Roof Blistering on Built-Up Roofing
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Why a Roof Membrane Might Split or Tear and What to do if it Does
If you see raised up spots that look like bubbles on your built-up roof your eyes aren't playing tricks on you. What you're seeing is something called roof blistering. Some roof blisters are so small you can't see them while others are large and can damage the protective surface of the roofing system.
Blistering on a built-up or mod bit roof is similar to asphalt shingle blisters in how they form. However, the chance of roof blisters can be even higher with built-up roofing because there are more layers of material, and it's usually installed on a low slope and flat roofs that are more prone to moisture problems.
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Types of Roof Blisters on Built-Up Roofs and Why They Happen
There's one type of asphalt shingle blistering, but with a built-up roof you can have two types of blisters that can form.
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Blistering in Between the Substrate and Membrane
Blistering between the membrane and bottom layer of the roofing system (substrate) happens when heat from the sun expands trapped moisture and/or air forming an air pocket. It's a result of the substrate having low permeability, which doesn't allow the heat to escape through the substrate. With no way to escape pressure builds and the blister grows. When this happens the membrane gets stretched. If it stretches too much the membrane won't go back to its original state once things cool down and the air pocket is there to stay. When the temperature rises the cycle will start again, causing the blister to grow.
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What Causes Blistering in Between the Substrate and Membrane
Roofs are going to take a beating from the sun no matter what material they're made of, but some materials are more UV resistant than others. The materials used for a roof membrane are resistant, however over time the UV rays degrade them. With each passing day the UV damage builds and makes the membrane more prone to tearing.
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Debris Collecting on the Roof Surface
When blistering is occurring that the lowest layer it's related to the substrate. As noted above, the permeability of the substrate plays a role. Other contributing factors include uneven substrates and insulation that isn't adhered fully to the substrate.
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Blistering in Between the Membrane Plies
When a roof blister forms between the layers of a built-up roof it's known as blistering between the membrane plies or inter-layer blistering. Trapped air, moisture or a combination of the two is the catalyst that will form blisters. When the heat rises the trapped air/moisture expands causing the blister to grow.
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What Causes Blistering in Between the Membrane Plies
When inter-layer blistering happens it's often a sign of installation issues, including:
- Skipping in the bitumen mopping
- Debris that was entrapped
- Felt that is curled
- Moisture in the roofing materials
- Not allowing enough drying time between layers
- Roofing layers that aren't bonded well

Other Contributing Factors for Roof Blisters
Research has shown that improper installation and application alone isn't the sole cause of most roof blisters. Even if everything is done correctly a blister can still form because no built-up roof system is 100% free of voids that can trap air and moisture.
Problems during installation may be the underlying cause but other contributing factors come into play that can increase the chance of a blister forming with or without installation issues.
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Extreme Temperatures and Fluctuations
Areas where the temperature fluctuates from hot to cold tend to have more roof blistering problems. The expansion and contraction from the temperature changes can cause voids where air and moisture can get trapped. Temperature fluctuations also increase the amount of condensation that's present and can get trapped to create a blister.
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Poor Ventilation
Poor attic ventilation or ventilation between the layers of the roofing can cause moisture build up. When trapped air and moisture have nowhere to go a blister can grow.
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Drainage Issues
If water isn't draining off the roof it will pool and possibly seep below the surface. Drainage issues can stem from the entire slope of the roof being off, lack of a drainage system that directs water off the roof or debris build up in gutters.
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Improper Maintenance
Regular maintenance helps correct and prevent many roofing issues, including the formation of blisters. Having the roof inspected regularly allows you to detect leaks that can allow moisture to get trapped below the surface. When you fail to take maintenance measures small leaks can turn into bigger issues.
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Hail Damage
In areas like Texas where hail storms occur there's a higher likelihood of roof blistering. Hail damage in the form of tiny tears can allow moisture accumulation in the lower layers.
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Heavy Foot Traffic
Heavy foot traffic alone may not cause air or moisture to get trapped, but it can make problems worse and increase the presence of blisters when there are issues with the substrate.

Repairing Blister Damage on a Built-Up Roof
If you notice blisters it's time to contact a roofer. A few small blisters on built-up or mod-bit roofing isn't typically a serious concern because the roof membrane should be elastic enough to handle the limited movement. It might affect the roof's appearance but not its performance. Even so, the problem should be inspected.
However, larger or more numerous blisters can cause roof damage and premature deterioration that harm the roof's integrity and reduce the roof's lifespan. Popped blisters can damage roof coatings and protective granules, allowing water to penetrate through. Roof blistering is also an indication that there could be other issues with the roof system that needs to be addressed, which is one more reason to have them repaired by a professional.
Repairing Unpopped Blisters on the Roof's Surface
You may not be able to prevent blisters completely, but they can be repaired. If there's unpopped shingle blistering that has impacted the roof's surface you'll need to reapply the protective coating in the affected area. It's also important to monitor the blisters moving forward to ensure they aren't getting larger.
Repairing Popped Blisters
If blisters pop you'll have to make more extensive repairs so that water damage doesn't occur. There are two ways to fix popped blisters:
- You can remove the entire blister and area around it. Next, you'll need to use layers of felt and asphalt to patch the removed portion.
- The other option is to cut the blister open, pull the roofing back, fill the void with asphalt, press the roofing material back down and patch over it.
Fortunately, roof replacement isn't typically needed if a blister pops. But there could be an underlying cause that needs further correction. For example, steps may need to be taken to ensure proper ventilation so fewer blisters form in the future.
LOA is a top-rated roofing company in Central Texas that knows how to handle roof blisters. If you aren't sure whether blistering is a serious concern schedule a free roof inspection. It's the easiest way to know what steps need to be taken to correct blistering. Contact our team today to schedule a free inspection.
