roofs thanks to their proven performance and long service life. Routine annual inspections to identify and address any issues are typically sufficient to keep the system in good working order.
Geographic location also plays a role in whether a built-up gravel roof might be the right choice. BUR systems perform well in regions with wide temperature fluctuations, high winds, or severe weather due to their multiple layers that create built-in redundancy and protection (although most manufacturers exclude severe weather events from coverage under their guarantees and warranties).
Finally, consider the building's function when choosing a roofing system. For specialized roofing projects like cold storage units or food manufacturing plants, a single-ply membrane system might make more sense due to specific performance requirements.
Adding gravel/rock to the roof system can add significant weight to the structure of the building. Consult a structural engineer with the project architect before proceeding with the design and loading of a gravel surfaced BUR assembly to ensure the building can handle the added weight per square foot.
BUR membranes have been around for years, and they remain a time-tested, reliable choice when it comes to protecting your building from the elements.
