How to Slope a Flat Roof For Drainage (Commercial Roofing)

Most commercial buildings have flat roofs. However, “flat roof” is a bit of a misnomer as these roofs aren’t 100% flat. If they were, they’d have constant water ponding, non-stop leaks, and other serious issues.

Therefore, all flat roofs are required to have a slight slope to allow for proper water drainage. They’re also commonly referred to as low-slope roofs — and they can be sloped to varying degrees.

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Below, we’ll discuss how to slope a flat roof for drainage and why it’s important.

The Importance of a Roof Slope

The roof slope, also known as the roof pitch, is the incline or steepness of a roof. This incline is typically expressed as a ratio of the roof’s vertical rise and horizontal run, meaning how much it rises vertically per horizontal foot. 

For example, a roof with a 4:12 slope rises four inches for every foot it extends horizontally. 

Ideally, a flat roof will have a minimum slope of one-quarter inch per foot, which would allow for a slope between one and 10 degrees. Of course, this ratio is minimal, which is why low-sloped roofs have a flat appearance.

The flat roof slope on commercial buildings serves several important purposes, such as:

• Providing flat roof draining. The slope of a roof is the most essential part of its water drainage system. Steeper-pitched roofs, like the roofing systems you see on homes, easily and quickly shed water and snow off the roof’s edge. Lower sloped roofs provide the same kind of drainage system to help commercial buildings avoid water ponding as they help to shed rainwater and snow, only they typically push the roof’s center or various other areas that lead to an open drainage system.
• Determining the materials that can be used. A roof’s pitch also determines the type of roofing materials and overall roofing system that will suit your commercial property best. There are several types of roofing materials that can be used for flat roofs, including PVC, BUR, EPDM, MBR, and TPO — all of which will affect the slope and how it performs over the course of the years.
• Saving money. As a business owner, you may have the option to choose between a flat roof or a steeper-pitched roof — even if you work in the rental property business. Opting for a lower-pitched roof is much more cost-effective as it requires fewer materials to be installed. 

How to Properly Slope a Flat Roof for the Best Drainage

When it comes to learning how to properly slope a flat roof for a well-functioning drainage system on your building, there are several factors you’ll want to consider:

• The roofing material. The most common types of flat roof materials include:
  • Built up roofing (BUR)
  • Polyvinyl Chloride (PVC)
  • Ethylene propylene diene monomer (EPDM)
  • Modified bitumen roofing (MBR)
  • Thermoplastic polyolefin (TPO)

Each type of roofing material comes with certain advantages and disadvantages. More importantly, they’ll each affect the slope of the roof — so you’ll need to choose based on your needs and the overall design of your commercial property.

• Year-round weather. If you live in an area that sees a lot of rainfall or a heavy snow season, you’ll want to consider investing in the highest-pitched roof you can (as far as flat roof pitches go) to ensure the proper shedding of all that extra moisture.
• Your roof accessories. Having skylights, vents, pipe boots, chimneys, etc., can all affect your roof’s performance. What’s more, it can affect how well your roof drains and prevents water damage.

Now, following the rule of thumb for a flat roof drainage system — which is to slope your flat roof at least one-quarter of an inch per horizontal foot — here’s the best way to slope for property drainage:

• Use the right underlayment. When you’re investing in a new roof for your business, you want it to last a long time and do a good job of preventing water seepage. In addition to acting as a water-resistant membrane, an underlayment that’s a bit thicker on one edge than the other will help to create more of a slope that allows your flat roof to shed water.
• Use pavers. If you have a mostly concrete roof, you can create a slope by having pavers installed up top. The pavers should have a thicker edge that’s also slightly higher than their thinner edge, which is very effective for water drainage.
• Opt for an upgraded drainage system. The most common type of drainage system for flat roofs is a gutter system that’s installed along the roof’s edge to collect water as it runs off. The water is then redirected from your business via its downspout, and the angles of the gutter system will directly affect the slope of your flat roof and how water is diverted away.
• Hire a reputable flat roofing contractor. It’s not a good idea to slope a roof yourself — or do any other kinds of repairs or installments, for that matter. It can be dangerous and you can easily do a poor job that leads to more costly repairs down the road. Professional roofing contractors that specialize in commercial properties will be able to provide you with the best options for your business’s needs. So, it’s best that you find a reputable roofer to take care of the job and guarantee the best possible outcome.

Need Help With Your Flat Roof?

Whether you need to re-slope your business’s roof or are looking to have a new roof installed, it’s best to consult with experienced and skilled professionals — and Trust Roofing has the most experienced and skilled roofing professionals around when it comes to flat roofing systems.

We’ll come out and inspect your property and walk you through all of your options so you can make a more informed decision. Reach out to us today to learn more about our commercial roofing services and to schedule your free inspection. 

We’ll also provide you with a free, no-obligation estimate, industry-leading warranties, and special financing options!