If you’ve noticed bubbles, blisters, or raised areas forming on a flat roof coating system, you’ve probably heard the most common explanation in the roofing industry:

“Moisture got trapped underneath the coating.”

While trapped moisture can occasionally contribute to roofing problems, it is far from the only cause of bubbling roof coatings, and in many cases, it’s not the primary cause at all.

In Arizona especially, blistering flat roofs are often related to a combination of:

• Attic pressure
• Poor ventilation
• HVAC air leakage
• Expansion and contraction
• Underlying roof movement
• Multiple coating layers reducing roof breathability over time

Understanding what actually causes roof coating bubbles matters because the wrong diagnosis can lead to:

• Unnecessary repairs
• Repeated coating failures
• Wasted money
• Unrealistic warranty expectations

Here’s a deeper look at why flat roof coatings blister, what’s cosmetic versus structural, and when bubbling may indicate a larger roofing issue.

What Is Roof Coating Blistering?

Roof coating blistering occurs when sections of the coating membrane lift and form visible bubbles or raised pockets across the surface. These can range from small, isolated bumps to large, widespread areas that affect entire sections of a roof.

On flat roofs, blistering often shows up on older systems that have been recoated multiple times or roofs that already had underlying issues before the latest coating was applied. In some cases, the bubbles remain unchanged for years. In others, they eventually dry out, crack open, or begin peeling, leading to more serious problems.

The key distinction is that not all blistering behaves the same way. Some is purely cosmetic, while other cases signal deeper issues within the roofing system. If your roof bubbled for the previous company’s application, simply recoating your roof alone will not fix the issue that caused the bubbling to occur in the first place.

On flat roofs, blistering is especially common on:

• older recoated roofs
• roofs with poor ventilation
• roofs with multiple existing coating layers
• roofs with previous repair systems underneath

The Common Myth: “There Was Water Under the Coating”

One of the most widely repeated explanations for roof blisters is that the contractor coated over moisture that became trapped underneath. While this idea sounds logical, it doesn’t fully reflect how modern coating systems actually work.

Most acrylic roof coatings are water-based by design. They contain water during application and are formulated to cure properly as that moisture evaporates. In many spray applications, additional water may even be introduced during installation. Because of this, the presence of moisture alone doesn’t explain why bubbles would appear long after the coating has cured.

In reality, blistering is more often tied to pressure and movement within the roof system itself. When air or heat builds up beneath the surface and cannot escape properly, it creates upward force against the coating—resulting in visible bubbles.

One of the Biggest Causes: Trapped Air Pressure from the Building

Many flat roofs constantly “breathe.”

This is particularly true in Arizona buildings where temperatures swing dramatically between day and night. Attics can become extremely hot, especially in our hot summers. Your HVAC system often runs continually to keep temperatures down, and attic spaces can become extremely hot. In many of our homes and commercial building in the Tucson area, the HVAC ductwork runs through these attic spaces beneath the roof structure. Over time dctwork develops leaks, air escapes into the ttic, and pressure build up underneath the roofing system. That trapped air naturally tries to escape upward.

If the roof has limited ventilation, multiple coating layers, aging seams, or reduced permeability, then the escaping air may push upward against the coating, creating visible bubbles or blisters. This is one reason bubbling can become worse after additional recoats are added over the years.

Why Multiple Roof Coatings Can Increase Bubbling

Every additional coating layer changes how a roof system breathes and expands. While coatings are designed to protect and extend the life of a roof, layering them over time changes how air moves through the system. As your coatings get layered over time, they thicken and the flexibility changes. This can trap pressure more easily and reduce the roof’s ability to release internal air movement. This increases the likelihood that bubbles will form, especially in older roofs that already have underlying ventilation issues.

This is one reason why some roofs seem to develop more blistering after each additional coat. The coating itself isn’t necessarily failing. Instead, the system beneath the roof coating is becoming less able to accommodate pressure changes.

Poor Ventilation Can Make Blistering Much Worse

Ventilation plays a major role in flat roof performance. When airflow is restricted, heat and pressure build up within the attic space, intensifying the stress placed on the roofing system.

In Arizona’s extreme temperatures, roofing systems may experience constant thermal expansion during the heat of the day followed by contraction during the cold temperatures overnight. This constant expansion and contraction cycle causes materials to shift repeatedly. Over time, that movement can weaken seams, stress coatings, and create conditions where blistering becomes more likely.

If ventilation is poor, the roof has fewer ways to release heat and pressure, and the result often shows up in the form of bubbles across the coating. Bubbles may become larger or appear more frequently.

This is especially common on older roofs with aging insulation and attic airflow issues.

Underlying Roof Defects Can Also Contribute

Sometimes blistering is connected to conditions beneath the visible coating layer.

Older roofs often contain hidden defects such as minor seam separations, small cracks, or previous repair areas that are no longer stable. Even if these issues were covered during a prior coating, they can continue to move over time. That movement eventually transfers upward, creating weak points where bubbles form.

This is why some roofs continue to develop blistering even after repairs are completed. The visible problem may have been addressed, but the underlying cause remains active within the system. Contractors may scrape, repair, and recoat visible blisters, only to see them reappear later because the underlying roof system is still moving.

Are Roof Coating Bubbles Dangerous?

Not necessarily. In many situations, blistering is largely cosmetic and does not immediately affect the roof’s ability to keep water out. Some roofs with bubbling can continue to perform for years without leaks.

The problem happens when blisters begin to deteriorate. As they age, they may dry out, become brittle, and eventually crack open. If someone steps on a dried bubble it can also break open. Once that happens, the protective barrier is compromised, and water can begin to penetrate the roof coating. You can imagine how much of a problem that can become if the monsoons hit!

We recommend that even cosmetic blistering should be evaluated periodically to ensure it doesn’t develop into a more serious issue.

Why Some Contractors Avoid Bubbling Roof Coatings

Bubbling and blistering roofs can become difficult long-term maintenance projects, especially for older roofs where multiple layers of coating exist underneath, ventilation issues may still be unresolved, or the roof structure itself contributes to movement and shifting. Even after repairs are completed, bubbles may return or new ones may form in other areas over time. Because of these ongoing factors, customer expectations can become difficult to manage if unrealistic promises are made about permanent results.

For that reason, we want to be completely honest and transparent upfront. While we can scrape as many bubbles as we can, and sometimes there thousands, they are likely going to return. Therefore, bubbling conditions are not covered under our warranty due to the unpredictable nature of these problems and the many underlying causes that may continue to affect the roof after repairs are made. It is important that customers understand these limitations and the potential long-term implications before moving forward. Our goal is to provide clear communication and realistic expectations rather than making guarantees that cannot responsibly be promised.

Signs a Blistering Roof Needs Professional Evaluation

Blistering should always be monitored, but certain changes indicate it’s time for a professional inspection. Call our team for a professional roof inspection if you notice:

• Blisters growing rapidly
• Soft spots beneath bubbles
• Cracking or splitting blisters
• Peeling coating
• Exposed substrate
• Recurring leaks
• Ponding water near blisters
• Widespread roof movement

A thorough evaluation can help determine whether the problem is cosmetic, repairable, or part of a larger system failure.

The Bottom Line on Blistering Flat Roofs

Roof coating bubbles are far more complex than the simple “trapped water” explanation many homeowners hear.

Some blistering is purely cosmetic, while other cases signal deeper concerns that require more advanced solutions. The key is identifying the true cause before attempting repairs. Without that understanding, even well-intentioned fixes can fail, leading to repeated issues and unnecessary costs.

If you’re dealing with blistering or bubbling on your roof and aren’t sure what it means, it’s worth having it looked at before it turns into a bigger issue.

We’re happy to take a look, explain what’s going on, and help you understand your options so you can make the right decision for your home or building. Give our owner Manny a call today to schedule a roof inspection and quote at (520) 260-4707.