Replacing a Drip Cap
That wooden edge over your windows serves a purpose
I know that wood drip caps are often used where the siding meets the top of a window, door, or water table. This is a nice detail, but when it rots out, how do you replace it?
— George, East Greenwich, R.I.
Norm Abram replies: A drip cap helps direct water away from the top of a window or door. Another version, called a water table, does the same thing along the bottom course of siding. These caps should be made of a rot-resistant wood such as heart redwood or red cedar (primed on all sides before you install it) or a rotproof cellular PVC. But if one ever does fail, replacing it isn't that hard.
First, remove the course of siding above the window, pry out the old drip cap, and clear away any debris. Then seal above the window with a 4- to 5-inch strip of self-adhering waterproofing membrane trimmed flush with the top of the window casing.
When you're ready to slip the new cap into place, run a generous bead of exterior-grade sealant along the joint where the top of the casing meets the membrane. Then put the drip cap in place and caulk along the joint where its top edge touches the membrane. Don't ever nail a drip cap to the wall; let the siding hold it in place. (Nails provide an avenue for water to reach the sheathing.)
If your walls are shingled, go ahead and install them; the job is done. If you're reinstalling clapboard, however, squeeze a final bead of caulk along the top of the cap, then bed the clapboard's bottom edge in it. Don't nail the clapboard in the area over the window; you'll split the siding. Instead, nail it on either side of the window, as shown in the illustration.