Repair Rotted Window Sill

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

Repair rotted window sill.

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

This old house general contractor tom silva shares some tricks of the trade for installing a new long lasting windowsill. 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. Repair any rotten spots on the old sill with epoxy. 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.

If the rot isn t extensive cut it out and use epoxy wood filler. Before working on the sill seal off the house interior from dust. Notch out the rotted areas of the nosing and replace with a matching wood patch. Sand the wood of the trim and wipe with a tack cloth to remove all the sawdust.

Buy more than one board if you think you ll need an extra attempt. Be sure you have waterproof construction adhesive and 3 1 2 inch deck screws on hand. Run a bead of waterproof marine grade adhesive along the old sill. Once the damaged stuff is off take a good look at what was behind it.

You will apply the adhesive to the back of the new window sill and then drill it into place. If your old window sill was rotted you likely will not want to reuse it. Surround the damaged area with wood strips to serve as a router guide. You will need to cut wood for a new window sill and then install it.

You will now need to use stain on the wood to seal it.