Wrapping exterior window trim is the process of combining 4 equally-shaped pieces of wood trim that in effect wrap around and decoratively frame the window. This is opposed to what is known as a butted casing where the edges of the side pieces butt up against the top and bottom pieces at a 90 degree angle. Wrapping involves using a miter saw to make a 45 degree angle cut on either end of each piece to create a trapezoidal shape. The length will vary depending on the size of the window.
Wrapping Exterior Window Trim
Along the jamb of the window on the edge of the 3/4 inch wood used, mark a reveal line. This ends up being the part of the jamb that is visible after the trim is in place. It could be 1/8, 1/4 or 5/16 of an inch. Draw this line in light pencil all around the jamb to give you a placement reference for the trim.
Measure from inside edge to inside edge on each side, from where the reveal line meets in the corner. Make your 45 degree angle cut on the miter and hold each corner up to the window to check your cut. Once you are satisfied it is flush, nail 2 finish nails in each piece.
If there are no gaps in the miter cuts, make sure the reveal line is visible all around the jamb and set a finish nail on the inside of the trim every 8 to 12 inches.