Repair Rotted Window Sill

Sand the wood of the trim and wipe with a tack cloth to remove all the sawdust.

Repair rotted window sill.

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. Buy more than one board if you think you ll need an extra attempt. Drill pilot holes every 16 inches or so through the new sill s front and back edges. Surround the damaged area with wood strips to serve as a router guide.

If you can t salvage the old sill for a template measure the sill space and draw a new sill template on your board. If your old window sill was rotted you likely will not want to reuse 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. If the rot isn t extensive cut it out and use epoxy wood filler.

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. You will apply the adhesive to the back of the new window sill and then drill it into place. 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. Rot that occurs deep inside of the sill is more difficult to repair.

Notch out the rotted areas of the nosing and replace with a matching wood patch. This old house general contractor tom silva shares some tricks of the trade for installing a new long lasting windowsill. You will need to cut wood for a new window sill and then install it. Repair any rotten spots on the old sill with epoxy.

Run a bead of waterproof marine grade adhesive along the old sill. Finish with a wood sealer on top of the stain where the window sill is exposed to the elements outside the house. See below for tools and a shoppin. You will now need to use stain on the wood to seal it.

Be sure you have waterproof construction adhesive and 3 1 2 inch deck screws on hand.