How to trim a round top window

Written by jonah morrissey
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How to trim a round top window
(Hemera Technologies/ Images)

Roundtop windows add a decorative element to your home. Installing trim around these windows can be an interesting woodworking project for a homeowner. The key to adding trim around a round-top window is to make a curved section of casing for the top of the window. The curve should be the same as the curve of the window. Adding this casing to the top of the window will enhance the flowing lines of the round-top window and complete the look on the exterior of your home.

Skill level:

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Things you need

  • 1-inch thick-by-4-inch-wide window casing, 96-inch lengths
  • 1-inch thick-by-(width required for the curve)-inch window casing, 60-inch length
  • Pencil
  • Mitre saw
  • Flexible curve ruler
  • Band saw
  • Fine-grit sandpaper
  • Window and door caulk
  • Caulk gun
  • Box, 10d finish nails
  • Hammer
  • Nail set
  • Wood putty
  • Putty knife
  • Primer/paint

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  1. 1

    Measure from the sill to the top of the window opening. This is the length for the side casing. Measure the width of the window opening at the top of the window. Add 7 inches to the width measurement and you will have the length of the top window casing.

  2. 2

    Measure, mark and cut two side casing boards to length out of the 1-by-4-by-96-inch boards. Measure, mark and cut one top casing board to length out of the 1-by-(width required for curve)-by-60-inch board based on your measurements for the window in step 1. Make the cuts with a mitre saw.

  3. 3

    Place the top casing board flat on a work surface. Form the flexible curve ruler to the curve of the window. Position the ruler end to end on the bottom lengthwise edge of the top casing board. Transfer the curve to the board with a pencil.

  4. 4

    Position the ruler end to end so that the centre is flush with the top lengthwise edge of the top board. Transfer the curve to the top edge of the top board with a pencil.

  5. 5

    Cut the curve out from the top and bottom lengthwise sides of the board using a band saw.

  6. 6

    Sand the cut edges until they are smooth using fine-grit sandpaper. Always sand with the grain of the wood.

  7. 7

    Apply a bead of window and door caulk on the back side of each side and top casing board. Place the side casing boards on each side of the window. Rest the top casing board onto the top ends of the side casing boards.

  8. 8

    Drive 10d finish nails every 6 to 8 inches through the casing boards into the rough opening framing of the house using hammer.

  9. 9

    Set the nails 1/16-inch below the surface of the casing with a nail set. Fill the holes with wood putty using the edge of a putty knife.

  10. 10

    Allow the putty to dry overnight and sand it flush with the surface of the wood using fine-grit sandpaper.

  11. 11

    Caulk around the seams of the casing and smooth out the caulk with the tip of your finger.

  12. 12

    Prime and paint the window casing, allowing at least 4 hours between the primer and top coat of paint.

Tips and warnings

  • Make a cardboard template of the curve if you are making several of the same curved trim boards at once.
  • Wear eye protection when cutting, sanding and installing window trim boards.

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