What is a Ballasted Roof System?

While ballasted roof systems aren’t as popular as they used to be, we still encounter a variety of such systems on existing buildings, and thought we’d explore the topic a bit more.

A ballasted roof means that the roof membrane is not anchored or adhered in any way to the decking material. It is, however, ballasted, generally with gravel. When the concept first came to the U.S., plenty of roofers scratched their heads and wondered how the roof stayed on without being anchored, especially during high winds or storms.

The gravel used in built up roofing (what the roofing contractors were used to) was not the same as that used in a ballasted roof. For built up roofing, the gravel was generally .25 - .5 inches in diameter. The stones used in ballasted roofing are generally 1.5 - 2.5 inches in diameter and applied in a thicker layer. The stones are therefore much more substantial and hold up well under wind tunnel tests. In many cases, ballasted roofing also uses pavers in places of heavy traffic or even over the entire surface of the roof.

Because a ballasted system is loose-laid, with plate and fasteners installed here and there to prevent excess movement, the installation is extremely simple and quick. It also has the advantage of being able to be installed during almost any kind of weather and temperature conditions. Since there are no fasteners, and no need for heat or sticking materials, there is less clutter and no smell during installation. In addition, the stones or pavers and virtually fireproof, making this one of the most fire resistant roofing systems on the market.

Ballasted roofs are durable and long lasting. The roof tends to flex a little better under the expansion and contraction of heat and cold, making it a bit more durable. Stones or concrete pavers protect the waterproofing layer from UV rays, hail, and foot traffic. The loose-laid layers in a ballasted roofing system are easily taken up making for easy repair. And at the end of the roof's designed lifespan, the lack of adhesives allows the roof to be totally recycled.

Another advantage is that the mass of the roofing materials absorbs heat during the day, but doesn’t pass it on the building below, thus helping to reduce energy costs.

This type of roof tends to be aesthetically pleasing as well, since it can also be used as an outdoor patio, with walkways, gardens and more.

The weight on a ballasted roof can be from 10 to 25 pounds per square foot, meaning the substructure must be designed specifically to carry more weight than other roofing systems. It can also be a bit challenging to find a leak with a ballasted roof system, and because the stones tend to collect a lot of dirt, patching it can be a bit difficult. The ballast stones (not pavers) can be moved around by the wind, and in some cases have caused dangerous flying projectiles during wind storms. For that reason, many roofers are moving to all pavers with this system.

As mentioned, ballasted roof systems are less common in new construction and replacement, but can be appropriate in some cases. We recommend consulting a reputable roofing contractor specifically experienced with ballasted roof systems should you need assistance with service, repair or installation.