Asphalt has been a traditional material used on Irish flat roofs for many years, generally applied hot in a liquid state to a minimal thickness of 20mm. In most cases, it is a durable material with a lifespan of 20 to 40 years depending upon the quality of the deck (substrate), application and the degree of exposure directly to sunlight. Deterioration is often indicated by the appearance of surface splits, cracking, and by surface bubbles – signalling trapped water vapour within the layer.