Base flashing height limitations originated with bituminous flashing systems and became more sophisticated with heavier mat weights, coatings and inclusion of granulated surfacing. Built-up roofing (BUR) material manufacturers quickly realized granulated bituminous flashings installed with hot asphalt or cement and attached at the top with roofing nails could slip and fall because of the sheet's inability to support its weight.
Some BUR material manufacturers settled on published maximum base flashing heights of 24 inches (610 mm). Anything installed above this height prompted membrane manufacturers to recommend special precautions. This change in height limitation and special precautions caused NRCA to remove the base flashing height limitations in The NRCA Roofing and Waterproofing Manual, Fifth Edition, in 2001, as shown in the detail.
Flashing height limitations for bituminous roof systems have remained at about 24 inches (610 mm) without special precautions, but the popularity, compatibility and lighter installed weight of thermoset single-ply sheets made single-ply manufacturers rethink this restraint. The average weight per square foot of 60-mil- (0.06-inch- [1.5-mm-]) thick, unreinforced EPDM is about 0.4 pounds per square foot (psf) (2 kg/m2) compared with about 1 psf (5 kg/m2) for one ply of granulated, polyester-reinforced SBS-modified bitumen sheet and a Type IV backer felt.