Technical Bulletin: Roof Blisters

Blister in Urethane Coating

Blisters can be an alarming site for any roofing contractor. Their arrival is usually a surprise and removing them can be a very difficult task. In this technical bulletin, we examine some of the common causes of roof blisters and offer some helpful information on how to avoid them.

What Causes Roof Blisters?

Blisters occur as a result of adhesion loss of the coatings product from the underlying roof substrate. The most common cause of blister formations is moisture penetration that occurs from below or above the roof surface. As external temperatures elevate, the trapped moisture expands allowing blisters to form between the coating layer(s) and the roof substrate. Because blisters can develop below or between coating layers, it is important to identify precisely which layers are affected before taking corrective measures.

Some blisters (osmotic blisters) occur when moisture permeates through the coating from above due to continuous exposure to water (e.g. ponding water). Moisture builds up between the coating layers, or the coating and the original roof membrane, and causes adhesion loss, ultimately resulting in the formation of blisters. Osmotic blisters are less likely to form on roof surfaces with adequate drainage.

Surface blisters can also be caused by moisture present within the coating product at the time of application. This happens when the coating dries so quickly that moisture is unable to completely evaporate before the coating cures.

What If Blistering Occurs?

Now that you understand some of the common causes of blistering, the next step is to determine the source of the moisture. Some likely scenarios are:

  • Osmotic blisters–blisters isolated to ponding water areas are due to moisture permeation from above.
  • Premature application of coatings over asphalt emulsion that is not fully cured
  • Roof coatings applied too thick over hot or moist surfaces
  • Premature top coat application before base coat is cured
  • Poor adhesion due to improper surface preparation or chemical interference
  • Moisture trapped within the existing roof membrane
  • Improper ventilation–moisture collects from the building interior
  • Premature application of coatings after heavy rainfall or pressure washing

What to Watch Out For

For roofing contractors, taking time to cut out and repair small blisters on a roof is not a welcome project. Fortunately, there are some preventative measures to help you avoid this situation. Here are a few things to watch out for:

  • Water excreting from the membrane as you walk the roof
  • Roofs with coating systems that have blistered before
  • Roofs that are poorly ventilated
  • Coating older foam roofs that have not been well maintained
  • Coating in extremely high temperatures can result in flash drying
  • Avoid coating in when frost or condensation is present
  • Coating over asphalt emulsion on a hot day can result in flash drying
  • Avoid coating if rain is imminent–or too soon after rain has occurred
  • Allow prior coating applications adequate time to fully cure
  • Store coatings properly to avoid moisture contamination


In summary, roof blisters are pretty common in the coatings industry and can occur for a number of different reasons. This article is intended to educate roofing contractors on some of the more common causes of blistering as it relates to fluid-applied roof restoration. Understanding what causes blisters can go long way towards preventing any unnecessary, and unwanted, repair jobs.


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