Ballasted Roof System
Ballasted roof systems have been installed since the early 1970’s. They are known for their installation speed and pleasing aesthetics. Most ballasted roof systems use gravel stone that is similar to what would be used in landscaping. However, other ballasted systems use pavers, which are easier to walk on and can be color coordinated.
A ballasted system is installed in the following way:
- The rigid insulation is loose-laid, with a plate and fastener installed here and there to prevent excessive movement.
- Then, the membrane is installed over the loose-laid rigid insulation.
- All membrane flashing and other detail work is completed.
- Finally, the ballast stone/or and pavers are laid down throughout the roof system. Most roofers typically installed 10-15 pounds of ballast per square foot.
Benefits of a Ballasted Roof System
- Aesthetically pleasing.
- Installs very quickly, since no fasteners or adhesives are used.
- Doesn’t produce any foul smell during construction.
- Since no fasteners are used, there is very little chance of hitting conduit or light systems that are close to the deck.
- Roofs with pavers as ballast can be used as outdoor patios.
Drawbacks of a Ballasted Roof System
- Adds a lot of weight to the roof system, typically 10-15 pounds per square feet.
- Extremely difficult to find a leak in this system, since the membrane is well hidden.
- Ballast stone traps a lot of dirt, making patching difficult.
- Ballast stone fractures and gets sharper over time, which can puncture the membrane.
- Ballast stone can crush the underlying insulation, creating weak spots in the roof membrane.
- Over time, the stones move around, creating “Bald spots” where the system isn’t held down at all.
- In windy areas, ballast stone can actually blow off the roof, creating projectiles below.
- Stone ballast is no longer allowed by many building codes. Pavers, however, are still acceptable.
If roof weight isn’t a concern, and aesthetics are a priority, consider a ballasted roof. Just be warned that, over time, as leaks start to occur, it will be extremely difficult to find and patch them. Also be aware that, in some states, such as Rhode Island, installing a roof system with ballast stone is not allowed by building due to the dangers of stone blowing off the roof, hitting people and objects below. In other states, like Massachusetts, ballast roof systems are illegal in certain towns because of the additional weight it adds to the building. For the headaches involved with ballasted roof systems, consider a mechanically attached or fully adhered roof system.
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