How to Trim a Window Exterior

Weather-Tight Seal

• Even though well-chosen window trim looks good, that's not its main purpose. Trim installed over siding provides the last barrier against inclement weather. Flashing installed before you lay in the window works with the trim to prevent water and air leaks inside the home. Beneath the siding, the vapor barrier should butt against the edge of the window. Caulk cracks and edges, then cover with a foil tape fitted against the top, bottom and sides of the windows before adding siding. Metal L-shaped flashing above the window can prevent water drips and leaks.

Measure and Measure Again

• To get window trim to fit tight against the window, you need to take accurate measurements of the window's dimensions. Make it a practice to measure each dimension twice. If you get a different measurement on round two, add a third round. Accurate measurements prevent waste and ensure your cuts are correct.

Casing Trim

• Simple trim begins with one-by-four exterior casing, cut in a box shape to fit flat around the window. This type calls for butting the casing and jambs up against the window's outer edges. You can miter-cut the corners at 45-degree angles or you can have the top and bottom boards extend beyond the window the width of the trim or beyond and sandwich the jambs between them. Install the trim with screws that are deep enough to penetrate the trim, the siding and sheathing, and the studs for the best results.

Exterior Sill

• For a different architectural touch, you can make the bottom or top trim perpendicular to the wall, instead of flat against it. This method uses the same trim around the window, but you need to screw the sill nose into the side trim from beneath or from the top, to ensure it does not come apart with weather. The jambs sit flat against the siding, while the sill nose juts out from the wall.

Visual Appeal

• The dimensions of the window, their placement on the wall, the type of siding and the architectural style of the home determine the best proportions of the trim to use. Standard trim comes in cut widths of 3 1/2 and 5 1/2 inches, which may not suit the design of your home. If you want more visual appeal, consider arches, decorative or routed trim, shutters, faux vents, cornice box, crowns or friezes above your windows, and an apron board beneath.