How to Install Window Casings


Installing window casing is similar to installing casing on a doorway. The main difference is that the bottom frame of the window needs to be trimmed while a doorway does not. There are several methods of installing window casing. The first method uses mitered corners; the second uses a decorative block at the corners.

Things You'll Need

  • Combination square Pencil 10-foot tape measure Wood glue Step stool Molding for door casing Plinth blocks--method 3 only Miter saw or miter block and saw Hammer Finish nails Countersink Paint-able caulk or wood putty

Mitered Corners

  • Set the combination square at 3/16 inches for marking. At each corner, mark the horizontal and vertical window frame pieces for a 3/16 inch reveal. Also mark the middle of each member, horizontal and vertical for the same reveal.

  • Cut the bottom casing for mitered corners. Measure the distance between the two marks on the vertical framing members at the bottom. Measure and cut a piece of casing to fit. The thin edge of the casing will be the shortest.

  • Position the bottom casing with the inner miter points in line with the vertical marks and the casing itself with the horizontal marks on the bottom window frame member. Nail in place with finish nails. Nail the casing to both the rough frame and the window frame.

  • Measure and cut the side casing pieces from the bottom casing inner miter point to the horizontal mark on the top frame member. Measure each side and make the cuts.

  • Position the casing pieces and align them with the marks made earlier on the side window frame members. Nail in place, with nails in both the window frame and the rough frame.

  • Measure and cut the top casing piece by measuring between the two side pieces already in place. Position the piece in place, aligning the mitered corners for a tight fit. Nail in place to the rough frame and the window frame.

Alternate Bottom Casing

  • Measure between the vertical marks at the bottom of the window frame. Add this measurement to twice the width of the casing material plus two more inches.
    Example: Measurement = 30 inches, casing is 3 inches wide.
    30 + 3 + 3 + 2 = 38 inches

  • Cut the casing on edge, rather than flat. Miter the ends with the saw set the same as if you were mitering the corners.

  • Cut matching pieces to "turn the corner." Glue these pieces to the ends of the casing with wood glue. Hold the pieces in place with painters tape for one hour.

  • Nail the casing in place, aligning with the horizontal marks on the bottom part of the window frame.

  • Measure, cut and install the remaining three pieces of casing as for mitered corners above, except that the bottoms of the side pieces are straight across cuts.

Plinth Block Installation

  • Set the combination square at 3/16 inches for marking. At each corner, mark the horizontal and vertical frame pieces for a 3/16 inch reveal. Also mark the middle of each member, horizontal and vertical for the same reveal.

  • Follow steps 1-4 for an "Alternate Bottom Casing" above.

  • Position and nail plinth blocks in the upper corners of the windows. Align the blocks as shown in the illustration with the marks made on the upper corners of the window frame. Use the combination square as a guide to insure the block placement and position is perfect.

  • Measure and cut the remaining three pieces of trim to fit between the two blocks, the left block and the bottom casing, and the right block and bottom casing.

  • Align the pieces with the blocks and marks and nail in place. Nail the pieces into both the rough frame and the window frame.

Tips & Warnings

  • Make precise measurements and cuts. Take measurements up close and looking straight on at the tape. Always measure the same way and when marking for cutting. Be precise in your measuring and cutting. Your work reflects the care taken during cutting and measuring. Always check to make sure you are cutting to leave your work piece the correct length. The blade of the saw removes 1/8 to 3/16 of an inch of material, more than enough to leave a big gap in your work if you cut on the wrong side or too close. A miter saw is much easier to use than a miter box. Don't count on the the ends of moldings to be square when you purchase them. Always cut a quarter to half inch off the ends before measuring them for cutting. Finish nailers save time, set the nails below the surface and are less likely to split the wood. Nail the casing to the rough frame and the window frame. Most windows need two nails at each end of each piece of casing and two more in the middle for a total of three in the window frame and three in the rough frame. For longer runs, add extra pairs of nails equally spaced. The time to make sure a window frame is square is before you begin trimming. Hold a 2-foot level against the top and sides of the frame. If the top is not level or the sides plumb, make adjustments. You'll find it nearly impossible to make mitered cuts line up on an out of plumb window frame, and the joints between blocks and casing will show gaps as well. If you will be painting the trim and window, use painters caulk to fill any gaps left when you are finished. Always paint with primer first, even if the casing came primed. The primer they use is cheap and you'll end up putting two coats on anyway.
  • Always wear safety glasses when cutting wood, pounding nails or using a nail gun. Always wear a dust mask when cutting wood. Hearing protection is a must when using a miter saw.

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  • Photo Credit Photo by Zach Adel, Illustrations by MJ Logan
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