How to Install Exterior Window Casing

Exterior window casing serves both a decorative and functional purpose. Casing augments the finished appearance of a home and, in many cases, ties a home's siding to its framing and seals small gaps around a window opening. Successful installation of exterior window casing requires only a basic knowledge of trim, molding and carpentry and helps an individual develop skills applicable to a wide range of carpentry tasks.

Things You'll Need

• Compound miter chop saw
• Level
• Tape measure
• Pencil
• Framing square
• Finish nail gun
• Fasteners
• Sealant

Instructions

1. Take measurements for the length of each piece of window casing. Do not assume the equivalence of opposite sides of the window -- manufacturing mistakes occur, as well as alterations and damage during installation. Stand on the exterior side of the window. Measure the window opening on the inside of the window frame from top to bottom on each side of the window. Measure the distance from the left side to the right side of the opening on both the top and bottom. Write down all measurements for future reference.

2. Understand how the measurements relate to the finished pieces of casing. Because you measured on the inside of the window frame, your measurements represent the "inside dimension" of each window casing piece. To understand the term "inside dimension", imagine this: if each end of a piece of casing is cut at an angle that tapers toward its opposite end, then that piece of casing forms a trapezoid. Excluding the angled ends, the trapezoid has one long side and one short side. Carpenters refer to the short side of this trapezoid as the "inside" -- this edge of the casing rests on the "inside" of the finished rectangle that installed casing creates.

3. Mark and cut the top piece of casing. A framing square allows you to lay out a perfect 45-degree angle. Butt the framing square to the flat edge of the casing. Place the square at the end of the piece of casing and use a pencil to mark a 45-degree angle across the piece. Use a tape measure and pencil to mark the opposite end of the dimension, reposition the framing square and draw another 45-degree angle to complete the piece's layout. Use a compound miter saw set at forty-five degrees to cut the casing at the marked positions.

4. Install the top piece of casing. Hold the casing flush with the inside edge of the window frame, position it so that its inside corners meet the inside corners of the window frame and use the nail gun to fasten it in place.

5. Continue to mark, cut and install casing pieces. After the top piece, complete the bottom piece. Before installing the side pieces, use a level to ensure that the edges of the top and bottom pieces are plumb with one another. Finish by installing the side pieces and sealing or caulking joints and gaps.

Tips & Warnings

• Apply a coat of primer and paint to all sides, edges and cuts on casing prior to installation. Coats of primer and paint prevent moisture and pest penetration and extend the life of your casing.

• If casing is to be installed properly the window must be plumb and square in its opening. Use a level to check the window for plumb and square prior to casing installation.