16 Flat Roof Terms You Need To Know

16 Flat Roof Terms You Need To Know

16 Flat Roof Terms You Need To Know

A flat roof is a complicated system comprising of trusses, supporting beams, underlayment, decking, and roofing material. A lot goes into keeping any commercial building standing strong against all weather elements. Whether you’re thinking about installing a new flat roof or replacing an old one, it’s important to understand common terminology to help you work with your roofing contractor. Here are some of the most important flat roofing terms you should know.

1. Aggregate

An aggregate is the mineral surfacing applied to built-up roofs to protect the top coating of bitumen against direct sunlight and to reflect solar radiation. It helps to reduce the maximum surface temperature, the short-time fluctuations of the black surface, as well as the seasonal range of temperatures.

2. BUR

Built-up roofing, also known as BUR, is the most known flat roofing material. It is composed of alternating layers of bitumen (asphalt) and reinforcing fabric and is finished with a top layer of aggregate, mostly stone or gravel. 

3. TPO

TPO (Thermoplastic Polyolefin) roofing systems are constructed with a single layer of synthetics and reinforcing scrim to cover flat roofs. TPO membranes are made in sheets 10, 12, or 20 feet wide. This roofing system has gained industry acceptance thanks to its naturally reflective surface which protects the roof against UV damage.


EPDM (Ethylene Propylene Diene Monomer) refers to the synthetic rubber derived from oil and natural gas. Its ease of installation and durability have spurred its growth and made it one of the most common commercial flat roofing system of choice in the US today. 

5. PVC

PVC Single-Ply Membrane is arguably the most popular flat roofing material. It’s even possible to install flat roofs with ONLY PVC membranes. Typically, a PVC membrane is a single layer of thermoplastic material with PVC seams heat-welded to form a watertight bond.

6. Modified Bitumen

Modified Bitumen roofing systems are single-ply systems which are mostly roll-applied to the roof surface. They are very similar to asphalt roofs but feature additional polymers in their materials to provide added protection from the weather elements and help in improving the lifespan of the product.

7. Roof Slope

A roof slope is simply the degree of slanting of the roof. Generally, the minimum slope for water to run off a roof is 1% (1/8″ per 1′). However, the minimum slope for a flat roof by building code is 2% (1/4″ per 1′). Although enough slope will allow rainwater to run off the roof freely, you will likely need a waterproof membrane for your flat roof.

8. Flashing

Flat roof flashing is a critical component of a roof and should not be ignored. Essentially, flat roof flashing comprises strips of metal bent at an angle of 90-degrees. Their main purpose is to act as a solid defense against rain, ice, snow, moisture, and even some debris.

9. R-Value

A roofing material’s R-value is the degree of its resistance to heat flow. For commercial flat roofs, it is important to know the R-value since many states require roof systems to have a minimum amount of thermal resistance. To determine the R-value of a roof system, your roofing contractor will take into account the membranes, insulation, underlayment, cover boards, and adhesives.

10. Substrate

A substrate is a surface upon which the waterproofing membrane or roofing material is applied. In a flat roofing system, a substrate may refer to the structural deck or insulation. 

11. Underlayment

Roofing underlayment refers to the water-resistant or waterproof material roofers install directly onto the roof deck. It is often applied under the rest of the roofing materials as an extra layer of protection from severe weather. 

12. Seam

A roofing seam occurs wherever the material panels meet. This could be with TPO, PVC, or metal roofs. Seams are typically the weakest point of the roof.

13. Storm Straps

Storm straps, or hurricane straps, are usually wrapped over roof trusses or rafters to fortify the roof and minimize wind damage. 

14. Single-Ply

Single-ply roofing systems are quite popular flat roofing materials, especially for projects with weight and load-bearing considerations. The single-ply membrane is durable and highly resistant to harsh climatic conditions, tears, and punctures. They are incredibly lightweight, typically weighing 4-7 pounds for every 10 square feet.

15. Torch-down

Torchdown roofing is simply modified bitumen combined with high-quality resins. This special combination ensures your flat roof is completely impermeable to rainwater and highly resistant to otherwise damaging UV rays. Torchdown roofing won’t melt or gather large puddles of rainwater, making it highly weatherproof.

16. Felt

Roofing felt is another popular and traditional method of creating flat roofs. Felt roofing, sometimes known as tar paper, is installed using hot bitumen to create a solid bond between the roof and the felt. 

For more than 40 years Bill West Roofing in Lee’s Summit, MO, has built a reputation for high-quality craftsmanship and exemplary customer service. If you are in need of a commercial flat roofing system replacement, our experienced roofers will ensure the best workmanship using the highest quality products. Contact us today to get a free estimate or to speak with one of our friendly roofing consultants.

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