Birds Did This?

Cody E. Tarbell, P.E.

Cody E. Tarbell, P.E.

Birds Did This?

Recently, EDT was asked to investigate the roof of a hotel property. Representatives of the hotel owner showed EDT the “large hail damage” they had found. However, hail damage is not what was discovered.

The hotel roof had a smooth-surface modified bitumen membrane with a protective metallic coating. Circular-shaped areas of absent metallic coating, often mistaken for hail damage, were observed on discrete sections of the roof. Other sections of the roof contained no such areas of damage. Further, no bruising or concentric-circle fractures characteristic of hail damage to the membrane were apparent at the areas of absent coatings.

hotel roof

hotel roof damage circular spots

The observed damage to the coating often exhibited a complete, or nearly complete, ring of absent coating with an undamaged center, a mark consistent with damage from bird droppings. Bird droppings are often concentrated in discrete sections of a roof, and not distributed across the roof surface like typical hail damage. Bird droppings contain uric acid, a damaging substance. Therefore, bird droppings can erode roofing materials, which can shorten the lifespan of the roof and potentially result in roof leaks.

Another recent examination revealed extensive bird dropping damage to asphalt composition shingles on a residence. Multiple roofing contractors notified the homeowner that their roof had been damaged by hail, but EDT did not observe any evidence of hail damage to the asphalt composition shingles.

Damage to an asphalt composition shingle from hail typically consists of a circular, semi-circular, or oval-shaped area approximately the same size as the impacting hailstone. In the circular-shaped area, the strike of the hailstone will embed about half of the granules into the underlying base mat and remove the remaining granules. In contrast, uric acid from bird droppings erodes the asphalt, and a complete or ring-shaped loss of granules often occurs, exposing the underlying base mat.  In addition, bird dropping damage is often concentrated at the hips and ridges of a roof, unlike hailstone strikes, which can be expected to have a uniform distribution across a particular roof slope.

Damage on residential roof shingles

circular pattern on residential roof shingle

For this particular residential roof, bird dropping damage had been an ongoing problem. In some of the damaged areas, the exposed base mat had weathered and deteriorated, indicating long-term exposure to sunlight and damaging ultraviolet (UV) rays. In the vicinity of the damaged areas, bird droppings were still present, indicating more recent deposits.  

Due to the nature of the materials, the effects of bird droppings are most damaging to roofing systems containing asphalt (e.g., modified bitumen, asphalt composition shingles). Roof systems such as metal panels, thermoplastics (e.g., TPO1 or PVC2), thermoset (e.g., EPDM3), wood shakes and other roofing materials that do not contain asphalt are typically safe from these aerial bombers.

Circular shaped damage on a roof may resemble hail damage but must be examined with an eagle eye to determine its true cause.

1Thermoplastic olefin/polyolefin
2Polyvinyl chloride
3Ethylene propylene diene monomer


About the Author

Cody E. Tarbell, P.E. is a consulting engineer with our Kansas City office. Mr. Tarbell provides structural evaluation and damage assessment of commercial and residential buildings and structures. You may contact Cody for your forensic engineering needs at or (913) 859-9580

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