Flashing a Window: New Construction

Cut the Opening

I start by making a level head cut in the housewrap...

...then slice two 45-degree cuts upward from the lower corners.

I finish with a vertical cut from the top down the middle.
Note: Don't make an "X-cut" at the top; this will direct water into the structure.

Secure the Housewrap

Next, I fold the flaps onto the jambs, trim them a bit shy of the interior face of the studs, and tape them to the framing.
Note: This makes the gap at the jambs much easier to foam.

Slope the Sill

I fasten a piece of beveled siding to the rough sill, with the thick edge toward the interior, to direct moisture outside.

Prep the Head

I create a head flap by making two 6-inch-long 45-degree cuts at the upper corners...

... then temporarily fold and tape the flap up out of the way.

Flash the Sill

After taping the housewrap to the studs, I install the sill flashing membrane, cutting it a foot longer than the width of the opening so it can extend up the jambs 6 inches.
Note: I prefer to use Tyvek FlexWrap, a butyl-based flashing membane. It has the self-healing characteristics of generic peel-and-stick but can also be stretched to form seamless corners.

After removing the release paper, I make sure the membrane is centered in the opening...
Note: Flashing membranes are meant to be installed at temperatures of 45 degrees or above. In colder weather, I keep the flashing in a warm place and cut pieces as I need them.

...then lower it onto the sill and press it into place, working from the middle toward the corners.

Seamless Corner

In the corners, I carefully stretch the flashing out to create seamless protection.

The flashing has a memory, so to prevent it from curling back, I drive a cap nail at the outer edge to hold it in place until the adhesive cures (24 to 48 hours).

Finally, I smooth the vertical portion of the flashing against the housewrap.

Caulk the Opening

Before installing the window, I apply a heavy bead of elastomeric latex caulk at the jambs and the head, but I never caulk the sill flange area.
Note: Should any moisture find its way into the rough opening, this caulk-free sill flange, coupled with the sloped sill, will provide a weep area for water to escape.

Install the Window

Next, I install the window, driving roofing nails through the preformed holes in the flanges, spacing them about 6 inches apart or per the manufacturer's directions.

Flash the Jambs and Head
Starting 2 to 3 inches above the window head, I apply flashing membrane over the jamb flanges, letting the tape extend at least to the bottom of the sill flashing.

I always use a J-roller to apply strong, even pressure to the flashing tape.

Next I apply flashing over the head flange...

... lapping it over the top of the jamb flashings.
Note: Applying the head flashing directly to the sheathing provides another level of protection. If any moisture were to find its way behind the housewrap and seep down to the window, it would encounter the head flashing and be directed over the windowhead flange, not behind it.

Lap the Head

Once I've applied the head flashing,

I fold the housewrap flap down over it...

...then tape the 45-degree corners with housewrap tape.

Skip-Tape the Head

I "skip-tape" (apply short pieces of tape with gaps in between) the lower edge of the flap in place.
Note: These skip-taped gaps act as weeps, giving any moisture that gets to the window head a way to escape.

Skip-Tape the Sill Flange

I also skip-tape the bottom (uncaulked) sill flange so that any moisture that gets in can weep out. Skip-taping also limits air leaks, though it's not entirely airtight.

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