Flat Roof Blistering and Bubbling: Causes and Repair

What Causes Flat Roof Blisters

Trapped moisture during installation is the most common cause of blistering. If the substrate, insulation, or membrane surface is damp when the membrane is adhered, that moisture becomes trapped in the assembly. When the sun heats the roof, the trapped moisture vaporizes and expands, pushing the membrane upward into a blister. Even small amounts of moisture, such as morning dew on the insulation surface or residual dampness from a rain event the previous day, can produce blisters once the membrane seals the moisture in place. This type of blistering often appears within the first year after installation and indicates a workmanship problem during the original installation rather than a material defect.

Inadequate adhesion between the membrane and the substrate allows air pockets to form, which expand in heat and contract in cold. Over many thermal cycles, these air pockets can grow into visible blisters as the repeated expansion gradually stretches the surrounding bond and enlarges the unbonded area. Poor adhesion results from insufficient adhesive application, contaminated bonding surfaces such as dust, oil, or moisture on the substrate, adhesive applied outside the manufacturer's recommended temperature range, or walking on the membrane before the adhesive has fully cured. Each of these errors creates a zone of weak or absent adhesion where air can accumulate and produce a blister over time.

Volatile gas emission from certain insulation types or substrate materials can create pressure under the membrane over time. Some polyiso insulation boards emit small amounts of gas as they age, a process called off-gassing, and this gas can accumulate under a fully adhered membrane. The gas has no escape path through a fully bonded membrane, so it collects in areas where the adhesion is slightly weaker and gradually inflates a blister. This type of blistering is more common with fully adhered systems than with mechanically fastened systems, which allow gas to dissipate through the fastening points and the slight gaps between the membrane and the fastener plates.

Water infiltration through existing defects can cause secondary blistering. When water enters through a seam failure, flashing gap, or membrane puncture and becomes trapped under the membrane, it creates moisture-driven blisters in areas away from the original entry point. These secondary blisters indicate an existing leak that is spreading water laterally through the assembly, and the blistering is a symptom of a larger problem. Repairing the blisters without finding and fixing the original water entry point will result in new blisters forming as water continues to enter the assembly and migrate to new locations.

When Blisters Need Repair

Repair is needed when: the blister is larger than 6 inches in diameter, the blister has cracked or torn at the surface exposing the substrate or insulation below, the blister is growing larger over time (compare photos taken during successive inspections to track size changes), the blister is located in a high-traffic area where foot traffic from maintenance workers or HVAC technicians could rupture it, or water is collecting around the blister base creating a secondary ponding problem that accelerates membrane deterioration in the surrounding area.

Monitoring is acceptable when: the blister is small (under 6 inches), stable in size between inspections, located in a non-traffic area away from walkways and equipment access paths, the surface membrane over the blister is intact without cracks, tears, or thinning, and no water is accumulating around it. Many small blisters remain stable for the entire life of the roof without ever causing a leak. Photographing blisters during routine inspections and comparing the images from year to year is the most reliable way to determine whether a blister is growing and needs attention or is stable and can be left alone.

The decision to repair or monitor should also account for the roof's remaining service life. A small, stable blister on a roof with 15 years of expected life remaining deserves monitoring, while the same blister on a roof approaching replacement age can be left alone entirely since the roof will be replaced before the blister becomes a problem. Repairing a blister on a roof that will be replaced within 2 to 3 years is rarely a worthwhile investment unless the blister is actively leaking.

Repair Methods by Material

On modified bitumen and BUR: Cut an X through the center of the blister with a sharp utility knife, making sure the cuts extend to the full edges of the blistered area. Peel back the four flaps of membrane, remove any moisture, debris, or deteriorated adhesive from inside the blister cavity, and allow the area to dry completely. If the cavity is wet, drying may take several hours in warm weather or may require returning the following day. Apply bitumen adhesive to the cavity and press the flaps back into their original position, rolling them firmly to eliminate any air pockets beneath. Apply a patch of matching membrane material over the repair area extending at least 6 inches beyond the cut lines in all directions. Torch-fuse or cold-adhere the patch following the membrane manufacturer's instructions, making sure the edges of the patch are fully sealed to the existing membrane surface.

On EPDM: Cut a single straight line through the blister and peel back both sides to expose the cavity interior. Clean the interior of the blister and the exposed substrate with EPDM-compatible membrane cleaner to remove any contaminants that would prevent adhesion. Allow the cleaned surfaces to dry fully. Apply EPDM primer to both the substrate and the membrane undersides, wait for the primer to become tacky (typically 5 to 15 minutes depending on temperature), then apply EPDM bonding adhesive to both surfaces. Press the membrane back into place, working from the center outward to expel any trapped air. Apply an EPDM patch over the repair with at least 6 inches of overlap in all directions, adhered with EPDM bonding adhesive, and sealed at all edges with lap sealant. Roll the entire patch firmly with a seam roller to ensure full contact between the patch and the existing membrane.

On TPO and PVC: Blistering is less common on thermoplastic membranes because heat-welded seams and mechanically fastened attachment systems allow more gas and moisture dissipation than fully adhered systems. When blisters do occur on TPO or PVC, the repair involves cutting open the blister along its longest axis, cleaning and drying the interior thoroughly, and heat-welding a patch of the same membrane material over the damaged area. The patch must be made from the same polymer type as the existing membrane (TPO patched with TPO, PVC patched with PVC) because TPO and PVC are not compatible for heat welding with each other. The weld must be made with proper temperature control, typically 900 to 1,100 degrees Fahrenheit at the welding tip, to ensure a permanent molecular bond between the patch and the existing membrane without overheating and damaging the material.

Prevention

Preventing blisters starts with proper installation practices. Ensure the substrate is completely dry before membrane installation, as even minor surface moisture can cause blistering once the membrane is sealed in place. Use a moisture meter to verify substrate moisture content before proceeding with adhesive application. Avoid installing membrane on damp or rainy days, and cover prepared surfaces with tarps if weather changes during the installation process. Follow the adhesive manufacturer's recommended application rates and open times precisely, as both insufficient adhesive and excessive adhesive can contribute to bonding problems that lead to blistering.

Mechanically fastened membrane systems are less susceptible to blistering than fully adhered systems because the fastening points allow trapped gas and moisture to dissipate through the small gaps around the fastener plates. If your building is in a climate with high humidity during the construction season, or if the substrate is concrete or other material prone to moisture retention, discuss mechanically fastened installation with your contractor as a blistering prevention strategy. Mechanically fastened systems also cost less to install because they require no adhesive, though they create more penetrations in the membrane that must be sealed by the overlap of the next membrane sheet.

Proper ventilation of the roof assembly reduces moisture-driven blistering by preventing condensation from accumulating on the underside of the membrane. In unvented assemblies, adequate vapor retarders on the warm side of the insulation prevent interior moisture from migrating through the insulation, reaching the cold membrane surface, and condensing into liquid water that creates blisters from below. The vapor retarder position and permeability rating should match the climate zone requirements specified in building code to provide effective moisture control without trapping moisture within the assembly.

For buildings where blistering has been a recurring problem through multiple roof installations, investigating the moisture source is essential before installing a new roof. The moisture may originate from below, through the building envelope, rather than from installation conditions. High interior humidity from cooking operations, swimming pools, laundry facilities, or manufacturing processes can drive moisture vapor upward through the ceiling and into the roof assembly continuously, creating conditions that produce blisters regardless of how carefully the membrane is installed. Addressing the interior moisture source with improved ventilation or a vapor retarder at the ceiling level is necessary to prevent blistering on the replacement roof.