Commercial Flat Roof Repair: Blisters On BUR Systems And What To Do With Them

Almost all conventional flat roof systems will show signs of blistering some time or another – even built-up roofing (BUR) that has a positive slope of at least 1/12 of an inch. As a commercial property owner, it is your responsibility to inspect the roof regularly for blistering and determining when professional roofing contractors need to be called in to repair the roof. This article will give you some insight as to how blisters are formed, and when they need to be repaired.

How Blisters are Formed

Regardless of the type of commercial flat roof installed, blisters are inevitable after some time. For BUR systems, blisters will begin to form in voids between the plies, the substrate and the membrane. The growth of the blisters is due to the evaporation of moisture and the expansion of the water vapors. When the substrate is not properly installed or when moisture is able to creep into the roofing layers, water vapors will begin to accumulate at certain points, and this is how the blisters are formed.

When Blisters Need to Be Repaired

Although blisters may look hideous, they are not always harmful and do not always need to be repaired. The general consensus is that opened blisters definitely need to be repaired; however, it is up to the contractor's discretion as to whether closed blisters need to be repaired immediately. At times, simply leaving the blisters be and monitoring their size may be the better judgment call. With that said, the blisters will require professional attention and be repaired if you see the following characteristics:

  • fatigue cracking in the blisters;
  • breaks in the blisters;
  • excessive gravel loss or loss of surface; and,
  • blisters in high traffic areas.

You should also take into account the size and the frequency of the blisters when making a judgment call as well. If a lot of blisters are present, they have the ability to really shorten the service life of the flat roofing system.

How Blisters are Repaired

Once you have determined that the blisters may need to be repaired, contact a commercial flat roofing contractor for a more thorough inspection. At times, the contractor may leave the blisters be until they worsen because repairing the blisters immediately may not be beneficial or helpful at all since the process for repairing the blisters is quite complex and tedious.

To repair the blisters, a professional roofing contractor will first prepare the surface by cleaning up any dirt and debris that has accumulated over time. The blistered area will then need to be cut and removed from the roof, and a thorough inspection will be conducted to determine the condition of the membrane. The insulation and the deck will also be inspected at this time, and any wet or damaged insulation will need to be replaced to prevent blisters from forming at the same location.

Once the blistered area has been removed, the surface of the membrane affected will be primed with an asphalt primer and allowed to dry. This will enhance the adhesive properties of the primer. Next, the same number of plies that were removed will be reinstalled using either bitumen or asphalt to the roof. Once this is finished, the surface will be recoated and be left alone for a period of time. This will allow all of the materials to properly set.


Not all blisters need to be repaired immediately. It is your duty to monitor the extent of the blistering to determine when a professional needs to be called in. It is crucial to find a licensed roofing contractor that has experience in working with flat roofs because the quality of the craftsmanship will play a large role as to when blisters will begin to form. For more information, contact a professional.