Any carpenter who has done remodeling work has suffered
through this scenario: You pull out an old flanged window
for replacement and recognize the telltale signs of a leaky window:
rotted framing and sheathing, even soaked insulation
infested with mold. That’s when a simple window swap-out
turns into a headache project, complete with a customer who’s
grouchy over a larger-than-expected bill.
Until recently, many flanged windows were installed by simply
cutting X’s in the house wrap and popping them in. But that
led to a lot of leaks—and a whole lot of unhappy customers.
Now, new materials and techniques make it possible to install
windows without any worry of callbacks. For only an extra 15
minutes of effort and a few dollars worth of materials, you can make any window installation watertight and lawsuit-proof.
It starts with the rough opening
The most important part of the process is
the “sill flashing” (Figure A and Photo
3). It’s a piece of preformed plastic flashing
or a site-built system (see “Site-Built
Sill Flashing,” below) that slopes away
from the opening, so any water that gets
behind the siding won’t collect under
the window and cause problems. But
rough openings provided by manufacturers
don’t generally allow for the thickness
of sill flashing. If you’re going with
premade plastic sill pans, add an extra
1/4 in. of height to the R.O. If you’re
building your own sill pan, add the thickness
of the siding plus 1/8 in. If you’re
dealing with an existing R.O., you can try
to dry-fit the window and see if there’s
enough space left over for shimming and
insulation. Otherwise, it’s a good idea to
tear out the 2-by sill framing and replace it
with 3/4-in. plywood, and then shim as
Figure A: Prep the Opening
Figure A: Prep the Opening
The most important part of weatherproofing begins before the window goes in.
After the house wrap is cut, tuck the bottom flap into the house and install sill
flashing. Then fold in the sides of the house wrap, but leave the top flap alone.
Cut the house wrap with care
Don't start hacking away at the house
wrap, especially if you're new to window
flashing. You're bound to miscut something
because it's tough to see the actual
opening. Instead, feel the window opening
through the house wrap and mark the cuts
first. Then slit the house wrap and tack the top and side flaps out of the way with a staple
or two (Photo 2). Drape the bottom flap
into the opening and staple it to the sill.
Keep in mind that the house wrap is your
last line of defense against any water that
finds its way past the siding. So if you do
blow any cuts, patch them with house wrap
Install the sill flashing
Some window manufacturers offer sill
flashing as options when you buy the window.
But if not, there are plenty of aftermarket
suppliers. Plan on using sill flashing. The one
we’re using comes in three parts:
two preformed corners and a center section
that you cut to length to fit. Start by
nailing the corners in place under the
house wrap at the sides and over the
house wrap below (Photo 4). Then cut
the center section to fit. The flashing is
easy to cut with tin snips. Pay attention
to the raised ribs, which provide a flat
surface where the window frame rests.
You have to cut the middle section so
that it overlaps the corners 1/2 in. Think
it through before you cut to make sure
the ribs don’t overlap. Add caulk at the
laps and nail the flashing into place with
roofing nails. Then use your thumb to
caulk the nail heads.
Time to slip the window in
Fold the house wrap sides in, staple
them to the room side of the trimmers
and king studs, then slice off the excess
on the inside of the house. Leave the top
flap alone for now. But seal the overlap
at the bottom corners with house wrap
tape. Then go ahead with a normal installation.
If you're new to the game, just follow
the window installation instructions.
Don't skip the corner gaskets if the window
comes with them (Figure B). They
keep out water, and leaving them off can
void the window warranty.
Figure B: Seal Around the Window
Figure B: Seal Around the Window
Seal around the window with flashing tape, lapping the
top piece over the sides. Then fold down the house wrap at the top,
and cover the diagonal slits at the top with house wrap tape.
Back to Top
Flashing tape is the first line of defense
Flashing tape (Photo 6) is a thick, pliable,
waterproof tape that goes under or
over nailing flanges, sill and top flashing,
and house wrap. Available wherever
you buy your windows, it’s usually sold in a 50-ft. roll of 4-in.-wide material. Use
4-in. if you’re siding right against the
window. But if you plan to surround the
window with trim, choose 9-in.-wide
flashing to protect behind the seam
between the trim and the siding. Cut the
tape to length with a utility knife and
peel a little bit of the backing free and
stick it to the top of the window. Then lay
it alongside the window and peel away
the rest of the backing as you smooth it
into place. Embed it with a laminate
roller for a good seal. For
the best adhesion in cold weather, hit
the tape with a heat gun while you roll.
Site-Built Sill Flashing
Twenty bucks doesn’t seem like
much for one window’s worth of plastic
sill flashing, but if you’re outfitting
a new house with 30 windows, 600
bucks can be sobering. Especially if
your competitor isn’t flashing at all!
But there is an alternative. For years,
carpenters have built their own sill
flashing from thin-beveled cedar siding
and flexible flashing tape. Rip the siding to the same
width as the sill and stretch the tape
at the corners to eliminate wrinkles.
Here’s some advice. Get your bid in
first and tell the customer about your
waterproofing system right up front.
It’ll come in handy when she asks XYZ
Construction if that’s part of his bid.
His deer-in-the-headlights look and/
or stammering about how it “isn’t
important” will give you a leg up
when it’s time to decide—you vs. XYZ
Construction. Who would you choose?
Replacing an Existing Window
The system we’re showing is for new
construction. Ideally, when you
replace an existing window with a
new one, it should be weatherproofed
using this same system.
Chances are, though, that the existing
windows are missing proper
flashing, house wrap or even felt. The
extent of weatherproofing you should
consider depends on the new window’s
exposure to weather. If the
window’s deep in an entryway and
has zero exposure, you can skip the
weatherproofing and just caulk
around it. If the top of the window is
near an eave but the bottom is
exposed to the elements, strip off the
siding and focus on flashing the bottom
half of the window, but ignore
the top. If the window’s out in the
open, unprotected by overhangs,
strip off as much siding as needed to
expose the opening and perform the
steps just as we show.