If I’m installing half-round windows or arched windows with a consistent radius, I take care of the wall finish first. Although this method works with all sorts of siding, let’s use shingles as the example. Instead of jigsawing each shingle to fit, I let the shingles overlap the rough opening. While doing this I make sure that no nails are driven within a few inches of the rough opening. This ensures an unobstructed space under the shingles for my flashing.
Next I tack a piece of thin plywood (1/4-in. plywood will do) over the area to be occupied by the window. This is my flat work surface. Then I attach a trammel to my router, and I screw it to the center point of the arch. Using a straight-flute bit, I make my first router pass deep enough to cut through the plywood. Then I continue with more passes until I’ve cut through the shingles. If the pivot point is placed correctly, the opening I get will fit my window and its trim.
David Hornstein, Arlington, MA