Repair Rotted Window Sill

Run a bead of waterproof marine grade adhesive along the old sill.

Repair rotted window sill.

Drill pilot holes every 16 inches or so through the new sill s front and back edges. You will need to cut wood for a new window sill and then install it. Let s look at how repairs and related costs to repair a rotted window sill change as wood rot is left to continue over time. Buy more than one board if you think you ll need an extra attempt.

Sand the wood of the trim and wipe with a tack cloth to remove all the sawdust. Finish with a wood sealer on top of the stain where the window sill is exposed to the elements outside the house. Surround the damaged area with wood strips to serve as a router guide. Be sure you have waterproof construction adhesive and 3 1 2 inch deck screws on hand.

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. You will now need to use stain on the wood to seal it. Rot that occurs deep inside of the sill is more difficult to repair. 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. Notch out the rotted areas of the nosing and replace with a matching wood patch. Once the damaged stuff is off take a good look at what was behind it. Place your old sill over the selected board and draw its outline with your carpenter s pencil.

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. See below for tools and a shoppin. 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.

Before working on the sill seal off the house interior from dust.