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The drying time of mastic asphalt varies from only 20 minutes for a first coat laid in winter. All the way up to approximately 2 to 3 hours in the summer months. And even longer if mastic asphalt is laid on a roof with insulation in the summer.
There are several factors that need to be taken into consideration. A mastic asphalt laid in the winter months on a concrete or screeded roof with no insulation probably only takes 20 minutes for the first coat to dry and no longer than an hour for the top coat. In the summer months when it is very hot much longer will be needed for the 1st coat and up to 2 or 3 hours for the top coat. Even after this time when walked on the finish will cause markings. Mastic asphalt laid on roofs with insulation this will take much longer to dry. And in summer you will probably not be able to walk on the roof until the next day. It is a good idea to paint the 1st coat of asphalt roofing with solar reflective roofing paint a day after it is laid. This will give you the opportunity to finish the upstands and then apply the second coat of mastic asphalt. This will have the added benefit of providing your roof with longevity. Mastic asphalt applied on any wooden substrates will take much longer to dry in comparison to other solid substrates.