Common Causes and Solutions for Blisters and Bubbles on Built-Up Roofs

Blisters and bubbles on a built-up roof (BUR) are common concerns for property owners and roofing professionals. These imperfections can signify underlying issues that may affect the roof’s longevity and performance. Understanding the causes, implications, and corrective measures for blisters on BUR systems is essential for maintaining a durable, weather-resistant roof structure.

What Are Built-Up Roofs?

Built-up roofs are popular flat or low-slope roofing systems composed of multiple layers of bitumen alternated with reinforcing fabrics. These layers create a thick, durable membrane designed to protect buildings from weather, UV rays, and mechanical damage. BUR systems are often finished with gravel or other surface treatments to extend durability.

These roofs are widely used in commercial buildings due to their strength and longevity, but they require proper installation and maintenance to avoid common problems like blisters and bubbles.

What Do Blisters and Bubbles on a Built-Up Roof Look Like?

Blisters on BUR surfaces appear as raised, rounded protrusions that look like bubbles beneath the membrane. They vary in size from small pinpoints to larger areas several inches across. These formations tend to be soft or spongy when pressed and may eventually rupture if left untreated.

Blisters and bubbles create an uneven roof surface that compromises its waterproofing ability, making these signs especially important to address promptly.

Why Do Blisters and Bubbles Form on Built-Up Roofs?

Trapped Moisture During Installation

One of the most common causes is moisture trapped in the roofing layers during installation. If the substrates or materials are wet or contain water vapor when the roof is applied, heat causes this moisture to expand, forming blisters.

Air Pockets or Delamination

During the layering process or adhesive cure time, air can become trapped between the bitumen and the ply sheets. These air pockets expand under temperature and form bubbles or blisters, especially in hot weather.

Poor Membrane Adhesion

Insufficient adhesion between layers or between the membrane and substrate allows membrane separation, creating empty spaces where moisture or air accumulates.

Substrate Moisture or Vapor Drive

Moisture rising from the building structure itself, called vapor drive, can migrate upwards and can create blisters once it encounters the hot membrane surface.

What Do Blisters on Built-Up Roofs Usually Indicate?

Blisters and bubbles typically indicate poor installation practices, trapped moisture, or material incompatibility. Their presence often points to conditions that compromise the roof’s watertight integrity.

They can signal:

• Excess moisture in roofing layers or substrates
• Inadequate drying of roof components before installation
• Subpar materials or adhesives
• Failure to properly cure or protect the bitumen layers during application

Ignoring these signs may lead to membrane rupture and therefore water infiltration, leading to costly roof damage.

Potential Risks Associated with Blisters and Bubbles on BUR

• Water Intrusion: Ruptured blisters allow rainwater to penetrate the roof system, damaging insulation and building interiors.
• Membrane Deterioration: Expansion and contraction weaken the membrane, risking further blister formation and cracking.
• Structural Damage: Persistent leaks can cause deterioration of roof decking, wood framing, and other structural components.
• Decreased Roof Lifespan: These defects shorten the effective life of the roofing system, increasing maintenance costs.

Addressing and Repairing Blisters on Built-Up Roofs

Inspection and Assessment

A thorough roof inspection by a certified roofing professional is crucial to evaluate blister size, distribution, and membrane condition. This assessment guides appropriate repair or replacement strategies.

Repair Methods

• Patching: Small, localized blisters can be cut open, dried, and patched with compatible roofing materials.
• Partial Membrane Replacement: Larger areas of blistering may require removal and replacement of damaged membrane sections.
• Roof Resurfacing or Coating: Applying reflective or protective coatings can restore roof integrity and reduce blister formation if underlying issues are managed.

Preventive Measures for Future Installations

• Use thoroughly dried and conditioned materials.
• Allow substrates to dry completely before membrane application.
• Ensure proper application temperature and curing time of bitumen layers.
• Apply quality adhesives to prevent delamination and air pockets.
• Implement vapor barriers where necessary to manage vapor drive.

How to Maintain a Built-Up Roof to Avoid Blisters and Bubbles

• Regular Inspections: Biannual professional inspections help detect early blistering before damage worsens.
• Prompt Repairs: Repairing small blisters immediately prevents them from growing larger.
• Roof Cleaning: Remove debris and contaminants that trap moisture or cause damage.
• Manage Drainage: Ensure roof drainage systems are clear to prevent water ponding, which can exacerbate blisters.

Comparing Built-Up Roofing with Other Roofing Systems Regarding Blisters

When to Consider Replacing a Built-Up Roof Due to Blistering

If blisters are widespread, recurrent, and accompanied by membrane cracking or significant water damage, roof replacement might be more cost-effective than repeated repairs. Replacement ensures comprehensive correction of underlying issues.

Consultation with a licensed roofing expert is recommended to evaluate roof condition and recommend the best course of action.