How to Replace an Aluminum Exterior Window Trim

Remove trim to be replaced with a pry bar. Pull out any nails and peel trim off the wood framing if it is attached with double-sided tape. Inspect the wood and replace any damaged parts; aluminum trim generally is fastened to wood brick mold on the top and sides and a sill across the bottom. Loosen nails holding aluminum siding if necessary to allow a piece to be pulled out. Reinstall siding before installing the new trim.

Install the sill cover first from the trim kit. Slide it under the brick mold and mark places with a pencil to outline the brick mold. Cut those sections out with tin snips so it will fit under the wood and flush against the edge of the window. Make end covers for the sill by cutting out a rectangle in the bottom of the trim plate. Nip the 90-degree angle at the top and fold the end piece over the wood sill and the top piece down over it to seal it. Fasten the end with an aluminum nail.

Cut the side pieces from the window trim kit to length and notch the inside edges so they fit around the brick mold; leave a top flap on the outside edge to bend over the top brick mold. Miter the top angles at about 45 degrees.

Trim a top piece to the width and notch out the bottom element so it fits snugly inside the two side pieces. Miter the ends to match the angles on the side pieces, allowing a slight overlap at the seam. Cut a flap on the top edge to fold down over the side trim.

Fasten all trim pieces with a single aluminum nail on each corner side and at the bottom of the vertical pieces, driven through the trim into the brick mold with a tack hammer. Add a couple of small nails on the inside edges of the trim if desired. Caulk thoroughly both inside and outside of the trim to seal it. Leave the bottom of the trim without caulk so any water that does get in can drain out.