Exterior Window Trim Installation

Accent your home with beautiful exterior window trim.

Exterior window trim frames the exterior side of your windows. It adds a decorative finish to cover the window framing and weatherproofing materials on your home. Flat casing is a basic exterior window trim and measures 5/4-by-4-inches. You can purchase the flat casing primed so that all you have to do is paint it once installed onto the window opening. An important consideration when installing exterior trim is ensuring that it is sealed adequately with window and door caulk.

Things You'll Need

• Tape measure
• Protractor
• 5/4-by-4-inch flat casing, 96 inches long
• Pencil
• Miter saw
• Window and door caulk
• Caulk gun
• Hammer
• Box, galvanized finish nails
• Nail set
• Exterior paint
• Paintbrush

Instructions

1. Measure up from the top edge of the sill to the bottom edge of the top of the window frame on each side of the window opening using a tape measure. This is the measurement required for the side casing. Place a protractor on the end of the sill and measure the angle of the sill. This is the angle required for the bottom ends of the side casing.

2. Place the flat casing boards on a work surface. Measure and mark two side casing boards to length, based on your measurement in step 1, using a tape measure and pencil.

3. Cut the side casing boards to length with a miter saw. Set the flat casing boards flat onto the table of the miter saw with the 5/4-inch edge against the fence. Slide the board over so that the blade lines up with the outside edge of the pencil mark. Guide the saw evenly through the wood to complete each cut.

4. Adjust the miter saw to the angle required for the bottom end of the side casing based on your measurement with the protractor in step 1. Cut one end of each side casing board to the required angle while maintaining the length of the casing.

5. Run a bead of caulk along the back side of each side casing board and along the bottom end.

6. Place one side casing board against either side of the window opening. The inside edge of the casing should be flush with the outside edge of the window frame. Make sure that the bottom end of the casing is tight against the sill. Hammer finish nails through the casing on each edge every 6 to 8 inches.

7. Measure the width of from the outside edges of the side casing at the top of the window opening. This is the length required for the top casing board. Measure, mark and cut a flat casing board to the required length for the top board using a miter saw.

8. Run a bead of caulk along the back of the top casing board with a caulk gun. Apply a bead of caulk to the top ends of the side window casing. Place the top window casing across the top of the window opening so that it rests on the top ends of the side casing. Hammer finish nails along each edge of the board every 6 to 8 inches.

9. Set the nails into the top and side casing approximately 1/16 inch by tapping them with hammer and nail set. Fill the nail holes with a small dab of window and door caulk.

10. Paint the window casing with a top coat of exterior paint.

Tips & Warnings

• Paint window trim in a color that will accent your siding.

• Wear eye protection when installing window trim.

References

• Home Addition Plus: Replacing Exterior Window Trim
• Hammer Zone: Building Traditional Casing for New Windows

Resources

• This Old House: How to Install Window Trim