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.
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