Repair Rotted Window Sill

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.

Repair rotted window sill.

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. Run a bead of waterproof marine grade adhesive along the old sill. Place your old sill over the selected board and draw its outline with your carpenter s pencil.

Finish with a wood sealer on top of the stain where the window sill is exposed to the elements outside the house. Drill pilot holes every 16 inches or so through the new sill s front and back edges. 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.

Buy more than one board if you think you ll need an extra attempt. If the rot isn t extensive cut it out and use epoxy wood filler. Repair any rotten spots on the old sill with epoxy. 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. Once the damaged stuff is off take a good look at what was behind it. See below for tools and a shoppin. Be sure you have waterproof construction adhesive and 3 1 2 inch deck screws on hand.

Sand the wood of the trim and wipe with a tack cloth to remove all the sawdust. You will need to cut wood for a new window sill and then install it. You will apply the adhesive to the back of the new window sill and then drill it into place. This old house general contractor tom silva shares some tricks of the trade for installing a new long lasting windowsill.

Notch out the rotted areas of the nosing and replace with a matching wood patch.