Design and Installation Deficiencies

  • Improper flashing (materials or methods);
  • Bubbles/blisters;
  • No cant strip at parapets. Built-up membranes should not turn upward 90 degrees. Corners should be softened by installation of cant strips.
  • Improper terminations at penetrations, walls, and roof edge;
  • Inadequate slope/positive drainage/signs of ponding;  
  • Inadequate drain method;
  • Impeded drainage
  • Inadequate flood coat thickness;
  • Less than 8" min. elevation above the surface (curbs, penetrations, etc). Common with re-covers;
  • Inadequate number of expansion joints; and

Weathering and Maintenance Issues

  • Improper repairs;
  • Failed or deteriorated flashing;
  • Drain blockage;
  • Craze-cracking (alligatoring: jargon, this term not used in narratives);
  • Flood coat weathered away (mat/reinforcement visible);
  • Base flashing exposed reinforcement;
  • Embrittlement and cracking;
  • Splitting due to wetting of organic reinforcement (typically happens due to rapid temperature drop);
  • Flood coat creep;
  • Membrane creep (inadequate attachment to deck);
  • Dissolved asphalt from AC condensate; and
  • Dissolved asphalt from greases/oils.


Wind, Hail and Mechanical Damage

  • Hail damage (spatter or impact damage including collateral damage);
  • Wind-caused ballast scouring near perimeter and corners;
  • Wind-dislodged cap flashing;
  • Wind uplift damage; and
  • Damage from wind-borne missiles




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, both contributing to blister growth.


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.



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.


Identifying the Number of Plies

Rolls come in standard and metric sizes: 

Standard- 36"         Metric 39 3/8"

2-ply standard:   17-inch ply exposure

2-ply metric:       18 9/16-inch ply exposure

3-ply standard:   11 1/4-inch ply exposure

3-ply metric:       12 7/16-inch ply exposure

4-ply standard:     8 1/2-inch ply exposure

4-ply metric:          9 5/16-inch ply exposure