Window frame flashings probably need to be repaired or replaced if you are experiencing leaks. A type of peel-and-stick membrane, window flashing prevents water from leaking around the window and into the house. Over time, siding or other exterior elements cut or wear away the flashing, creating a need to replace it. In some cases, such as with a double-hung window, homeowners place additional pan flashing along the bottom edge of the hole for the window before a window is installed to prevent leakage.
Remove any other nails and screws around the window, if necessary. Peel back the old flashing and remove it from the window if necessary. Remove the old window out of the window hole. In some cases, you may have to peel back the siding slightly to remove the window.
Measure the inside, bottom corners of the window. Cut a piece of flashing membrane to fit into each corner. Apply the flashing in each corner over the seam where the larger pieces of flashing meet. The flashing patches should overlap the other pieces. This process will help reduce the chance of leakage.