Blisters and Bubbles on a Built-Up Roof Usually Indicate Moisture, Poor Installation, or Aging

The presence of blisters or bubbles on a built-up roof (BUR) is a common concern for property owners. These raised areas can signal moisture intrusion, air entrapment, or deterioration of the roofing material. Understanding the causes, signs, and appropriate actions is essential to prevent leaks and extend roof life. This article explains what typically causes blisters and bubbles on BUR systems, how to assess their severity, and practical repair and prevention strategies for a safe, durable roof.

Causes Of Blisters On A Built-Up Roof

Blisters on BUR membranes arise when air, vapor, or moisture becomes trapped beneath the surface, or when the asphalt and reinforcement layers separate. Common causes include:

• Moisture Entrapment: Water or humidity trapped under the membrane during installation or from a compromised vapor barrier can form pockets that push the surface upward.
• Improper Adhesion Or Seams: Inadequate bonding of plies or poor sealant around penetrations creates weak spots where air accumulates.
• Hot Weather Installations: Excess heat can cause asphalt to soften and expand, trapping air or moisture under the membrane as it cools.
• Aging And Oxidation: Deterioration of bitumen with UV exposure reduces elasticity, encouraging blister formation under stress.
• Inadequate Ventilation: Roof decks without proper ventilation trap moisture, increasing blister risk in multi-ply systems.
• Improper Maintenance: Grading, debris, or ponding water can contribute to localized blistering over time.

Signs And Severity Of Blisters

Assessing blister severity helps determine repair needs and urgency. Indicators include:

• Size And Depth: Small right-now blisters may be monitored, while large, rapidly growing blisters suggest active moisture ingress.
• Color Changes: Dark or wet-looking areas can indicate trapped moisture beneath the membrane.
• Texture: Soft, spongy spots that indent under pressure point to underlying moisture or delamination.
• Leak Symptoms: Visible drips or staining following rain confirm compromised areas requiring prompt attention.

When To Repair Or Replace

Decision factors depend on blister characteristics and roof condition. Consider these guidelines:

• Minor Blisters: If blisters are small, localized, dry, and non-progressive, a targeted repair with patching and resealing may suffice.
• Extensive Blistering: Widespread, deep, or rapidly expanding blisters with evidence of moisture or delamination typically require more extensive repair or full BUR section replacement.
• Structural Concerns: Signs of rot, sagging decking, or long-standing leaks indicate a systemic issue that warrants a professional evaluation and possible roof replacement.

Repair And Replacement Options

Professional assessment is essential, but common approaches include:

• Patching And Resealing: Clean, dry patches with compatible bitumen-based patches or solvent-based sealants can seal small blisters.
• Hot-Applied Patching: For larger blisters, hot-applied asphalt or ply repair with reinforcement fabrics may restore continuity.
• Layer Replacement: Damaged plies or reinforcement layers are cut out and replaced, often combined with resealing.
• Full BUR Section Repair: In severe cases, replacing the affected BUR section ensures long-term integrity and reduces continued moisture intrusion.
• Deck Inspection And Ventilation: Address underlying issues by inspecting deck condition and improving ventilation to reduce future moisture buildup.

Prevention Strategies

Preventing blister formation starts with proper design, installation, and maintenance. Effective measures include:

• Quality Materials: Use compatible asphalt, felts, and cap sheets designed for BUR systems to ensure reliable bonding and flexibility.
• Proper Ventilation: Ensure adequate roof deck ventilation to minimize trapped moisture during and after installation.
• Weather-Appropriate Installation: Schedule installations during favorable temperatures to prevent premature aging and air entrapment.
• Careful Flashing And Detailing: Seal penetrations, edges, and terminations thoroughly to avoid moisture ingress.
• Regular Inspections: Conduct routine commercial or residential roof inspections, especially after heavy rainfall or temperature swings, to catch blisters early.
• Drainage Maintenance: Keep drains, scuppers, and gutters clear to prevent ponding, which increases blister risk.

Identifying A Qualified Repair Plan

Because BUR systems vary in composition and age, a qualified roofing contractor should assess blistering. Key steps in their process include:

• Non-Destructive Evaluation: Visual inspection, moisture meters, and infrared cameras help locate hidden moisture pockets without full teardown.
• Core Sampling: In some cases, removing small samples offers insight into ply integrity and moisture levels.
• Finite Repair Plan: A documented plan outlines materials, patch areas, and anticipated lifespan, reducing the likelihood of repeated failures.
• Warranty Considerations: Review system warranties and manufacturer guidelines to ensure repairs align with coverage terms.

Common Questions About BUR Blisters

Homeowners and facility managers frequently ask about BUR blisters. Answers to common questions include:

• Are blisters always leaks? Not always, but many blisters increase the risk of leaks, especially during rain or freeze-thaw cycles.
• Can I fix blisters myself? Minor patches may be DIY for some systems, but most BUR repairs require professional equipment and knowledge to avoid further damage.
• Will replacement be necessary? Depending on blister severity, age, and deck condition, replacement of affected areas or full BUR replacement may be indicated.