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...
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