Starting from the roof deck, a built-up roof membrane system may include the following components:

  1. A slip sheet
  2. Air or vapor retarder;
  3. Rigid board roof insulation;
  4. Cover board;
  5. Primer;
  6. Interply bitumen or adhesive;
  7. Layers of base sheet, plies, cap sheet, or other surfacing materials; and
  8. Membrane flashings.

Base sheets, plies, bitumen, cap sheets, float coats, and flashings must be of compatible materials that can vary by manufacturer and by basic material/methods choices. Inspectors typically will not be able to determine whether proper materials have been specified by the designer and proper installation procedures been followed by the applicators. Many opportunities for mistakes exist.

Inspectors will concentrate on looking for obvious installation mistakes, damage, and deterioration.

The primary factors affecting the performance of built-up roofs are:

  1. Proper product specification for the conditions to which the roof will be exposed;
  2. Proper application procedures;
  3. Material quality (especially for materials for which no manufacturing standards exist);
  4. Environmental factors; and
  5. Quality of maintenance.




Over the long-term, asphaltic materials lose volatile compounds to evaporation. As this happens the material shrinks. Because of direct exposure to sun and wind, shrinkage takes place at the membrane surface faster than it does in the underlying membrane. The unequal shrinkage rates between materials bonded to each other creates tension that is relieved by cracking. This cracking typically appears as random pattern cracking that is widespread and fairly uniform across the roof. The same cracking patterns are sometimes visible in concrete and soil, for the same reasons.

This condition will develop first in the surface layer of the membrane, but with time will propagate downward into the membrane and will eventually develop into a split, which can allow leakage. If applied before splits develop, recoating the roof can extend its lifespan.

In the past, this condition has been called “alligatoring”, but this term should be avoided in inspection reports, since it is a colloquial term a client will probably not understand.


Roof-deck Movement

As wood roof decks contract with loss of moisture, membranes that lack sufficient flexibility may split above panel joints.



As wood roof decks expand as they absorb moisture, membranes that lack sufficient flexibility may split above panel joints. Buckling can also be caused by insulation that has detached from the substrate.



Blisters are common in field-applied coatings like paint or built-up roofs. In a roof system, they are raised surface areas above voids filled with entrapped air or moisture. Even under ideal conditions, some voids will always be created during the built-up roof installation process.

Voids can result from:

  • Uneven mopping;
  • Trapped debris;
  • Curled or wrinkled felts;
  • Uneven off-gassing; or
  • Bitumen bubbling.


As roof temperatures rise during the day, pressure inside a blister also rises and adhesion to the asphalt weakens.

Blistering of built-up roofs takes two forms:

  1. Blisters between the base sheet and the roof deck.

Blistering occurs when the roof deck (substrate) is of low permeability. If roof temperatures rise too quickly for trapped air or moisture to escape through the substrate, a blister will form and grow in size until the roof reaches maximum temperature.

This condition is sometimes made worse with bitumen that has a low softening temperature. As roof temperatures rise and bitumen softens, it becomes less able to resist the pressures developing inside blisters, increasing the rate of blister growth due to stretching of the membrane or loss of bond around the blister perimeter.

Blister size also affects the bond around the blister perimeter. As a blister doubles in size, the lifting force around its perimeter also double, increasing the rate of bond failure and the blister growth rate.

Constant swelling and shrinking of the blister will eventually lead to the development of cracks and loss of granules that protect asphalt from UV damage.

Membranes applied directly to cellular foam insulation tend to suffer more blistering than membranes applied over other bases materials. This may be caused by off-gassing of the insulation but may also be the result of voids created by failed adhesion.


  1. Blisters between membrane plies.

Air and/or moisture will almost always be entrained into the membrane during topping off process (final application of bitumen as the weather-resisting surface). Blisters exhale air/moisture vapor during the warm day and inhale during the cool night. Bitumen membranes stretch easily when they are warm, but when they cool, they become stiff and resist returning to their original shape. This process creates a vacuum that pulls air and moisture vapor into the blister through microscopic cracks in the bitumen and felt. As the amount of air/moisture vapor increases, during the day, the increased pressure inside the blister will exceed the strength of the asphalt bond around the blister perimeter and the bubble will grow.


Treating Blisters

A few small blisters aren’t really a concern. If cracks or bare spots appear, they can be covered with a cold-process asphalt coating (roofing cement) and sprinkled with gravel.

Blisters larger than about 18” should have the perimeters marked and should be monitored to identify any growth.

Large blisters 3-5’ should be repaired, especially if they are in paths of travel, worn, or cracked.

A roof with widespread, large blisters may be at or near the ends of its useful life.



Common problems are inadequate slope, debris accumulation, and improper placement of drains or scuppers. Sediment from water pooling in front of scuppers is a common sight.



The following factors have the potential to create problems:

All Membrane Types

            Slope and Drainage:

  • The National Roofing Contractor’s Association (NRCA) standard for adequate slope is that there be no water ponded on the roof 48 hours after rain has ceased and drying conditions have existed.
  • Slope is typically provided by:
    • Sloping the roof structural framing or using tapered sleepers under the roof deck.
    • Using tapered rigid board insulation designed to provide slope.
    • Proper location of drains, scuppers, and gutters.


Storage and Transport

  • Materials may be damaged or deteriorated
  • Storage areas must be clean of gravel and other debris. Debris damaged bitumen-based rolls can be the starting point for the development of blisters.
  • Bitumen-based rolls must be stored on end to avoid damage.
  • Moisture-absorbent materials must be transported and stored using a method that will keep them dry.
  • Water-based materials should be protected from freezing.


Application: Weather Conditions:

  • Application of roofing materials should be postponed if precipitation is forecast.
  • Temperatures below 40° F require special materials/installation methods.
  • Moisture in materials may cause blistering. If precipitation occurs before installation is complete, materials should be allowed to dry completely before installation continues.


Construction Traffic:

  • Completed roof sections should be protected from excessive foot traffic and should not be used as staging/work areas. In some cases, permanent walkways should be installed.


Polymer-modified Membrane Application:

  • Curbs, drains, and roof penetrations should be in place before roofing begins.
  • Plies should be laid shingle fashion, overlapping at both ends and along their lengths.
  • Plies should be laid so that water does not run against the overlaps.
  • Plies that are tightly curled may not lie flat after installation.
  • Some system manufacturers recommend rolling with a weighted roller after installation.
  • Proper temperature ranges for both bitumen and substrate are important.


Roof-mounted Photovoltaic (PV) Arrays:

  • PV arrays subject roofs to increased structural loads, temperatures, and UV radiation.
  • Ballast should not be used to resist array uplift.
  • Arrays should not be adhered to mechanically attached membranes.
  • Arrays should not be mounted on loose-laid, ballasted membranes.