Q. What's the best way to flash an arch-topped window?

A.Builder and designer Carl Hagstrom replies: The slickest way I've found to flash a circle-top window is to use DuPont FlexWrap, a flexible flashing tape from the makers of Tyvek housewrap. I refer to FlexWrap as "peel-and-stick on steroids." Like generic peel-and-stick, FlexWrap is self-healing (it seals around fastener penetrations), and it has a very aggressive butyl adhesive backing. It costs about $2 a lineal foot.

FlexWrap stretches over two times its original length (no, that doesn't mean you have to buy only half as much) and will easily conform to any radius circle-top window. I also use FlexWrap for the window sill flashing -- the flexibility allows me to fan out the corners and make a one-piece sill flashing.

The sequence of installation is critical in a properly flashed window. First, the window opening is cut out following an inverted "Y" pattern (the head piece of the old "X" pattern cut directs moisture toward the interior and shouldn't be used), and a head flap is cut and taped up and out of the way above the window. Next, the one-piece FlexWrap sill flashing tape is installed, followed by the window unit installed in a bead of elastomeric latex caulk. Do not caulk the sill flange -- that would trap inside the opening any moisture that finds its way onto the rough sill. An uncaulked sill flange provides a weep area for moisture to escape. Even if the sill is level, shimming the unit provides free space for the water to exit more easily. A fat bead of caulk at the interior side of the sill creates a dam to force water to the sill weep.

Jamb flashing tape is installed next, followed by the FlexWrap head flashing tape. To avoid a reverse lap, the head flashing should adhere directly to the sheathing, not the house wrap. The head flap is then folded over the head flashing tape and taped in place. A second piece of housewrap can be cut and inserted in a slit cut above the head flap. This second piece extends at least 12 inches beyond the head flap cut and is backup protection for the head flap taped seam.