Quantcast Differential Movement

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MIL-HDBK-1001/5A
c)  Consolidation of as many openings as possible into
a smaller number of larger openings;
d)  Use bituminous base flashings, i.e., modified
bitumens or glass fiber flashing felts, at parapets, walls,
expansion joints, and curb framing access hatches, vents, or
equipment openings.
e)  Location of flashed joints above the highest water
level on the roof, with drainage away from flashed joints;
f)  Allowance for differential movement between base
and counterflashing;
g)
Firm connection of flashings to solid supports;
h)  Contouring of bituminous base flashings with cants
to avoid sharp bends;
i)
Avoidance of surface-applied cap flashings.
j)  Always avoid pitch pockets always in new
construction and whenever possible in reroofing.  When required,
fill with a crowned pourable sealer.
5.3.2.1
Differential Movement.  Flashing details must provide
for differential movement among the different parts of the
building.  Where there is any possibility of differential
movement between the roof deck and vertical elements, do not
anchor base flashings to the intersecting vertical elements.
Anchor the counterflashing to the wall, column, pipe or other
flashed building component.  Review detailing carefully.
5.3.2.2
Contour Flashings.  With built-up flashing materials,
use cants with 45 degree (100 percent) slope.  Cants should also
be used with modified bitumen flashings.
Wood cants (southern yellow pine, Douglas fir, or
equivalent), pressure-treated with water-base preservatives may
be preferable to fiberboard cants where they brace the right-
angle joint between vertical and horizontal nailers at equipment
or wall curbs.  Avoid fiberboard cants for torch-applied
modified-bitumen flashing.
5.3.3
Foot Traffic.
Refer to par. 10.7.
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