How To Repair Exterior Rotted Wood On A Window Sill

Rotted wood on an exterior window sill? Here’s how to repair

Rotted wood on an exterior window sill is necessary to repair so the rot doesn’t spread to the rest of the window frame or into your home’s walls. Exterior window sills see the best and the worst of being outside year round because they are horizontal — so they can get soggy from rain and snow, and then become a crumbling mess from the hot, dry sun.

Inspect for rotted wood

First Inspect your window sill to see the extent of the exterior rotted wood damage. If the rotted wood is minor, you can dig out the spongy, soft  rotted wood with a screwdriver or chisel. Dig deep and poke around to ensure you’re getting all of it until only healthy hard wood remains. Use wood filler compound to fill the holes, gaps and grooves and let it harden completely. Then sand smooth, prime and paint.


If the exterior window sill has more than about 10 per cent rotted wood, it’s better to replace the whole piece.


Tools required:


  • Tape measure
  • Hammer
  • Drill
  • Pry bar
  • Chisel
  • Saws: Handsaw, circular saw or reciprocating saw
  • Shims
  • Nails
  • Screws
  • Caulking
  • Primer
  • Paint

How To Repair Exterior Rotted Wood On A Window Sill Midpost Collage

With a utility knife, cut through the caulking that filled the gap between the sill and the window trim. Pull out any nails or screws that were holding it in place as well and use a chisel or flat pry bar to pull it free. If it’s particularly difficult to remove use a saw to cut it out in pieces.


However, try to preserve the window sill size and shape as much as possible because it can be used as a template for your new sill. Some hardware stores sell wood that is specifically for window sills that have a pre-bevelled edge. Choose cedar or pressure-treated wood since it’s most resistant to wet weather.


When the window sill is cut to fit the interior opening, as well as the notches that make it a few inches wider than the window on the outside shelf, lay it in place. Attach it both from the top and bottom. Drive galvanized nails from the top into the wall frame. And from underneath, use deck screws inserted upwards into the vertical window trim for added strength against the weight of snow and ice.


When securely in place, use caulking to fill all the gaps around the window trim and exterior walls so that no water can get in, as well as the tops of nails and screws. Then prime and paint using a product specifically designed for exterior protection.


Your window sill is now ready to receive some decor love — whether that’s window boxes full of flowers in summer, or twinkling holiday lights in winter. If you would prefer to hire a professional to repair  your exterior rotted wood window sill, contact, call 416.494.9095  or click here for a FREE quote.

Closing Photo on How To Repair Exterior Rotted Wood On A Window Sill

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