by Steve
Easley
Drainless Designs
Designers love intersecting roof planes, but the intersections must
be capable of handling a tremendous amount of water: a 1-inch rain
on a 2,000-square-foot roof will deposit about 1,250 gallons of
water. If that water is obstructed by any intersecting roof, wall,
dormer, or eaves detail, and thus does not have a clear path to
drain away, that water will back up and probably find its way
inside the home. In many states, the statute of limitations is 10
years for construction and design defects. Can the design or
construction practice you are using last this length of time
without a problem?
Callbacks typically result from three things: poor design, faulty
construction, or the actions of a clueless homeowner. While this is
true everywhere, coastal environments are a whole lot less
forgiving of errors. Wind-driven rain, combined with longer periods
of wetting and shorter periods when materials can dry, means
moisture gets driven deeper into building cavities and the
materials stay wet longer. The result is a much higher incidence of
moisture-related problems.
In my work for home builders, I walk a lot of sites, looking for
potential problems and explaining the best practices that will fend
off an expensive callback. Usually my job includes addressing a
company's construction managers — and sometimes the entire
crew or invited trade partners — explaining the problems and
advising what to do to avoid them. It's critical that this
information gets to everyone, because a home is only as good as the
knowledge and attention of your worst sub or employee.
Over the years, I've seen the same mistakes made over and over
again. Here is a collection of some frequent blunders on exterior
details, with a few simple solutions that will help fend off
moisture problems.
Ridge intersecting eaves. The problem with this
design is that all the water that runs off the higher roof is
collected into the gutter. Note that there is no downspout, so all
the water that runs off the higher roof runs toward the poorly
flashed ridge cap of the intersecting roof. The solution is to drop
the ridge so the eaves and the gutter of the higher roof can run
uninterrupted above the lower roof ridge. And, of course, pitch the
upper gutter to a downspout at the corner.
Horizontal valleys. Whenever a valley runs
into a horizontal plane, the water flowing down that valley will
become trapped. If there's enough water at one time (or it turns to
snow), it's likely to back up under the shingles and flashing. A
whole series of horizontal valleys has been created between these
condo units (top). Such poor-draining roofs means the builder is
more likely to face a lawsuit from the homeowner's association.
Another common mistake is a roof sloping into a vertical wall
(bottom), creating a horizontal valley condition that leads to
leaks.
Poorly conceived roof intersections.Wherever
two roofs of different pitches intersect, it's important to detail
the intersection so that water can shed away from the structure. In
this example, there's no way for water to drain off the lower roof
(in the foreground) without getting funneled into the wall, where
the eaves of the second roof blocks its way.
Design solution. Here's one example of how to detail
intersecting roofs to avoid a horizontal valley. By keeping the
gable ends on different planes, the valley is allowed to run
unbroken, so water can drain away freely.
Reverse Shingling
Treat your walls just like roofs. Few builders would ever install
roof shingles without an underlayment or flashing, and they would
never lap the bottom shingles over the courses above. In the same
way, walls should always be covered by a secondary moisture
barrier, and that layer should be installed "shingle fashion" so
the upper layers overlap the lower layers. Too often, however,
layers of flashing and housewrap are "reverse shingled."
Reverse-shingled step flashing.Each piece of
step flashing shown in these two cases (top and bottom) is applied
over the one below it, which is correct. But they are all applied
over the housewrap. Best practice calls for installing the step
flashing over the wall sheathing, followed by a piece of flexible
peel-and-stick flashing over this, so that the sheathing itself
functions as a tertiary moisture barrier. Housewrap — the
secondary barrier — should overlap the peel-and-stick, and
then cladding — the primary defense — would overlap the
housewrap. The wall shown in the photo on the right has the added
problem that water has a chance to become trapped where the roof
meets the brick veneer — an example of the drainless design
described previously.
Reverse-shingled housewrap. If you follow the arrow above,
you can see that the secondary moisture barrier is
reversed-shingled — the bottom layer overlaps the top layer,
which means that any water that gets in behind the cladding can
easily be funneled into the wall rather than draining away from it.
When wrapping a wall, always start at the bottom so each successive
layer overlays the layer below.
Wall Flashing Woes
After a heavy rain you can learn a lot about moisture flow by
looking at the rain patterns on cladding materials. The tremendous
moisture load visible after a heavy rain underscores the need to
flash these heavily loaded intersections carefully. Some of the
biggest moisture loads are where walls and roofs intersect.
No kickouts. The concentrated water flowing
off a sloped roof can soak side walls (top), concentrating the
moisture load right at vulnerable window headers (not a great
design). The solution is to divert the water away from the wall
using kickouts — small pieces of sheet metal placed at the
end of the step flashing and sealed to the wall and roof sheathings
with some peel-and-stick flexible flashing (bottom).
Missing flashings. The window at right has neither sill
flashing underneath the windowsill (to direct water that leaks
through the window to the exterior) nor step flashing underneath
the roofing shingles where it intersects the wall (to protect
against splashback). On the dormer below, step flashing should be
installed underneath the shingles on the roof and underneath a
water-resistive barrier on the wall of any sloped roof that
intersects a wall.
Wrong WRB Details
All claddings leak, and the more wind pressure you have, the more
water leaks in. Rather than attempt to fight this fact using caulk
to try to make the cladding watertight, the best strategy is to
design the wall system to drain quickly and freely. This is
accomplished with a well-detailed water-resistive barrier (WRB)
that provides a second line of defense. The guiding principle for
detailing this barrier is to make sure that water flowing across it
always travels down and away from the building.
Like all cladding, vinyl siding leaks. By design,
it is made with weep holes to allow for drainage, but it needs a
good drainage plane behind it to protect the wall. In this example,
the secondary moisture barrier is doing its job, shedding enough
water that wet spots are clearly visible underneath the vinyl
siding.
Unprotected wall penetrations.It takes only
minutes for wall components to get wet, but it takes days weeks or
months for them to dry out. If the wetting rate exceeds the drying
rate of building materials, mold and decay can occur. Notice that
there is no flashing around the air conditioner line set and
electrical wires coming through the wall (top). It would be very
easy for water to run down the wires and flow directly into the
opening. The proper way to handle this penetration (bottom) is to
use a flashing boot that is tightly sealed with an acrylic-based
tape or flashing materials.
Incomplete coverage. Water-resistive barriers
must be installed correctly. The rips and gaps in the WRB shown
here will allow any water blowing through to leak into the wall at
the corners. The housewrap should be overlapped approximately 12
inches at the corners and sealed with an acrylic-backed tape. Note
that the tape used for the window flashing is not an approved
flashing material. It will probably fail as a result of
differential movement.
How to grow mold.This thermograph looks at
the corner of the house from the inside. The fan of a blower door
is depressurizing the house (sucking air out the door), so cold
outdoor air is being drawn in through cracks in the exterior wall.
The blue areas show where cold air is infiltrating underneath the
wall plate and around an electrical outlet. This cold air will
reduce the surface temperatures, thereby increasing the surface
relative humidity and the chance for mold growth. The cure includes
a breathable exterior housewrap with the seams taped as well as
sill seal and other air-sealing measures — all elements of a
good air-barrier system.
Unsealed wall penetration.Cutting a large
round hole in the housewrap (top) defeats its purpose. The solution
is to X-cut the housewrap, then slide the vent through the X-cut
and seal around the pipe with an acrylic tape before securing the
face plate (bottom).
Overfastening. Every staple through the
water-resistive barrier is a potential water leak, so don't get
trigger happy, as this installer did (top). In this case, water can
actually be seen dripping off the ends of the staples inside the
wall cavities (bottom). Best practice calls for a staple or cap
nail every 12 to 18 inches along the vertical stud.
No capillary break.Synthetic stone (above)
creates a unique challenge: Unlike brick veneer, which is built in
front of an air space, synthetic stone is buttered right to the
wall and has no air space to promote drainage. Use two layers of
housewrap before installing the lath. The outer layer protects the
inner layer from bonding to the mortar holding the stone in place,
which allows the inner layer to function as the drainage plane. The
natural separation between the layers provides a capillary break to
reduce the chance for water wicking through.
Poor Window and Door Flashing
While peel-and-stick flashing has become fairly popular, there is
still enormous confusion in the field about the correct way to
apply it around windows and doors. The biggest problems involve the
flashing sequence. The sill of any opening must be flashed before
the window or door unit is installed. Then install the window using
a compatible sealant, apply jamb flashings, and finally, add a head
flashing.
How a window looks to a water drop.This is
what water running down a wall "sees" as it approaches the head of
the window, underscoring how very important it is to properly flash
at the window head. Sealant alone won't keep the water out. If the
sealant ever separates and fails, you would have a perfect entry
point for water to invade the structure.
Proper window flashing. The sequence of flashing a window
is critical. Begin with a sill flashing that covers the rough sill
and has a bottom flap that laps over the top of the housewrap as
shown. This will ensure that any water that leaks through the
window drains out. The jamb flashing should go on after the window
has been installed, and should lap over the window flange to
prevent wind-driven water from getting past the window into the
rough opening.
Improper head flashing. The head flashing on this
window is reverse-lapped with the housewrap. Before applying the
head flashing, a flap needs to be cut in the housewrap so the head
flashing can adhere directly to the wall sheathing and cover the
nail fin. Once the flap is folded back down over the head flashing,
water draining down the wall above the window will drain outside,
not behind the head flashing.
Why "splines" don't work.Applying jamb
flashing before the window is installed meets the requirements of
"Method B" of the ASTM window and door installation standard, but
in my opinion it's not the best way to flash a window. You can
still end up with small gaps between the nail fin and the flashing
as in this instance, which allow water to penetrate.
Improper sill flashing. Notice that the black
sill flashing is stuck to the housewrap and then stuck over the top
of the nail fin below the window. This is a mistake. The sill
flashing always goes on before the window is installed.
Site Problems
Along with roof details, poor site drainage can dump an enormous
quantity of water into homes. Builders can best prevent problems by
first siting and grading to keeping runoff away from the home, and
then by installing good perimeter drainage to carry groundwater
away. But the homeowner must remain conscious of these issues as
well.
Steve Easley is president of BMI, a company
that provides training on building science issues and consults with
builders on field issues. All photos are by the author.
Dreadful drainage. In this coastal subdivision, water
running off one series of houses drains right into another. There
is not enough natural drainage away from the lower structures to
prevent subslab moisture.The homeowner factor. Imagine the force of this
sprinkler and the amount of water that is pouring into this wall.
This is a perfect example of why redundancy is so important. The
only way to avoid problems caused by the homeowner irrigating this
wall is to make sure a WRB behind the brick is impeccably detailed,
the drainage cavity and weep holes remain open, well-draining
backfill is put in, and good perimeter drainage is
installed.
Maintaining drainage.When I inspect a home
for moisture problems, I look for signs of lack of maintenance. For
example, here the downspout is disconnected from the leader, so all
the water that runs off the roof is being funneled near the
foundation. This can create a tremendous moisture load on the
home.