How to Do Aluminum Capping

Pry up a row of siding above the window, using a small pry bar. Clean the underlying wooden surface with a steel brush to remove any debris. Wipe the surface with a piece of dry rag.

Cut the aluminum capping according to the width of the window with metal shears, but leave an extra inch at each end. Apply flashing tape on top of the window, then slip the capping under the siding. Press the aluminum capping against the flashing tape.

Nail the capping onto the wall, but make sure that the cap slopes away from the window. Apply caulk along the top edge of the aluminum capping, then apply roofing tar over each nail head.

Cut the extended end of the aluminum capping with metal shears, then fold the aluminum sheet downward along the edge of the window. Apply exterior-grade sealant along the edge of the drip cap to prevent moisture from penetrating.

Replace the siding, but provide a clearance of 1/4 inch between the capping and the bottom of the siding to prevent water from seeping under the siding. Apply caulk on both ends of the capping.