An exterior sconce should rest tightly against the wall without any gaps that might let in water. But it’s not easy to get a weather tight fit against siding with overlapping pieces, such as shingles, clapboards, and vinyl.
One solution is a rigid-foam mounting block, notched on the back side to mirror the profile of the overlap. The notches fit on top of the siding as snugly as a puzzle piece; no cutting is needed. These blocks fit over vinyl or wood siding with a 4- or 5-inch-wide exposure and ½– or ⅝-inch-thick butt edges. (If your siding doesn’t match these measurements, you’ll need to make your own block.)
To install the light, simply caulk around the top and sides of the block’s back face, and press it against the siding. Leave the bottom uncaulked so that any water that seeps in can escape. Now pull through the wires from the receptacle box, connect them to the fixture, and screw the light to the electrical box in the wall. Finally, caulk around the sides and top of the lamp base where it meets the block and the sides and top of the block where it meets the siding.
Illustration by Harry Bates