Blistering is a common problem in built-up roofing (BUR) systems, particularly on commercial buildings exposed to Florida’s heat, humidity, and frequent temperature changes. A roof blister appears as a raised bubble or pocket beneath the roofing surface. While small blisters may not immediately cause a leak, they can weaken the roofing system and eventually lead to membrane damage, water intrusion, and costly repairs.
Understanding what causes blistering in built-up roofing can help commercial property owners identify problems early and maintain a longer-lasting roof.
What Is Blistering in Built-Up Roofing?
Built-up roofing typically consists of multiple layers of roofing felts reinforced with asphalt, often finished with a protective surfacing layer such as aggregate or a coating. Blistering occurs when air, moisture, or other materials become trapped between layers of the roofing system.
When the trapped material expands because of heat, it can create pressure between the layers. This pressure causes the roofing membrane to separate locally and form a raised area, or blister.
Common Causes of BUR Blistering
1. Trapped Moisture
One of the most common causes of blistering is moisture trapped within the roofing assembly. If the roof deck, insulation, or roofing materials contain moisture during installation, the moisture can become enclosed between impermeable layers.
Florida’s hot temperatures can cause trapped water to turn into vapor. As the vapor expands, pressure builds beneath the membrane and can create blisters.
2. Poor Installation
Improper installation can contribute significantly to blister formation. Inadequate asphalt application, insufficient bonding between roofing plies, poor surface preparation, or improper installation conditions can leave voids between layers.
These voids can become areas where air or moisture accumulates, increasing the possibility of blistering.
3. Excessive Heat
Commercial roofs experience significant temperature fluctuations. Dark roofing surfaces can become extremely hot under direct sunlight. As temperatures increase, trapped air and moisture expand.
Repeated heating and cooling can place stress on the roofing membrane and gradually enlarge existing blisters.
4. Inadequate Ventilation
When moisture vapor cannot escape from the roofing assembly, pressure can build beneath the membrane. Poorly designed or inadequate ventilation may increase the risk of vapor-related blistering, especially in assemblies containing moisture-sensitive materials.
5. Aging Roofing Materials
As a built-up roof ages, its materials can lose flexibility and adhesion. Repeated exposure to ultraviolet radiation, rain, wind, temperature changes, and foot traffic can weaken the roofing system.
An older BUR membrane may therefore be more susceptible to blistering and other forms of deterioration.
6. Improper Repairs or Multiple Roof Layers
Previous repairs and additional roofing layers can sometimes trap moisture between roofing components. If an existing roof is covered without properly evaluating its condition, moisture or damaged materials may remain beneath the new system.
Are Roof Blisters Always a Serious Problem?
Not necessarily. Small, stable blisters that remain intact may not immediately require removal. However, damaged, expanding, or numerous blisters should be evaluated by a qualified commercial roofing professional.
If a blister cracks or becomes punctured, water can enter the roofing assembly. This can lead to wet insulation, deterioration of the roofing felts, interior leaks, and more extensive structural or roofing damage.
How Can Blistering Be Prevented?
Proper installation and regular maintenance are the best defenses against BUR blistering. Roofing contractors should ensure that substrates are properly prepared, materials are installed under appropriate conditions, and moisture is not trapped within the assembly.
Building owners should also schedule regular commercial roof inspections. Early detection can help identify blisters, membrane deterioration, drainage problems, and other issues before they become expensive repairs.
Final Answer
Blistering in built-up roofing is primarily caused by trapped moisture or air between roofing layers, poor adhesion, excessive heat, inadequate ventilation, aging materials, and improper installation or repairs. Florida’s hot climate can accelerate the expansion of trapped moisture and air, making regular inspections especially important.
If your commercial BUR roof has visible blisters, do not automatically puncture or remove them. A professional roofing inspection can determine whether the blister is stable, requires localized repair, or indicates a larger moisture or adhesion problem within the roofing system.
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