Roofing

Although many authorities advise that multiple layers of asphalt shingles can be acceptable there are definate reductions in longevity resulting, up to 50% of life can be lost by each subsequent thickness of shingles applied over another. This also reduces the possibilties of inspection / replacement of flashings and sheathing when not removing the worn out shingles. Did you know also that when sheathing has to be replaced a building permit is required as this is a significant part of the buildings structure.

Warranties for shingle materials are for the large part worthless and not claimable and if claimable may only support replacement materials so therefore the specifically "failed" components only would be offered as replacement. However 50% of failed shingles can be attributed to inferior installation workmanship and design issues  can be picked out as concerns on many others. Weaved covering of valleys can give reductions of life of approx 5 years so metal valleys will ensure maximised life and effective return for investment.