Blistering, Ridging, and Splitting in Commercial Membranes

Blistering, ridging, and splitting are surface distortions in commercial flat roof membranes caused by trapped moisture, entrapped air, or thermal stress acting on the membrane and the layers beneath it. These conditions range from cosmetic issues that can be safely monitored for years to active failures that require immediate repair. The key to managing them effectively is understanding which conditions are progressing toward failure and which are stable.

Building owners who see blisters or ridges on their roof often assume the worst — that the entire system is failing. In many cases, that assumption is premature. A small, stable blister on an otherwise sound 10-year-old modified bitumen roof may never require repair. A rapidly growing blister on a 3-year-old membrane, however, may indicate a serious installation defect that needs prompt attention. The difference lies in size, growth rate, and whether the membrane surface over the blister is intact.

Blistering

What Causes Blisters

Blisters form when moisture or air trapped beneath or within the membrane expands under heat, pushing the membrane upward into a bubble. On a 140-degree roof surface in direct summer sun, trapped moisture converts to vapor and expands with enough force to separate membrane layers or lift the membrane from its substrate. The blister inflates during the heat of the day and partially deflates as the roof cools at night. This daily cycling gradually enlarges the blister over time.

Sources of trapped moisture include wet insulation installed during construction, adhesive applied in humid conditions, and moisture that has entered through a previous leak and been sealed over during a repair. Air entrapment occurs when adhesive coverage is incomplete, leaving voids between the membrane and substrate. In multi-ply systems like BUR and modified bitumen, blisters can form between individual plies when interply adhesion is imperfect — these interply blisters are the most common type and are often the least concerning.

Assessing Blister Severity

Blister severity is determined by three factors: size, membrane integrity, and growth rate. Small blisters (under 6 inches in diameter) with intact membrane surfaces that have not changed size in 12 months are low-priority items that should be monitored at each inspection but do not require immediate repair. Large blisters (over 12 inches), blisters with cracked or torn surfaces, and blisters that are measurably growing between inspections require repair to prevent water infiltration.

The foot traffic test is a practical field assessment for blister severity. Step carefully onto the blister (on a cool morning, not when the membrane is hot and soft). If the blister feels firm and does not compress, it may be a solid void filled with expanded insulation or adhesive — not a true moisture blister. If the blister compresses and feels spongy, it contains trapped moisture or air and is a true blister. If the surface crackles or gives way under foot pressure, the membrane over the blister is compromised and repair is needed immediately.

Repairing Blisters

Blister repair on single-ply membranes involves cutting the membrane at the blister, allowing the trapped moisture or air to escape, drying the area, and installing a membrane patch that extends at least 6 inches beyond the blister in all directions. The patch is heat-welded (TPO/PVC) or adhered (EPDM) to create a permanent repair. If the insulation beneath the blister is wet, the wet insulation section must be removed and replaced before patching. Cost: $200-500 per blister repair including insulation replacement if needed.

On modified bitumen and BUR roofs, interply blisters are repaired by cutting an X-shaped incision through the blister, folding back the flaps, removing any moisture, applying new interply adhesive, pressing the flaps back down, and installing a cap sheet patch over the repair area. This method works well for isolated blisters. When blisters are numerous and widespread (more than 10 per 1,000 square feet), individual repair becomes impractical, and a full recover or replacement should be evaluated.

Ridging

What Causes Ridging

Ridging is a linear raised area in the membrane surface, typically running in straight lines that correspond to insulation board joints, cover board edges, or deck panel seams beneath the membrane. Ridging occurs when thermal movement causes the substrate joints to shift, pushing the membrane upward along the joint line. It is most common on mechanically attached systems where the membrane has limited ability to absorb movement, and on fully adhered systems installed over insulation boards with tight joints that do not allow for thermal expansion.

The severity of ridging depends on the ridge height, the membrane condition at the ridge crest, and whether the ridging is static or progressive. A ridge of 1/4 inch or less with intact membrane is a cosmetic issue that requires monitoring but no repair. A ridge of 1/2 inch or more creates a stress point where the membrane can crack or split at the crest, particularly on older membranes that have lost flexibility. Progressive ridging — ridges that are visibly higher or more numerous than they were a year ago — indicates ongoing substrate movement that will eventually compromise the membrane.

Managing Ridging

Minor ridging on membranes less than 10 years old is typically a monitoring item. Document ridge locations and heights with photographs at each inspection. As long as the membrane at the ridge crest remains intact and flexible, the ridge is not compromising waterproofing performance. Avoid foot traffic on ridged areas, as stepping on a ridge concentrates your weight along the stress line and can cause the membrane to split.

Ridging that has progressed to surface cracking requires repair. The repair involves cutting the membrane along the ridge, relieving the substrate movement by creating a gap in the insulation joint, and installing a membrane strip over the cut that is wide enough to allow for continued minor movement without re-ridging. On roofs with widespread ridging, a recover with a new membrane layer is more practical than individual ridge repairs.

Splitting

What Causes Splitting

Splitting is the most serious of the three conditions — it is an actual crack or separation in the membrane that creates an open path for water infiltration. Splits occur when thermal contraction, substrate movement, or building structural movement exceeds the membrane's elongation capacity. Single-ply membranes have high elongation (200-400% for TPO, 300%+ for EPDM), so splitting in these membranes indicates extreme stress concentrations. Modified bitumen and BUR membranes have lower elongation (3-5% for standard APP, up to 40% for SBS), making them more susceptible to splitting in cold weather.

Splits typically appear along lines of maximum stress: at insulation board joints, at the base of flashings, along building expansion joints, and at transitions between different structural sections. A split that aligns with an insulation board joint indicates that the joint is opening wider than the membrane can accommodate. A split at the base of a flashing indicates that building movement is pulling the flashing away from the wall. Understanding the stress source is essential for designing a repair that will not simply split again in the same location.

Repairing Splits

Split repair must address the underlying movement, not just cover the split. Simply patching over a split without relieving the stress that caused it guarantees that the patch will fail at the same location. The standard repair approach involves cutting the membrane along the split to relieve tension, replacing any wet insulation beneath the split, and installing a reinforced membrane strip that spans the split with enough width (12-18 inches minimum) to distribute the movement across a wider area.

For splits caused by building movement at expansion joints or structural transitions, a flexible expansion joint cover should be installed. These prefabricated covers accommodate 2-4 inches of movement in any direction and cost $25-50 per linear foot installed. They are the only reliable long-term solution for splits at structural movement points. Attempting to repair these splits with standard membrane patches will result in repeated failure.

When to Monitor vs. When to Repair

The decision to monitor or repair depends on whether the condition is compromising waterproofing performance today or is on a trajectory to compromise it in the foreseeable future. Use these guidelines to categorize each condition at inspection:

• Monitor only — small blisters (under 6 inches) with intact surfaces, ridging under 1/4 inch with no surface cracking, and conditions that have been stable for 12+ months
• Schedule repair within 90 days — blisters over 12 inches, ridging over 1/2 inch, any condition showing measurable progression between inspections, and any condition in a ponding area where standing water will accelerate deterioration
• Repair immediately — any split (open membrane), any blister with a cracked or torn surface, and any condition that is actively admitting water into the roof assembly

Document all blistering, ridging, and splitting conditions with dated photographs, measurements, and location maps at every inspection. This documentation creates the timeline that distinguishes stable conditions from progressive ones. A blister that has been 4 inches in diameter for three consecutive inspections is a different risk than a blister that grew from 4 inches to 8 inches in six months. Without documentation, every condition looks equally urgent — or equally harmless — and neither assumption leads to good maintenance decisions.

Prevention During Installation

Most blistering, ridging, and splitting conditions originate during installation, making quality control during the roofing project the most effective prevention strategy. Insulation must be dry when installed — moisture meters should confirm moisture content below manufacturer thresholds before any insulation board is placed. Adhesive must be applied at the correct coverage rate and allowed to flash off (partially cure) before the membrane is placed into it. Insulation board joints should include a 1/4-inch gap to accommodate thermal expansion.

Weather conditions during installation matter significantly. Adhesive applied below 40 degrees Fahrenheit does not develop full bond strength. Membrane installed in direct sun on a 100-degree day may develop thermal blisters within months if the adhesive does not cure completely before the membrane is subjected to heat cycling. Manufacturer-certified installers are trained to manage these variables, which is one reason NDL warranties require certified installation crews.