How to Trim an Exterior Window With Wood

Mark the reveal on the exterior face of the window jamb. Either use a tape measure or set a combination square to the desired depth and mark the reveal every 6 inches on all four sides of the window and in each corner.

Measure from reveal mark to reveal mark across the top of the window. This measurement is the length of the top trim board measured from short point to short point.

Cut the top board by setting the miter angle to 45 degrees on the right side of the saw. Place the trim on the right side of the saw and cut. This will leave the top of the trim, that is pushed against the fence of the saw, longer than the bottom side of the board. The top point is called the long point and the bottom point is known as the short point.

Measure the desired distance from the short point and make a mark along the bottom corner of the trim.

Flip the saw to the other 45 degree mark on the left side of the saw and slide the trim to the left of the saw blade until the blade and the mark line up. Cut the board on this point to complete the top trim board.

Repeat this process for the side pieces and the bottom piece of casing.

Fasten the miter joints by nailing through the side of one trim piece into the other piece.

Nail the top piece in place by lining the bottom of the board up on the reveal marks and nailing through the casing into the window jamb. Be careful not to nail too close to the edge of the board so as not to risk splitting the wood.

Install the remaining three pieces of casing. Apply wood glue to the mitered ends to create a secure joint.

Place a piece of flashing over the top of the window trim. The flashing should extend to the edges of the trim. Nail the flashing to the wall, not to the top of the trim.