How to Repair a Rotted Windowsill


Replacing a rotted or damaged windowsill is pretty simple: Take out the old sill, cut a replacement sill and install it.

Things You'll Need

  • Finish Nails
  • Measuring Tapes
  • Wood Putty
  • Lumber
  • Hammers
  • Pry Bars
  • Saber Saws
  • Take some overall measurements of the windowsill for width, length and thickness.

  • Remove the sash channel trim and side pieces of casing trim to get access to the sill. Work loose any trim with a small pry bar. Rest the pry bar on a small block of scrap lumber to keep it from marring other trim or walls.

  • Pull all nails left in the walls or window jamb. Remove nails in the trim with pliers, pulling them through the back of the trim.

  • Remove the sill piece itself, trying damage it as little as possible. (The easiest method is to use the old sill as a template for the replacement.)

  • Take the old sill to your local lumberyard or home improvement center. Purchase replacement lumber and 6d or 8d finish nails.

  • Trace the pattern of the old sill directly on the new lumber - or make careful measurements and transfer those to the new lumber.

  • Cut out the new sill with a saber saw. Use a fine-toothed (finish cut) blade to make the cuts as smooth as possible. You might have to do some sanding, especially for exposed end grain cuts.

  • Fit the new sill in place and use a couple of nails to hold it there. Replace the trim you removed with a couple nails for each piece. Once you are satisfied with the fit, nail all pieces securely. Countersink all nails with a nail set.

  • Fill all nail holes (old and new) with wood putty. Paint or stain to match your decor.

Tips & Warnings

  • If you plan to paint the windowsill, you can save a little money by using pine or another inexpensive wood, instead of oak, ash or mahogany.

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