An important but less commonly used detail in the roofing
industry is for proper installation of base flashing with a new
lightweight insulating concrete (LWIC) or poured gypsum roof deck.
Roofing and Waterproofing Manual, Fifth Edition, provides a
detail for installation of a built-up roof system's base flashing
where a wall meets an LWIC or poured gypsum deck in BUR-5A, "Base
flashing for vented base sheet."
The unique and critical element of the detail is proper
installation of the vented base sheet horizontally under the
membrane and vertically behind the base flashing. The purpose of
the vented base sheet is to facilitate venting and drying of
residual moisture vapor (generally not liquid moisture) from the
hydration process of an LWIC or poured gypsum roof deck. A roof
assembly that incorporates LWIC or poured gypsum needs to be
detailed and constructed to allow moisture to travel horizontally
in the roof's field and vertically at base flashings; therefore, a
vented base sheet cannot be fully adhered to a deck. NRCA recommends a vented base
sheet be mechanically attached to an LWIC or poured gypsum roof
deck with suitable fasteners.
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