Repair Rotted Window Sill

Place your old sill over the selected board and draw its outline with your carpenter s pencil.

Repair rotted window sill.

Once the damaged stuff is off take a good look at what was behind it. Small cracks or chipping paint on the window sill can be handled easily by scraping all loose paint and applying a high quality exterior paint in under 30 minutes assuming you have the tools and materials you want to include getting materials out and cleaning up. Sand the wood of the trim and wipe with a tack cloth to remove all the sawdust. You will now need to use stain on the wood to seal it.

If your old window sill was rotted you likely will not want to reuse it. Trim the back edge of the new sill as needed so that its lower edge fits tightly against the siding and its back edge fits against the old wood. Notch out the rotted areas of the nosing and replace with a matching wood patch. If you can t salvage the old sill for a template measure the sill space and draw a new sill template on your board.

You will apply the adhesive to the back of the new window sill and then drill it into place. Surround the damaged area with wood strips to serve as a router guide. Before working on the sill seal off the house interior from dust. Rot that occurs deep inside of the sill is more difficult to repair.

You will need to cut wood for a new window sill and then install it. Buy more than one board if you think you ll need an extra attempt. Finish with a wood sealer on top of the stain where the window sill is exposed to the elements outside the house. Repair any rotten spots on the old sill with epoxy.

See below for tools and a shoppin. Drill pilot holes every 16 inches or so through the new sill s front and back edges. If the sill was pretty rotted there s a good chance that water worked its way in behind it and rotted siding and possibly even framing members. This old house general contractor tom silva shares some tricks of the trade for installing a new long lasting windowsill.

If the rot isn t extensive cut it out and use epoxy wood filler.