• In this video, This Old House general contractor Tom Silva explains how to install window trim.


    1 Hold casing against each window side, and make reference marks along the wall.
    2 Measure distance between the marks, then add 2 inches to determine the length of the window stool.
    3 Cut the stool to length using a power miter saw with the blade set to 7 degrees.
    4 Hold stool in place against the windowsill and use scribers or a pencil compass to mark where it meets the window sash and wall.
    5 Trim the stool along the scribed lines using a jigsaw.
    6 Set the stool in place, then use the scribe to mark the stool a second time.
    7 Trim the stool once more with the jigsaw to ensure a tight fit.
    8 Attach the stool to the windowsill using 4d finishing nails.
    9 Mark and cut the side casings to length with the miter saw set to 45 degrees.
    10 Nail casings to the window’s side jambs using 4d finishing nails.
    11 Lay a piece of casing on top of side casings, upside down, and mark where each end extends past.
    12 Miter-cut the head casing to 45 degrees.
    13 Glue and nail the head casing to the window’s head jamb using 4d finishing nails.
    14 Use 8d finishing nails to secure the outer edges of the side and head casings to the wall.
    15 With the saw set to 45 degrees, cut the apron to length from a piece of window casing.
    16 Cut 45-degree returns from a length of casing.
    17 Glue the returns to the ends of the apron.
    18 Nail apron to wall beneath the window stool using 8d finishing nails.
    • Difficulty: Moderate to hard
      Requires good carpentry skills—and ample patience—but installing trim isn’t difficult.
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      Video Directory

      Selected Topic/Section

      Tools List

      • miter saw
        Power miter saw,
        used to cut the moldings to length
      • 100-foot tape measure
        Tape measure
      • pencil compass
        Scribe or pencil compass,
        used to mark the window stool for trimming
      • jigsaw
        used to trim the stool
      • pneumatic finish nailer
        Cordless nail gun with 4d and 8d finishing nails,
        used to attach moldings (could also use hammer and nailset)

      Shopping List

      1. Window stool,

      flat molding that attaches to interior side of windowsill

      2. Window casing,

      interior molding applied around the top and sides of window, and used to form the apron below

      3. Carpenter’s glue,

      applied to mitered joints of window casings

      4. Rag,

      for wiping off excess glue