the choice of flashing material is often governed by price,
other factors to consider are the ease of installation and the
expected life of the material once it is embedded in the wall
system. Metal flashings are often used on commercial jobs, but
the two most common types of flashing for most residential jobs
are EPDM and self-adhering rubberized asphalt. The BIA
recommends that most nonmetal flashings should be at least 30
mils thick, which means polyethylene, housewrap, and #15
asphalt felt are not acceptable flashing materials. Aluminum is
unacceptable because it corrodes when embedded in mortar, which
becomes very alkaline when exposed to moisture. PVC flashing
has been known to deteriorate as well, raising concerns about
its expected life.
Remember that the drainage wall system only works if all the
materials are properly installed, and if the flashing is
designed to last as long as the wall itself. Consult the
flashing manufacturers for recommendations on which product to
use in a specific wall system.
flashing is not continuous around the whole building - for
example, above a window, or wherever stepped flashing is needed
- the flashing must include end dams (Figure 8).
End dams are required
in any through-wall flashings that are not continuous, such as
step flashings or window heads and sills.
To create an end dam, turn the flashing up sideways, against
the adjacent head joint, to create a pocket. An end dam will
prevent moisture from traveling off the end of the flashing and
into the adjacent air space. The BIA recommends that end dams
should be at least 2 inches high.
codes require weepholes above all through-wall flashings. The
sole purpose of a weephole is to allow water that has been
collected by the flashing to exit the wall system. Since it is
impossible to eliminate all mortar droppings between the brick
and backup, the weepholes must allow for drainage even when
some mortar has dropped onto the flashing.
There are three common types of weepholes: open head joints,
rope wicks, and weep inserts. Open head joints are formed by
omitting the mortar from the vertical joints immediately above
the flashing every 24 inches (Figure 9).
9. The most dependable type of weephole is the
open head joint.
They're simple, they're cheap, and they work. In
standard-size brick, it takes 2-1/4 inches of mortar droppings
before an open head joint gets clogged. (A 2-1/4-inch chunk of
mortar creates more problems than just a clogged weephole; the
odds are mortar has also clogged the air space above, impeding
the proper drainage of the wall system.)
Rope wicks are the next most popular type of weephole.
Although they do not allow as much flow as an open head joint,
they are often chosen to avoid the appearance of an open hole
or the shadow created by an open head joint. The best type of
rope for this purpose is cotton rope, because nylon,
polypropylene, and polyester do not wick. Wicks should be
spaced 16 inches on-center - more closely spaced than open head
joints, to make up for their smaller size. The rope should be a
minimum of 10 inches long, so that when the outer end of the
wick is installed flush with the exterior face of the brick,
the excess can be draped in the cavity or attached to the
backup wall. The extended length of wick in the air space will
reduce the likelihood of mortar droppings covering the entire
There are also several different types of weep inserts
available. These are typically made of plastic, and are
designed to fit into a head joint. The smaller the insert, the
more easily it can be clogged. Plastic tubes should not be
used, because the openings on either side of the tube are only
1/4 to 1/2 inch, and are too easily clogged.
Weepholes must always be above grade to allow the wall to
drain. Remember that the grade can change over time. For
instance, when homeowners install several applications of
mulch, the grade may end up above the weepholes, preventing
proper drainage of the air space. In most cases, through-wall
base flashing and weepholes should be 4 inches to 6 inches
above grade. The air space below the base flashing should be
filled solid with grout or mortar.
Most people are familiar with the visible base flashings and
counterflashings that are installed where the roofing meets a
chimney. These visible roof flashings prevent water from
traveling between the roofing and the brick - but they don't
stop any water that is already in the chimney, such as water
that may enter through the brickwork or the chimney cap.
Through-chimney flashing is designed to prevent moisture that
has penetrated the chimney from traveling below the roof.
Through-chimney flashing operates on the same principles as
through-wall flashing. Through-chimney flashing has to be
installed above the roof flashing, even if that means putting
it a few courses up, so that the collected water is led to the
Flashing should also be installed under a concrete chimney
cap. This prevents moisture from entering the top of the
masonry, and also prevents staining or contamination of the
brick. The flashing can be laid in place above the top course
of bricks, before the concrete cap is poured.
A mortar wash does not form an adequate chimney cap. A
mortar wash will shrink and crack as it cures, especially when
the mortar is brought to a feather edge (see "Troubleshooting
Common Chimney Problems," 7/98). Mortar is very similar to
concrete, only with smaller aggregate. Remember, concrete
usually needs something to give it some tensile strength and to
prevent cracking - for example, 6x6 mesh or other
For chimney applications, choose a flashing material that
can hold up to the potentially high temperatures expected when
the flue is in use. Use metal flashing at the chimney cap.