What Is the Purpose of Gravel on a Flat Roof?

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A flat or low slope roof is often not a suitable surface for shingles or other roofing materials because moisture can’t flow off them. One solution to this is a built-up, or tar and gravel, roof, where many layers of hot asphalt or bitumen, tar paper and a waterproof membrane are applied and then covered with gravel. The gravel both protects these layers and acts as a ballast.

Tar and Gravel Roof Structure

  • A tar and gravel roof has several layers, including the roof deck, a vapor barrier, insulation, a waterproof membrane and gravel. The roofing membrane, or roofing bitumen, serves as the primary waterproofing component. It is heated and applied to the roof as a liquid. Roofing membranes are typically either petroleum-based asphalts or coal tar pitch. Three or more layers are applied with either asphalt-coated sheets or felt between the membrane to add reinforcement and stabilization. When gravel is added to the final layer, some of it is embedded into the hot bitumen while the remainder provides a protective coating for the membrane.

Gravel Layer

  • Gravel or another type of aggregate, such as slag, on a low slope or flat roof surface adds weight to the material beneath. A good covering of gravel acts as a securing layer, preventing the underlying material, which protects the roof from moisture, from being blown off. Flat roof surfaces are subjected to intense UV rays from the sun. Without a protective coating such as gravel, the asphalt beneath can become extremely hot. The sun can also soften the membrane covering and eventually make it brittle, which leads to cracks and leaks. Light-colored gravel, in particular, can help reflect heat from the building, lowering cooling costs as well. Gravel can also offer some fire protection because it is a fire-resistant material.

Advantages

  • Tar and gravel roofs will last between 10 and 20 years and are rated by how many layers are applied during installation. Typically, a roof with 3 to 5 layers of bitumen and felt is considered standard. This type of roof coating is lower in cost compared to other flat roof solutions and offers resistance to heat conduction between the interior and exteriors of your home, which saves on heating and cooling costs.

Disadvantages

  • Tar and gravel roofs are durable but often cannot be installed by the homeowner. They’re also heavier than other roofing materials, and the joists in your roof system may require reinforcement to withstand this added weight. Tar and gravel roofs may develop leaks primarily around flashing, chimneys and eaves, which are difficult to find or repair without removing the gravel layer. Wind may blow the gravel and expose part of the membrane surface to the elements, which may also lead to leaks as the membrane deteriorates. Often a repair requires removal of at least the surface layers and reapplication. For these reasons, tar and gravel roofs shouldn’t be installed in areas that receive significant amounts of rain or snowfall.

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