Step 1: Prep the jambs

Tom Silva trimming out a window
Photos: David Carmack
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Hold a straightedge across the window; check that each jamb is flush with the wall. If they protrude, plane them down to the wall. If the jambs fall short of the wall, measure the largest gap between the straightedge and each jamb. Then, from 1x stock, rip jamb extensions as wide as necessary and ¼inch thinner than the jamb thickness. Holding the strip against the jamb, drive 1 ½inch 18-gauge pneumatic nails (or 4d finish nails) every 8 to 10 inches. Plane off any excess.

Spread the legs of a compass ΒΌ inch and rest the point on one of the jamb's lower inside corners. Using the inside face of the jamb as a guide, draw a reveal line along the edges of all three jambs.

Tip: For a tight fit between the casing and jamb, plane a slight bevel into the wall-side edge of the jamb.

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    Tools List

    • finish hammer
      20-ounce finish hammer
    • hand plane
      Hand plane,
      for trimming jamb extensions
    • 16-foot tape measure
      Measuring tape
    • router
      Router,
      for making stool profile
    • random orbit sander
      Random-orbit sander,
      or hand sanding block
    • jigsaw
      Jigsaw,
      for notching stool and profiling apron
    • pneumatic finish nailer
      Pneumatic nailer,
      for attaching casing
    • pencil compass
      Compass,
      for transferring marks and reveal lines
    • table saw
      Table saw,
      for cutting rabbet in bottom of stool
    • try square
      Square,
      for marking casing, stool, and apron
    • utility knife
      Utility knife or chisel,
      for trimming drywall and adjusting fit of stool
    • miter saw
      Miter saw,
      for crosscutting trim pieces to length
    • nailset
      Nail set (to recess heads of hand-driven nails
    • drill
      Drill with 1/16-inch-diameter bit,
      for making pilot holes before hand nailing

    Shopping List

    1. WOOD MOLDING

    2. CARPENTER'S GLUE

    3. PENCIL

    4. PAINT OR STAIN

    5. 120-GRIT SANDPAPER

    6. WOOD PUTTY

    7. 4D AND 8D FINISHING NAILS