Flat Roof / Built Up Roof

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1. Weathering Issues

Weathering is a natural process through which a built up roof ages or deterioration. The rate of weathering of your commercial roof depends on climate: exposure and amount of sunlight, rainfall, wind, and even hail.

Weathering (deterioration) affects properly manufactured roofing materials that make up your commercial built up roof, but defects or deficiencies in the materials or manufacturing process will accelerate deterioration.

Common weathering anomalies include embrittlement of flood coat, erosion, and splitting. Other anomalies found on built-up roofs include maintenance-related issues.

Visual explanation of the installation of a flat roof

Built-up roofing membranes usually are applied over insulation, although they can be applied directly to the deck.  Both must provide a flat, smooth surface.  Ultimately, the roofing membrane and insulation must be secured to the structure, most often to the deck.

Other components of the roofing assembly may include vapor retarders to prevent condensation within the roofs system.Each component of a roofing system must be considered carefully as nuances of behavior and chemical incompatibilities may affect the performance of the roof.  Building codes govern roofing design and installation.

2. Hail Damage

Built-up roofing is damaged by impacts of hailstones when its bitumen or reinforcements are broken, or protective surfacing is removed and materials not so intended are exposed. Hailstones can damage membranes when the hailstones are large enough and have sufficient densities, hardness, and speeds to puncture, tear, or bruise the reinforcements where the hailstones strike.

Impact-caused punctures, tears, and bruises can be felt by hand. Hence, it is important not only to examine membranes visually, but also to hand-manipulate them. Bruises in roofing feel similar to bruises in apples and indicate ruptures in the reinforcement. Impact-caused fractures initiate in regions of maximum tensile strain on the bottom of the membranes and propagate toward top sides. The severity and frequency of the hail-caused damage in roofing determine its subsequent repair.

hail damage on built up roof 2
hail damage on built up roof

The decision to repair or remove and replace roofing is made based on economics. The following are general rules of thumb regarding the impact resistance of membranes. For a given type of built-up roofing membrane:

  • 1. The thicker the membrane, the greater the impact resistance.
  • 2. The stiffer the underlying support, the greater the impact resistance.
  • 3. The more weathered the membrane, the less the impactresistance.
  • 4. Aggregate-ballasted membranes are more impact resistant than bare membranes.

Close up of hail damaged built up roofing ballasted with gravel. Note that the gravel ballast was not driven through the membrane plies, but ejected from the impacted area.

The dark black color of the asphalt exposed by the hailstone impact contrasts with the weathered, gray colored asphalt, indicating the impact was caused recently.

Close up view of hailed caused bruise and spalled flood coat in built up roofing with aluminum coating. For smooth built up roofing in relatively good condition, the expected threshold for damage is 2 inch hard hailstones with perpendicular impacts.

The pattern of circular fractures in the membrane are substanially larger than the size of the impacting hailstone due to soft support conditions.


Wind can damage roofing both directly and indirectly. Direct wind-caused damage is the scouring, distorting, tearing, or peeling away of materials which results from positive or negative (sometimes termed uplift or vacuum) pressures on material surfaces.

Obviously, direct wind-caused damage occurs first in regions of higher (magnitude) pressures such as windward roof edges and corners. Regions experiencing lower (magnitude) pressures, areas away from windward roof edges and corners, are not damaged unless the wind speeds increase further. The progression of direct wind-caused damage extends from the scouring gravel ballast, to distorting or tearing away metal edge flashings, to ballooning and peeling of the membrane from the roof. Indirect wind-caused damage involves the impact of wind-borne objects. These wind-borne objects may scuff, mar, bruise, tear, or puncture the membrane.

Loose gravel ballast has been scoured from built up roofing in the negative pressure region along the roof edge.

hail damage on built up roof

Examples of Wind and Hail Damage on Commercial Roofs

National Claims Institute is the authority in this field
Built Up Roof Hail Damage 1
Built Up Roof Hail Damage 2
Built Up Roof Hail Damage 3
Built Up Roof Hail Damage 4
Built Up Roof Hail Damage 5
Built Up Roof Hail Damage 6
Built Up Roof Hail Damage 7
Built Up Roof Damage
Built Up Roof Hail Damage 9
Built Up Roof Damage 10
Built Up Roof Hail Damage 11
Built Up Roof Hail Damage 12
hail damage on built up roof
Built Up Roof Hail Damage 15

Examples of Mechanical Damage on Commercial Roofs

National Claims Institute is the authority in this field
Built Up Roof Mechanical Damage



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