COMPOSITION
FLASHING |
Composition flashing or base flashing
is used because the bending radius of
present built-up roofing materials is
generally limited to 45°. To allow
for this bending radius, all vertical
surfaces must have cant strips installed
between the roof and the vertical surface.
When you are inspecting your built-up
roofs, look for the 45° angle at
the horizontal and vertical point. If
this 45° angle/cant is missing and
you have composition flashings, the
area will be prone to severe splitting
and leakage of water into the assembly
or into the facility interior.
|
TYPES
OF
MATERIALS AND
PROTECTION |
Most commonly used today are fiberglass
Type IV and a granule surfaced modified
asphalt membrane. All flashings should
be a minimum of two plies, with the
Type IV glass as the backer. By simply
coating your new or existing flashing
with a reflective coating, you will
prolong (as much as 1/3) their serviceability
by blocking out the deteriorating effects
of the ultraviolet rays of the sun.
|
| HEIGHT |
The height of the base flashing should
not be less than eight inches (8")
and not higher than fourteen inches
(14") above the finished roof surface.
Walls or parapets requiring flashing
higher than fourteen inches (14")
should receive special moisture proofing
above the fourteen inch (14") height.
A wood nailer strip or suitable detail
allowing mechanical fastening of the
base flashing at the top must be provided.
Masonry surfaces should be primed with
asphalt concrete primer before installing
the flashing system.
|
| METAL
FLASHING |
Since metals have a high coefficient
of expansion, metal flashing should
be isolated from the roof membrane wherever
possible to prevent metal movements
from splitting the membrane. Flashing
details that require metal flanges to
be sandwiched into the roof membrane
should be avoided wherever possible.
|
| COUNTERFLASHING |
For all walls and projections that
receive composition base flashing, metal
counterflashing should be installed
in the wall above the base flashing.
The design of this detail should be
two-piece, allowing installation of
the counterflashing after the base flashing.
The counterflashing should extend down
a minimum of 4" over the top of
the composition flashing.
|
| CAP
FLASHING |
Metal coping is often used to cover
the top of a parapet wall in lieu of
masonry copings. If your composition
flashing system is the full height of
the parapet wall, extend this flashing
over the top to the outside edge of
the parapet. This will ensure no interior
leaks should water enter at the metal
coping joints.
|
METAL
EDGE
GRAVEL STOPS |
These should be raised above the waterline
by using tapered cants and wood blocking.
When this is not possible, the metal
flanges for low profile gravel stops
should be set in mastic on top of the
completed built-up roof membrane and
nailed at 4" intervals to the wood
nailer. The metal flange should be primed
with asphalt primer and felt flashing
strips applied 12" and 18",
respectively. Interior drainage is recommended,
and edges should be raised whenever
possible. Reference National Roofing
Contractors Association.
|
DO
I HAVE A
PROBLEM? |
When inspecting your built-up roof
flashing, keep the following in mind:
- Unless you have elastomeric flashings,
you must have a 45° angle at all
horizontal and vertical points.
- Where built-up roof and metal
meet, the difference of expansion
and contraction of the two dissimilar
materials creates problems leading
to leaks from fatigued flashing
membranes that have failed.
- Check all perimeter and projection
flashings carefully; 80-90% of a
roof's problems originate at these
points.
- When inspecting your roof's metal
edge, brush back the gravel at each
metal edge joint and look for splits
in the flashing at this point.
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