How to Finish Window Trim

Select your wood supply for your windows. For the exterior side of the window some 1-by-4-inch clear fir is ideal, even if you plan to paint the wood. Make sure you have enough on hand to finish the job, for the size of a window affects how much wood you will need. Perhaps by buying your lumber in different lengths, you can save a little on your purchase. Careful calculations should reveal just how many boards you do need.

Check the jamb and exterior sheathing to make sure everything is tight and firm. Trim is usually installed before the siding is put up, so you ought to have the opportunity to examine everything closely.

Install the two side pieces first if you are going to butt your joints. Cut both side pieces with square ends. The length should begin at the bottom edge of the top piece of jamb and run to the top edge of the bottom piece. If you wish to set your trim back a little bit (this is very advisable) then add 1/2 inch to each piece.

Prime the two pieces on all sides and nail them into place using long galvanized finish or casing nails. Set each piece back 1/4 inch from the edge of the jamb and split the extra half-inch of length, so part of it falls at each end.

Measure and cut the two top pieces. Be sure to square off each end at exactly 90 degrees. Both the top and bottom piece should run from outside edge to outside edge of the pieces that you just nailed into place. Paint these first and then nail in the same manner as before.

Add a piece of drip cap molding to the top piece of trim, especially if you live in a place with a high or normal rainfall. Nail this piece to the top surface of the window trim so that it can disperse water away from the side of the building along the top of the window.

For mitered corners, cut your trim at a 45-degree angle on a miter saw. When you do this, you can still cut the two side pieces as before, except the cuts will be 45-degree angles at each end. It is also a good idea to also set the mitered trim back by 1/4 inch from the edge of the jamb, so figure in an extra 1/2 inch for each piece. Go ahead and nail the two side pieces, but you will be marking the cuts on the top and bottom piece instead of setting the saw to 45 degrees. Once the angles of the two cuts are penciled in, then go ahead and cut the piece with a circular saw, being sure to follow the pencil marks carefully. Then prime and nail until place. Then add a drip cap if necessary.

Set all nails and then caulk the holes and all seams with an outdoor-grade white caulk.