Flashing and Trimming a Window

To prevent window leaks — along with the mold growth, rot, and callbacks from irate clients that go with them — the author has developed a "bombproof" method of keeping window openings reliably watertight. Here, the head flashing is being applied, but the process starts with planning, precutting flashing strips, and applying them from the rough sill and working upward along the side jambs to the header.

Assuming that housewrap is present and properly installed, the first step is to cut it back from the rough sill and side jambs about 3 inches, or the width of a level.

At the head of the rough opening, cut a flap of house wrap as shown and tuck it up out of the way.

Transferring the required dimensions of the flashing pieces to a plywood story board makes it easier to cut them accurately to length.

Fold each piece of flashing material twice lengthwise so it's short enough to cut using a standard framing square for a straightedge.

Trim the flashing material with a utility knife using a framing square for a guide.

Cut "bowtie" strips, which are applied to seal corner seams, from flashing material scraps.

Bow ties are narrow toward the center and widen toward the ends so they can wrap neatly into tight corners and seal effectively.

Start at the bottom of the window opening. Adhere a flashing strip to the sheathing and housewrap as shown.

Make a cut at the corner to create a flap.

With a cut where the rough sill joins the jamb at the far end, half of the flashing strip is folded down onto the sill. The process is repeated at the other corner. If you're working alone on a window, do one corner at a time to prevent the sticky flashing membrane from wrinkling during application.

One end of a bow tie is adhered to the flashing membrane outside the opening, the other half folded to the inside to seal the joint between the rough sill and jamb.

When applied correctly, the narrow center section of the bow tie will turn neatly up the corner of the opening.

Apply side flashing from the top. First align the edge of the membrane with a pre-drawn pencil guideline on the housewrap measured to position the strip correctly, then peel away the backing and adhere the strip to the housewrap and sheathing.

Cut slits at the bottom and top of the opening to create a flap. Fold it into the opening against the rough jamb.

Press the side flashing flap into the bottom and top corners as shown.

Apply the head flashing flush with the outer edge of the rough header.

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