Q: A photo in the “Flexible Flashing” article in JLC’s April issue shows accordion-style flashing around a circle-top window. How do you flash around the curved window if you have only straight flashing tape?
A: Mike Guertin, contractor and presenter at JLC Live, responds: As with every detail like this, proper installation begins before the window goes in. After running the housewrap (WRB) over the opening and making the center and sill cuts for the rectangular part of the window, make a horizontal slit across the mull line (where the curved and rectangular sections meet) of the rough opening. Cut the WRB along the circular part of the RO, and extend the cut for the mull line out about 3 inches on both sides of the opening with the cuts angled upward. Continue the cuts vertically up about 8 to 10 inches past the top of the RO. Fold the top flap up and temporarily tape it above the opening. Detail the WRB and sill pan flashing for the lower rectangular section as you would for any window. Install the window and run flashing tape along the sides of the window, letting it extend a few inches past the mull line.
For the curved window on top, there are many brands of flexible flashing tape (like the one in the article) that make for a seamless installation. However, in a pinch, you can revert to the method we used before those so-called accordion tapes were available: layering strips of straight flashing tape along the arc of the window. The goal is to use as few pieces as possible; otherwise, the layers will build up, preventing your trim and siding from lying flat.
One way to use fewer pieces is to cut the window arc out of the straight flashing tape. I’d use 6-inch or wider tape so that you still have plenty of coverage even after cutting the arc. Start with a piece about 18 to 24 inches long and place it over the curved window frame. Trace the curve of the window, making sure that you leave at least 4 inches of tape at the deepest point of the arc. Then use scissors or a sharp knife to cut along the curved line. Apply pieces starting from the outer (lower) edges on both sides of the curved window, working upward to the top of the curve. The number of pieces used depends on the width of the flashing tape and the size of the window, but often you can flash a curved window this way with just three or four pieces.
Next, roll the flashing tape against the window flange and the sheathing, and fold down the top flap of WRB. Many installers simply tape over the slits to complete the installation, but I weave extra pieces of WRB into the vertical slits to create a mechanical lap as well, and then I tape all the joints. That way I’m not depending on the tape alone to complete the WRB around the window—an inexpensive and easy way to ensure greater durability.
A word of caution: Be sure to check the compatibility of the flashing tape with the window brand you’re installing. Asphalt-based tapes can’t be used to flash some windows because the asphalt can react with plasticizers in the window’s vinyl frame, flanges, or gaskets. Butyl and acrylic adhesive tapes are usually compatible.