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Rocks are placed on commercial flat roofs for many different reasons:

  1. If the roof is what is called a ballast roof, it is not fastened or adhered down throughout the whole roof. The membrane is most likely an EPDM membrane that is fastened at the edges only and being held down by the weight of the rock on top of it. The rock is called ballast. This system is typically an 8-year system and needs to be replaced once the integrity of the roof is gone.
  2. Various types of rock are embedded into asphalt roofs to add strength to the roof and help it last longer.
  3. Pea gravel is sometimes placed on top of a smooth BUR asphalt roof to protect the roof from the sun's harmful UV rays. This protects the asphalt from "alligatoring", or cracking, from exposure to the sun.
  4. Small #11 roofing granules are sometimes used on foam roofs by embedding them into the top layer of an elastomeric roof coating. This is done to add additional strength to areas that may have more traffic, and also to prevent birds from pecking at the foam on the roof.

There are some benefits to having rock on a flat roof, especially if it is a black rubber or asphalt roof, as it adds additional reflectivity and protects the substrate from UV rays. However, with the advancements in elastomeric roof coatings, it is much better to go with a highly reflective white roof coating over rock. Look into the Reflective Roof Coating Institute's study on reflectivity and energy efficiency.

Jason Popovich

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15y ago

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Q: Why is rocks placed on a flat commercial roof?
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