How to Repair a Flat Roof on a Commercial Building

by Steve Coffey  // in Roofing


Any damage or disturbance in a commercial building or warehouse can instantly impact operations, and the bottom line soon thereafter. Making necessary repairs quickly and correctly ensures no service interruption.

If your commercial building’s roof springs a leak, it is all hands on deck to find the cause and correct the problem. Commercial building owners should be aware of common causes of flat roof leaks, how to make a lasting repair, and when a roof repair or replacement is the better option.

A flat rooftop of a multicolor apartment complex with a city skyline in the background.

Types of Commercial Flat Roofs

Flat roofs have different design requirements than sloped roofs—there are several types of flat roofing systems available for commercial buildings. The following types of flat roofs are most often used in commercial applications.

  • Built-Up Roof (BUR) – Built-up roofs are popular on commercial buildings, often referred to as “tar and gravel roofs.” Roofing felts, also called ply sheets, are reinforced with fiberglass or organic materials. The final surface can be covered with a layer of gravel for added protection.
  • Modified Bitumen – A modified bitumen roof is similar to a BUR with slightly modified material layers. Both types of roofs can be installed using a “hot” method with a torch, but the modified bitumen can also be installed using a “cold” process. The “cold” process uses adhesive sheets that can be rolled out and directly applied to the substrate, reducing installation risks associated with hot asphalt.
  • Single Ply Rubber Membrane – Also known as ethylene propylene diene monomer (EPDM) roofing, single ply rubber membrane roofing uses sheets of rubber glued down or anchored with fasteners. These roofs do not require hot torches for installation as they are mechanically fastened.

Other types of commercial flat roofs on the market include a hybrid combination of BUR and modified bitumen, thermoplastic membrane, and more.

Causes of Flat Roof Leaks

Leaks will inevitably appear on any commercial flat roofing system over the course of its usable life. Some of the reasons leaks occur include:

  • Installation Errors – Even after a newly installed roof has been inspected for quality, installation errors can still exist, only to be discovered later on down the road. For example, poorly installed drainage, flashing, or seals can lead to leaks.
  • Weather Damage – Extreme weather, beyond an average rain or wind storm, can cause damage to a building’s flat roof system. Severe hail, hurricanes, tornadoes, torrential rain, or lightning storms can lead to torn sheeting, damaged flashing, or punctured holes. 
  • Open Penetrations – Any opening in a roof, like those for pipes, vents, or ducts, can create a roof leak if the penetration is not adequately sealed or if the seal wears away over time.
  • Poor Drainage – A bit of a misnomer, a flat roof isn’t actually flat. A flat roof generally has a pitch of one to ten degrees so water can run off the roof or toward the nearest roof drain. If a flat roof isn’t slightly sloped, water can pool, causing the roof to sag and possibly leak.
  • Long-Term Exposure – Most flat roofing systems have a lifespan of around 20 years. After that time, the roof will break down from excessive exposure to elements like the sun, pollutants, and chemicals, which can end up causing leaks.

How to Locate Leaks in a Flat Roof

Before making any repairs to your flat roof, you should locate all of the leaks in an effort to make all necessary fixes at one time. Inspect all the flashing, collars, penetrations, and previous patches for decay or gaps that could allow water inside. Check the roofing material seams for edges that are no longer tightly sealed. Finally, look for low spots in that roof that may have pooling water.

Using a hose, run water on top of any potential leak spots you located. Keep the water running for several minutes and check the location from inside for dripping water. If you want to be more cautious about repairing every possible leak, you can wet down every square foot of your roof. Spray section by section and check for leaks inside during the process.

A man in a white t-shirt and a red bandana on a flat rooftop using a large paint roller to apply a waterproof coating.

Repair Options for How to Repair a Flat Roof

Once you have located all of the leaks you want to repair, you have a couple of options for the repair method. Before beginning any repair procedures below, make sure your roof is clean by removing all dirt, dust, and debris and dry so the fix will hold.

Membrane Patch

Materials Needed:

  • Utility knife
  • Roofing cement
  • Trowel
  • Roofing material
  • Aluminum-fiber coating
  • Broom/brush

A roof patch is most commonly used to repair a leak in a commercial flat roof. Start by using a utility knife to cut and remove the damaged, leaking section of the roofing membrane. Be sure to only cut away the membrane material and not the roofing felt layer underneath. Allow the exposed area to dry completely before proceeding.

Apply roofing cement with a trowel to the area where you removed the membrane—achieve complete coverage of the roofing cement from edge to edge. Next, cut a piece of roofing material that perfectly fits the cutout area and install the patch on top of the wet cement.

Repeat the process by cutting another patch from roofing material approximately 6 inches larger on all sides than the first patch. Apply roofing cement on top of the first patch and install the second roofing layer on top. Lastly, apply a third and final layer of roofing cement on the second patch, extending the application across the edges to the existing roofing material. Once the cement has dried, cover the patch with an aluminum-fiber coating using a broom or brush.

Roof Coating

Materials Needed:

  • Aluminum-fiber coating
  • Broom/brush

As an alternative to or in conjunction with a single patch, you can opt to coat your entire roof with a layer of aluminum-fiber coating. This coating will seal minor leaks and help extend the life of your commercial flat roof by preventing future leaks. In addition to repairing and protecting your roof, an aluminum coating will reflect light from the sun, allowing you to potentially save energy costs in the summer.

Prep your roof by cleaning and drying the surface. You might want to work in sections, so the surface remains prepped before you begin coating. Apply a thin coat of aluminum-fiber coating across every surface with a broom or a brush. Follow the manufacturer’s recommended instructions for the number of coats and necessary drying time. 

How to Prevent Flat Roof Leaks

Roof leaks caused by age and weather are inevitable, but you can take steps to reduce the frequency of leaks that require a repair.

Keep Up With Routine Maintenance

Like any part of a building, a roofing system requires regular upkeep to perform and last. Keep your flat roof clean—wash the surface a few times a year and remove debris, especially after storm events. Make sure your roof drainage is properly working and promptly repair any leaks that do occur.

Consider Warranty and Insurance Coverage

When your roof springs a leak, the repair might be covered by your roof warranty or building insurance. Depending on your manufacturer’s warranty agreement, leaks due to installation failures are typically repaired at no cost to you. Remember to keep up with recommended maintenance procedures to retain these coverages.

A strong maintenance program will keep your commercial building and business operations running smoothly. Executing the plan requires hiring skilled professionals, fixing any problem at the source, and keeping up-to-date with useful industry trends. Subscribe to our newsletter for the latest construction happenings delivered directly to your inbox.

Flat Roof Repair FAQs

Should You Repair or Replace a Damaged Commercial Flat Roof?

Generally speaking, you should repair your roof rather than replace it if less than 25% of the surface is damaged. Once over a quarter of the roof requires repairing, replacing the entire commercial flat roof is typically more cost-effective.

Can a Patch Fix A Roof Leak Problem?

Yes, a patch can be used to fix a roof leak. You will remove the damaged section of your roofing membrane and replace it with a new patch to extend the life of your roof. Remember that roof patches are temporary and should not be considered a long-term solution. Once over 25% of your roof requires a repair, it is time for a complete roof replacement.

About the author Steve Coffey

I am passionate about the building materials industry helping companies all throughout the channel see success and exceed the expectations of their audiences

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