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5 Need-to-Knows About Flat Roofs

Would you like to buy a home with a flat roof? Do you already own one? Even though flat roofs are more common in commercial buildings, they’re also popular for many single- and multiple-family residences.

What should you know about flat roofs if you’re new to them?

1. Flat roofs aren’t flat.

Every roof must slope, to some degree, for water to drain. With a flat roof, however, the pitch may be as little as 1/4- to 1/2 inch per linear foot. Since a flat roof isn’t actually flat, some prefer calling them “low-slope” roofs.

A house with a flat roof is often higher in the center and uses gutters and downspouts to collect roof water and divert it away from the home. Scuppers are another option—large square openings in the roofline with downspouts directly below the scuppers to carry the water away.

2. Flat roofs offer several advantages.

Since a flat roof is constructed without trusses, it may cost less to build one. However, there are significant differences in the cost of materials used to finish the roof.

Walking on flat roofs is easier and safer than on pitched roofs, making installing, inspecting, and maintaining a flat roof simpler.

You might be able to use the roof for outdoor living space or additional storage. You’ll also have more options for installing an array of solar panels.

3. There are drawbacks to flat roofs, too.

Flat roofs often weigh more than pitched roofs and may require additional structural support, especially if the roofing material is heavy.

Of course, you’ll also need to factor in the weight of any rooftop features, like a deck—plus snow accumulation, if applicable, in the winter.

Homes with flat roofs don’t have attics, so there’s less storage space. Flat roof systems typically include a layer of insulation below the roofing material, but the lack of an attic limits your ability to add insulation later.

4. Three types of flat roof systems are most common.

Overlapping shingles work well on pitched roofs since gravity readily pulls water downward. In contrast, low-slope roofs require a continuous waterproof surface so standing water can slowly drain or evaporate. The most common systems are:

Built-up roofing (BUR)

Alternating layers of roofing felt and hot asphalt (or tar) are built up, and the top asphalt layer is covered with gravel to reflect the sun and protect the lower layers. This is the traditional method of finishing a flat roof. But it’s also the heaviest and may require extra structural support.

Modified bitumen (MBR)

A lighter-weight alternative to BUR, modified bitumen is an asphalt-based material with a mineral coating, similar to asphalt shingles. MBR comes in long rolls. When the sheets are unrolled, a torch is used to heat the underside, which helps the material adhere to the base layer.

Membrane (or single-ply) roofing

Sheets of a flexible, waterproof membrane (often rubber) are rolled out. Then the edges are sealed. This is typically the lightest weight option. Unfortunately, it’s relatively easy to puncture a membrane roof, but repairs are pretty simple.

There are pros and cons to each flat roofing system, including differences in costs and maintenance. So, research options to decide which approach works best for your application and budget.

5. It’s essential to hire experienced and reputable roofing professionals.

All roofs can leak. However, since water drains more slowly from a flat roof, you want to make sure it’s installed correctly. Typically, this is not a DIY job.

Be sure to find an installer who stands behind their work and can help you make the best decisions for selecting and maintaining your roof.