Repair Rotted Window Sill

Finish with a wood sealer on top of the stain where the window sill is exposed to the elements outside the house.

Repair rotted window sill.

Sand the wood of the trim and wipe with a tack cloth to remove all the sawdust. Notch out the rotted areas of the nosing and replace with a matching wood patch. Be sure you have waterproof construction adhesive and 3 1 2 inch deck screws on hand. 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. 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. If the rot isn t extensive cut it out and use epoxy wood filler. See below for tools and a shoppin.

If you can t salvage the old sill for a template measure the sill space and draw a new sill template on your board. 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. Place your old sill over the selected board and draw its outline with your carpenter s pencil.

You will apply the adhesive to the back of the new window sill and then drill it into place. 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 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. Before working on the sill seal off the house interior from dust.

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. Run a bead of waterproof marine grade adhesive along the old sill. Surround the damaged area with wood strips to serve as a router guide.

Drill pilot holes every 16 inches or so through the new sill s front and back edges.