What Is A Low Slope Roof?

Jessica Allen
by Jessica Allen

One of the most significant differentiators between roofs is the slope, or the roof’s vertical rise over its horizontal run. Roofs with a low slope are nearly flat, while roofs with a high slope are steeper. But what constitutes a roof as a low-slope system?

OSHA standards say that a low-slope roof’s slope is less than or equal to 4:12, or four in twelve. This means a vertical rise of four inches or less over a horizontal run of 12 inches. Low-slope roofs are also called flat roofs, and the three main types are built-up, modified bitumen, and single-ply.

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What Is The Normal Slope Of A Roof?

For residential buildings in North America, the normal slope has historically been between 4:12 and 9:12. 4:12, said as “four in twelve,” means a rise of four inches over a run of 12 inches. Meanwhile, a slope of nine in twelve means a rise of nine inches over a run of 12 inches.

For asphalt shingles, which is the most common roofing material, the minimum slope is 2:12. The reason for this minimum slope is gravity. Water that collects on the roof will run off with this minimum slope of 2:12.

What Is A Low-Slope Or Flat Roof System?

The terms “low-slope roof” and “ flat roof” are interchangeable. They both refer to a roof that is only slightly pitched and nearly flat. However, no roof should be completely flat because it needs a slight slope so that water runs off.

Generally, a low-slope or flat roof has a slope of less than 4:12, or four in twelve. This means for every horizontal foot, the height of the roof rises by less than four inches. However, definitions vary; some say a low slope roof must have a slope less than 3:12.

Most of today’s low-slope roofs have a continuous membrane covering bonded with adhesives or heat welding. Some low-slope roofs consist of soldered interlocking panels of tin or copper. This construction is meant to resist pools of standing water.

There are three main types of low-slope roof systems: a built-up roof, a modified bitumen, and a single-ply.

Built-Up Roof

A built-up roof is made up of multiple layers of roofing felt or tar paper. Hot asphalt or coal-tar pitch (also called bitumen) creates a watertight membrane. Then, this membrane is surfaced using a gravel coating embedded in more bitumen.

Built-up roofs have a long history of performance, but they’re not used as commonly as they once were. They come with some environmental concerns and can be expensive.

Modified Bitumen Roof

A modified bitumen roof uses a heavy polyester or fiberglass mat for strength. Chemically-modified asphalt is used on top of the mat. This type of roof has a range of application methods and excellent waterproofing characteristics.

Modified bitumen roofs also have high tensile strength and a proven record of high performance. The cost is competitive considering the lifespan of the roof, and many modified bitumen systems come with long-term warranties.

Single-Ply Roof

A single-ply roof is manufactured as an entire membrane roof within factory-controlled conditions. This type of roof is usually white, which makes it heat-reflective. Most single-ply roofs are Energy Star products, which means they’re highly energy-efficient and can reduce cooling bills.

Another benefit of single-ply roofs is that they’re environmentally friendly. They’re also highly durable, strong, waterproof, and come at a competitive price. There are many top coating options to choose from, plus a range of application methods.

The three most popular types of single-ply roofs are TPO, PVC, and EPDM. TPO, or thermoplastic polyolefin, stands up to inclement weather and has great flexibility. PVC, which stands for polyvinyl chloride, is similar to TPO but can withstand a wider range of chemicals. EPDM, or ethylene propylene diene terpolymer, is a rubber product that can hold up to decades of hot/cold cycles.

Low Slope Roof Advantages And Disadvantages

Let’s dive into the benefits and drawbacks of installing a low-slope roof.

The Advantages

We’ll start off by talking about the benefits of low-slope roofs.

Improved Heating And Cooling

First, low-slope roofs improve heating and cooling when compared to steep-slope roofs. This is because, with a low-slope roof, there’s less attic space, which contains excess air. This excess air decreases the HVAC system’s efficiency.

Easier Maintenance

It’s much easier and safer to perform maintenance on a low-slope roof in comparison to a steep-slope roof. There’s less of a chance of slipping and falling since it’s easier to walk on roofs with lower slopes.

Cheaper Installation

Low-slope roofs require fewer materials and labor hours to install, making them more affordable.

Solar Panel Compatibility

Roofs with low slopes are compatible with the latest solar panels.

Modern Design

Low-slope roofs mesh well with the current contemporary design schemes for homes.

Less Wasted Space

With steep-slope roofs, there are usually attics that may or may not be usable. Low-slope roofs do not create attics and thus come with less wasted space.

The Disadvantages

Now, let’s cover some of the downfalls of low-slope roofing systems.

Increased Chance Of Water Damage

Unfortunately, water, ice, and snow sit on low-slope roofs longer than on steep slope roofs. If homeowners don’t remove the buildup, leaks and water damage can occur.

Need To Remove Debris Regularly

A buildup of debris can damage the roofing material of a low-slope roof, so it must be removed regularly.

Not Compatible With All Roofing Materials

Low-slope roofs aren’t compatible with all roofing materials. For example, traditional asphalt shingles often cannot be used on low-slope roofs.

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Related Questions

What does OSHA consider a low-slope roof?

OSHA’s definition of a low-slope roof is a roof with a slope less than or equal to 4:12.

Can you shingle a low-slope roof?

It depends on the exact slope of the roof. Some professionals will install shingles on a roof with a slope as low as 2:12. However, special procedures must be followed when installing shingles on a low-slope roof.

Why are the standard pitched roof treatments not acceptable for a low-slope roof?

The main issue is that low-slope roofs are more affected by water than steep-slope roofs. They’re much more susceptible to the buildup of standing water, which can lead to leaks and water damage.

Jessica Allen
Jessica Allen

With a lifelong passion for writing plus strong enthusiasm for home improvement and DIY projects, joining the team at Upgraded Home was an easy choice. Jessica Allen likes to share helpful information with current and aspiring homeowners. Aside from writing, Jessica loves doing yoga, playing the piano, and dabbling in graphic design.

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