A.Jim Carlson responds:
A skylight opening on a low-slope or flat
roof must have a raised curb to allow for proper flashing and
The raised curb should be at least 8 inches above the
finished roof surface (see illustration, next page).
Depending on the roofing material, you may need to install a
cant strip at the base of the curb. A cant strip provides a
more gradual transition for the roofing membrane to turn up
from horizontal to vertical.
The field plies of the roof membrane should be turned up the
vertical face of the curb, above the head of the cant strip.
However, the turned-up field plies are not adequate curb
flashing by themselves. To properly flash the skylight curb,
membrane base flashing should be installed before the skylight
is set in place. The base flashing reinforces the membrane at
the curb-to-roof intersection.
Base flashing is simply made from strips cut from rolls of
the roofing material, or from membrane flashing stock,
depending on the roof system you are using. Feather the base
flashing plies onto the field of the roof, with the uppermost
ply extending approximately 4 inches beyond the toe of the cant
Once the curb has been flashed, the skylight and
counterflashing can be installed. Counterflashing is usually
made of sheet metal, and should be wide enough to extend over
the base flashing a sufficient distance to keep out wind-driven
rain and snow. Most manufactured skylight assemblies have a
metal flange that serves as counterflashing. Depending on the
specific climate, you may need to insert an additional metal
"counterflashing skirt" to extend the pre-made flange.
Jim Carlson is deputy director
of Technology and Research for the National Roofing Contractors