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Flashing and Trimming a Window

Flashing the rough opening

Flashing and Trimming a Window

Flashing the rough opening

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    Emanuel Silva

    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.

  • Figure 1. Once the housewrap has been cut back from the sides and bottom of the rough opening by the width of a level (left), a flap along its upper edge is temporarily tucked out of the way (right).

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    Figure 1. Once the housewrap has been cut back from the sides and bottom of the rough opening by the width of a level (left), a flap along its upper edge is temporarily tucked out of the way (right).

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    Emanuel Silva

    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.

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    Emanuel Silva

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

  • Figure 2. Transferring the dimensions of the flashing pieces to a plywood story board makes it easier to cut them accurately to length (A). Each piece is then folded lengthwise twice (B) so its short enough to cut against a standard framing square (C). Corner pieces, or bow ties  measuring about 112 inches by 8 inches overall  are cut from scrap material (D, E).

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    Figure 2. Transferring the dimensions of the flashing pieces to a plywood story board makes it easier to cut them accurately to length (A). Each piece is then folded lengthwise twice (B) so its short enough to cut against a standard framing square (C). Corner pieces, or bow ties measuring about 112 inches by 8 inches overall are cut from scrap material (D, E).

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    Emanuel Silva

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

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    Emanuel Silva

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

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    Emanuel Silva

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

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    Emanuel Silva

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

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    Emanuel Silva

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

  • Figure 3. After the sill flashing has been adhered to the sheathing and housewrap (A), a vertical cut at the corner forms a flap that is folded against the sill (B). A partial cut at the far end is snipped through when the flap has been folded down halfway along its length (C).

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    Figure 3. After the sill flashing has been adhered to the sheathing and housewrap (A), a vertical cut at the corner forms a flap that is folded against the sill (B). A partial cut at the far end is snipped through when the flap has been folded down halfway along its length (C).

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    Emanuel Silva

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

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    Emanuel Silva

    Make a cut at the corner to create a flap.

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    Emanuel Silva

    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.

  • Figure 4. One end of a bow tie is adhered to the flashing membrane outside the opening, the other half folded to the inside (left). When applied correctly, the narrow center section will turn neatly up the corner of the opening (right).

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    Figure 4. One end of a bow tie is adhered to the flashing membrane outside the opening, the other half folded to the inside (left). When applied correctly, the narrow center section will turn neatly up the corner of the opening (right).

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    Emanuel Silva

    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.

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    Emanuel Silva

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

  • Figure 5. Side flashing is applied from the top; the backing paper is peeled away as the material is aligned with the pencil line on the housewrap (A). A precut slit at the bottom creates a flap thats folded into the opening and pressed into the corner (B, C). Head flashing is applied flush with the edge of the opening (D).

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    Figure 5. Side flashing is applied from the top; the backing paper is peeled away as the material is aligned with the pencil line on the housewrap (A). A precut slit at the bottom creates a flap thats folded into the opening and pressed into the corner (B, C). Head flashing is applied flush with the edge of the opening (D).

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    Emanuel Silva

    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.

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    Emanuel Silva

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

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    Emanuel Silva

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

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    Emanuel Silva

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

Because window flashing disappears from sight once the window itself is in place, it’s easy to rush through that part of the job and move on to something else. Windows are sometimes installed with no flashing at all, and in my work as a remodeler I often see windows that are installed right over the housewrap, with the adhesive membrane applied over both the housewrap and the window flange. That’s better than nothing, but not by much — it can’t stop any water that gets behind the housewrap above the level of the window, since by then water will already have gotten behind the flashing as well.

To prevent window leaks — along with the mold growth, rot, and callbacks from irate clients that go with them — I’ve developed a bombproof method of keeping my windows reliably watertight. It includes several tips and tricks I’ve learned from other builders, and I’ll probably continue to tweak and improve it in the years to come. The key is to take accurate measurements and work methodically; cutting flashing by eye and sticking it in place freehand won’t give you consistently good results.

Flashing the Rough Opening

Assuming that the housewrap has been properly lapped and fastened to the sheathing, my first move is to cut it back from the sides and bottom of the opening by the width of my level, using the level itself as a straightedge. I leave it flush with the edge of the opening at the top (see slideshow). I then make a 6-inch cut at each of the top corners, angled back from the opening at 45 degrees, and temporarily tuck the resulting flap under itself to get it out of the way.

To make it easier to put the flashing on straight, I use my level again to draw plumb and level lines at the sides and bottom of the opening. The distance the lines are from the opening depends on the width of the trim, since the flashing needs to extend far enough to protect the joint between the casings and siding — but 6 inches is usually fine.

Cutting the flashing membrane. There are two separate sets of flashing pieces: the inner pieces that are applied to the sheathing and fold back over the framing, and the outer pieces that cover the window flanges. I cut both sets at the same time, determining their lengths by measuring directly from my markings on the housewrap. To calculate the width of the inner pieces, I add the distance the lines are set back from the opening to the depth of the framing, plus the thickness of the sheathing. For the 2x4 framing shown here, that came to 6 inches plus 3-1/2 inches plus 1/2 inch, or 10 inches even. The outer flashings are 3 inches or so wider than the window flanges themselves.

For accurate cutting, I transfer those measurements to a story board — fashioned from a piece of plywood about a foot and a half wide by 8 feet long — that I attach to my work table. I prefer Grace Vycor flashing because it’s premarked at 6- and 12-inch intervals, which simplifies measuring. I ordinarily buy it in the 12-inch width. Once I’ve cut the main flashing pieces to length and width, I cut a couple of “bow ties” to seal around the corners of the opening.

Flashing the sill and sides. The sill piece is applied first. If the rough openings have been sized to allow it, I’ll tack a full-length piece of clapboard to the rough sill before applying the membrane, creating an outward slope that will cause any water that somehow makes its way past the window to drain to the outside. In practice, though, this isn’t always possible — I had to omit the clapboard from the job photographed here because including it would have left insufficient clearance at the window head.

To apply the membrane, I first peel back enough of the backing to accurately position one end, then peel off the rest, keeping the bottom edge aligned with the pencil line on the housewrap. Next, I cut most of the way through one end of the flap that extends above the opening — starting from the corner but leaving a narrow connecting piece at the top — before completely cutting the other end free and folding it down against the sill. Once I’ve worked my way halfway down the sill, I cut through the last bit at the far corner with a quick jab of my utility knife and fold down the rest of the flap. This two-stage approach makes the material easier to control and helps prevent the formation of wrinkles and air bubbles.

Finally, I apply a bow tie to each bottom corner, which serves to seal the small pinhole where the side flashing will later fold over the sill flashing. Although this area is often ignored, it’s a common source of leaks. When properly applied, the bow tie catches the end of the horizontal flap stuck to the sill, and turns the corner to extend a fraction of an inch up the trimmer stud.

Side and head flashing. The side pieces are applied in much the same way as the sill piece, except that I start at the top and work down. But one important preliminary step comes first: When the side pieces are still on the story board, I cut a vertical slit through the bottom of each to create the flap that will be turned up and stuck to the horizontal sill flashing.

Once I’ve pressed the membrane against the sheathing and housewrap, I carefully fold the side flap into the opening and stick the slitted section flat against the sill, using a layout square as a guide to form a neat inside corner. I then work my way back upward, pressing the side flap against the trimmer and cutting it loose at the top when I’m partway along, similar to how I installed the sill flashing.

With both pieces of side flashing in place, I’m ready to install the head piece. This is easy, because there are no folds to make — the membrane is just stuck to the sheathing above the window, its lower edge flush with the opening and the ends lapped over the side flashing.