The three-layer microporous roofing underlay with high UV resistance developed by Web Dynamics is described. Membranes may be exposed to UV radiation ( sunlight ) before the tiles are laid. Accelerated ageing studies conducted by Web showed that relatively short exposure times degraded some underlays, leading to cracking and leakage. Durability was found to be dependent upon a number of factors, including film gauge and polymer content. Materials containing microporous films originally developed for the hygiene market were considered to be unsuitable for roofing applications.