How to Replace Window Flashing With a Wood Exterior

Remove the trim from the outside of the window frame, using a pry bar. If you remove it with care, you can reuse the trim.

Pry off siding starting at the highest point on the wall above the window where you need to replace window flashing. Again, handle each piece with care so you can reuse it. Either stack the pieces in the order you remove them, or number them on the back with a marker. If you need to remove the flashing on both sides of the window as well as above and below it, remove that siding.

Use the pry bar to remove the old flashing from the house. Dispose of the old flashing.

Examine the sheathing for signs of rot. Look for soft spots in the wood. If the wood is rotten, cut out the damaged sheathing with a circular saw and replace the sheathing with exterior grade plywood. Cover the plywood piece with builder's felt attached with 1 1/2-inch roofing nails.

Measure and cut, using a utility knife, a piece of self-adhesive builder’s felt 24 inches longer than the window’s height for each side of the window. Measure and cut two pieces 24 inches wider than the width of the window. This product typically comes in rolls 18 to 24 inches wide.

Peel the backing off the builder’s felt and adhere a width piece to the existing builder’s felt or house wrap so it’s centered underneath the window. Place the two vertical strips centered along the sides of the window. Set the final short piece in place centered on top of the window. Run your hand firmly over all the seams to seal them.

Cut metal flashing the same length you cut the strips in Step 5. Attach it to the perimeter of the window, using 1 1/2-inch roofing nails. Start with the base of the window, attach the sides next and finish with the flashing over the window.

Cover the flashing with self-adhesive waterproof membrane cut to fit the length and width of the flashing. Seal all edges by running your hand over them.

If any of the exterior siding shows signs of rot, replace the boards with new ones. Then reattach the siding starting with the lowest boards and working your way up the wall using nails similar to those you pried out earlier. Reattach the trim around the window.