How to Replace a Rotted Wooden Window Sill

Written by mark morris
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How to Replace a Rotted Wooden Window Sill
Most window sills can be replaced with pine lumber, using the original as a template. (flowers on the windowsill image by Daria Miroshnikova from

Wood is an incredibly versatile building material. Properly installed and maintained, wooden window frames can last for a century or more in good working order. Woods two most notorious enemies are sun and moisture. Of these two, the greater threat is moisture, which causes rot. Rotted wooden window sill become soft and spongy, no longer able to hold weight and virtually worthless.

Skill level:

Things you need

  • Hammer
  • Pry bar
  • Pine lumber
  • Pencil
  • Jigsaw
  • Stain
  • Primer
  • Clear finish
  • Paint
  • Paint brush
  • Finish nails

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  1. 1

    Lift the lower sash of the window to its full height and brace it in place with a piece of lumber, if needed, to keep it up. Raise the storm window to its highest position and remove the screen from the window to prevent damage in the demolition process.

  2. 2

    Strike the bottom of the rotted window sill, which is the horizontal plate at the bottom of the window that sticks out into the room, on its bottom side with a hammer, knocking it up. Continue striking the rotted sill until it shifts upward enough to allow a pry bar.

  3. 3

    Pry the rotted window sill the rest of the way out of the frame, beginning with one side. Continue prying along the width of the window until the entire sill is loose. Lift it out of the window.

  4. 4

    Check the sub sill and frame below the sill for signs of rot. Tear out and replace the 2-by-4 sub sill if it appears soft. Test this by pressing the point of a screwdriver into the wood. If it sinks in more than 1/16 inch deep, pry the old 2-by-4 out with the pry bar. Cut another 2-by-4 to the same length and install it into the same position, driving 16 d nails down through it, into the ends of the short jack studs at either end of the piece.

  5. 5

    Position the rotted sill on a piece of 1 inch thick pine lumber and trace the outline onto the pine. Cut out the outline with a jigsaw, staying as close to the line as you can. Sand the edges and both faces of the new sill you cut out of the pine with 150 grit sandpaper.

  6. 6

    Apply a coat of stain if the window is stained, or primer, if the window is painted. Use a brush and spread the finish as evenly as possible. Wipe excess stain away with a soft, lint free rag. Apply the finish to both faces and all edges to seal the sill. Allow the finish to dry for at least two hours before proceeding.

  7. 7

    Paint a painted sill with at least two coats of semigloss, interior latex. Apply two coats of clear finish in your choice of sheens over stain. Follow the label instructions regarding application and drying time between coats for best results. In general, use a fine bristle brush, working in long, straight strokes, keeping the finish as smooth as possible. Allow each side to dry before turning over or applying a second coat. Allow the finish to cure completely before proceeding.

  8. 8

    Pull any old nails or screws from the bed of the window frame where the sill will sit. Sweep out any debris. Position the sill into the window and tap into place with a rubber mallet. Use a wood rasp to shave down any edge that is too tight for a perfect fit. Drive at least four 2 inch finish nails through the sill into the frame below to secure it.

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