UV and Heat Degradation of Roofing

Sunlight and heat are among the main causes of roof aging. Ultraviolet (UV) radiation breaks chemical bonds in organic materials such as asphalt and plastics, while high temperatures speed up these reactions and cause repeated expansion and contraction. The combined effect shortens the service life of many coverings, especially in hot, sunny climates.

How UV damages materials

In polymers, UV light and oxygen together cause photo-oxidation: chain reactions that break long molecules into shorter ones, making the material brittle and eventually causing cracking. The process is self-accelerating once radicals form. Research on polymers notes that temperature often has a larger effect on degradation rate than UV exposure alone.

• Polyolefins (the base of many TPO membranes) and PVC need stabilizing additives to resist UV. PVC degradation can cause yellowing and browning without stabilizers.
• UV stabilizers absorb UV light and dissipate it as low-level heat; typical loadings are 0.05 to 2 percent of the material.
• Carbon black screens UV, which is one reason black EPDM rubber membranes are UV-stable.

Asphalt roofing

Asphalt oxidizes and hardens with exposure to heat and UV. Mineral granules on shingles and cap sheets shield the asphalt from sunlight; as granules wear away, aging accelerates. Inspection guidance notes that hot climates drastically reduce asphalt shingle life.

Heat and thermal movement

Dark roofs can become far hotter than the air. The US EPA notes that roof surfaces can reach as much as 37 °C (66 °F) above air temperature, and Lawrence Berkeley National Laboratory has measured black roofs about 30 °C (54 °F) hotter than white roofs on summer afternoons. Large daily temperature swings cause materials to expand and contract, stressing seams, fasteners and flashings.

Other materials

• Metal roofing: the metal itself is not affected by UV, but paint and coating systems can fade and chalk, and coating chemistry affects colour retention.
• Wood: UV breaks down lignin at the surface, causing greying and erosion.
• Clay and natural slate: inorganic and largely unaffected by UV.
• Coatings and sealants: elastomeric coatings, mastics and sealants weather and need periodic renewal.

Reducing UV and heat damage

Measures include reflective (cool) surfaces, UV-stabilized membranes, protective granules or ballast, adequate ventilation under steep-slope roofs, and regular recoating of coatings and sealants.