Planning on Re-Roofing? - September 30th 2006
Some roofing contractors and roofing manufactures suggest that new roofing membrane can be installed over the old. This is a "No No!" In my opinion it must all come off including the flashing around chimneys, vent flashings as well as replacing all roof vents.
This will enable the applicator to inspect the substrate i.e. Roof sheathing, Eave Flashings, Chimney Counter Flashings and Roof Venting.
Roof venting is very important. For every 200 feet of attic (roof cavity) you must have 1 square foot of venting (144 square inches). After calculating the required square inches of venting 50% (half) should go into the roof or ridge the other 50% (half) into the soffit.
Good, Better, Best: consider at least a 30-year membrane applied over #15 asphalt paper (over entire roof) with a 90lb roof starter strip. In cold climates consider ice shield as a starter strip.