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Top 5 Causes of Commercial Roof Leaks

If you’re seeing water from your ceiling after a rain shower, there’s a good chance you have some issues with your commercial roof leaking. 

However, there are some other not-so-intrusive indicators that you can look for which will let you know it’s time for a roof inspection.


  • Discoloration: If you see discoloration on the ceiling or walls, it can be the result of a roof leak. It could also indicate water damage within the wall structure itself.
  • Odor: A consistent smell of mildew can be caused by mold growing as a result of a moist environment. Mold can also cause health issues if not treated. Many times it is a roofing issue that is the cause.

Once you identify there is a problem, here are the top five causes of commercial roof leaks

1. Poor or No Drainage

poor drainage on tpo roof
Water and vegetation buildup in the corner of the building's roof

If you have standing water on your roof, the issue can be narrowed down to a problem with your roof’s drainage system. 

The drainage of your roof is managed by two options: 

  • Gravity-fed system: This is where the roof’s slope draws water towards the main drainage system. Tapered crickets may be installed to help eliminate ponding and divert water towards the drains (See Pictures Below). 
  • Siphonic system: symphonic drainage system is where the drain is fitted with a cap that prevents air from entering the drain creating a lower atmospheric pressure which in response, sucks water in and allows the water to move without the need for slopes or gravity.
Roof drainage diagram
Tapered crickets between drains to eliminate ponding of water
Tapered crickets behind curbs and HVAC units

Without proper roof drainage, standing water can breakdown the barrier membrane overtime causing seepage into the roof’s substructure and eventually into the interior of the building. 

In addition, standing water will create additional weight on your roof which can lead to structural strain and further damage. Structural damage from ponding can cause tens of thousands of dollars in damage and can result in the roof caving in. 

2. Open Penetrations in the Commercial Roof

seal separating around roof penetration
Seal coming apart around roof penetrations

Roof vents, HVAC units, drains, gas lines, pipes and other systems are all very common on flat commercial roofs. All of these systems come through the roof’s membrane which are referred to as penetrations. 

Commercial roof leaks can take place at the intersection of these roof penetrations over time or after any damage caused by high winds, equipment repairs, etc.

Areas of penetrations should be properly sealed during the initial roof installation. One thing to look for is whether the penetration has the proper prefabricated boot or sheet metal flashing that conforms to the penetration. 

In the picture above, part of the boot is separating from the roof and will allow water to get under the boot and into the building causing your commercial roof to leak. This is an example of when a penetration needs to be resealed. 

3. Metal Flashing Issues

Where the roof conjoins with other roof sections (e.g. skylights, chimneys, walls, and roof edges), flashing is used to seal off the possibility of water entering these locations. 

One thing to note before looking for damaged flashing is to make sure it is installed and done so correctly.

One common issue is not installing flashing where a masonry wall and roof meet. In the image below, adding reglet flashing (left side) can help prevent the water that is absorbed in the wall from going below the new roofing membrane (note the blue shades in the wall and roof). In this example, adding reglet flashing can drastically increase the life of your roof and prevent future commercial roof leaks. 

Adding Reglet Flashing (left) hinders the absorbed water from the masonry walls from reaching the new roof like the image on the right shows..

Another main concern with flashing is it getting damaged. This can happen in a few ways. First, changing temperatures can expand and contract the metal. Over time this may create a gap where water can enter.

Secondly, wind and hail can cause damage to the metal, stopping the flashing from being able to do what it is supposed to do.

One thing to watch for is the design of the metal. Some contractors may use home-made edge metal that is not designed to handle high winds and hail. You want to make sure your roof contains quality, engineered metal designed to last for the next 30 years.

4. Membrane Damage

Membrane separating
Membrane separating
Membrane tearing

All commercial roofs will have some sort of waterproof membrane layer as the primary water barrier. 

Damage to this barrier can come in the form of heavy foot traffic, natural occurrences such as hail, wind, falling branches that can puncture or tear the membrane as well as damage due to mechanical issues such as HVAC and other non-roof related maintenance issues.

One common issue is when standing water collects on the roof. Those puddles also collect dirt and debris. As the water evaporates, the dirt and debris stay and create a dark spot on the roof. The sun raises the temperature in these areas, especially on white roof membranes like TPO and PVC. The rising temperature breaks down the membrane causing commercial roof leaks and failures (learn more here).

Most often, damages can be successfully repaired with patchwork done by a trained commercial roof repair technician. 

In other instances, a larger membrane section may need to be repaired. The insulation may need to be replaced. There also could be structural work needed depending on how long the repair goes unattended.

5. Commercial Roof's Age

old epdm roof with ballast
Old EPDM roof with ballast

A commercial roof’s lifespan can vary based on the type of material used and whether or not you invest in a regular maintenance schedule. 

In general, a commercial roofing system can last up to 15 to 20 years. You can check out this article to give you an idea of how long you can expect from your commercial roof: How Long Does a Commercial Roof Last

When the time comes for a roof replacement, commercial building codes allow up to 2 roof systems to be installed on your building. After one roof over, at least one roof will need to be removed prior to replacing it with a new roof system. This makes keeping your roof well maintained a top priority to insure you get the full life out of your roof. 

infographic explaining the top 5 causes of commercial roof leaks

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For the past decade, WNC Roofing has been solely focused on commercial roof repair, maintenance, and replacement for commercial and industrial businesses throughout the Upstate SC and Western North Carolina regions. Our team of service technicians are trained to identify and repair all commercial roof types in a Safe Smart and Professional manner.

Our “Fixed right the first time. Every time” mantra provides all of you with the confidence of turning a reactive roofing problem into a proactive plan to will eliminate future commercial roof leaks and minimize your roofing expenses.

Give Us a Call at 800-704-7765 or request a repair today.