How to Repair a Rotten Window Sill

You may be unfortunate enough to discover that you have a window sill where the wood has rotted. You will need to replace the window sill because it not only looks bad, but because it will also slowly disintegrate. It might seem like a challenging job but replacing a window sill isn’t as big a task as it first appears.

Step 1 - Measuring

Start by measuring your old window sill for length, depth and width. At the lumber yard, select an appropriate piece of wood and cut to size.

Step 2 - Accessing the Sill

To remove the rotted window sill, you have to be able to get to it first. To do this, begin by taking out the trim on the sash channels and the casing trim. You’ll probably need a pry bar to do this. Be sure to work slowly. To ensure you don’t damage any other wood with the pry bar, rest it on a piece of wood before you exert any force.

When all the trim has been removed, pull out all of the nails using a pair of pliers. When you’ve done this, check the jamb of the window and the walls for other nails and remove these as well. You’ll now have complete access to the window sill. Be careful when you remove it, trying to keep it in one piece if at all possible (it might not be possible if the wood is very rotten). By doing this, you’ll have an easy template for your new window sill.

Step 3 - Cutting the New Window Sill

If you can keep your old window sill intact, you can simply trace the design on your new piece of wood. If not, try to reconstruct it as closely as possible by taking measurements. When you’ve done this, mark them on the new window sill. Now, cut out the shape of the window sill with a saber saw, making sure you use a very fine-tooth blade to achieve the finest cut. Sand down smooth when you’ve finished.

Step 4 - Fitting the Window Sill

Put the new window sill in place and hold it in place with two finish nails. Start refitting the trim, putting it back in the reverse order to which you removed it. Just use a couple of nails to hold each part in until you’re certain the fit is good. Complete by using more nails, all of which must be countersunk, to secure the trim and window sill fully. Fill each of the nail holes with wood putty. Once it’s dried, sand it down even with the wood.

Sand the wood of the trim and wipe with a tack cloth to remove all the sawdust. You will now need to use stain on the wood to seal it. Finish with a wood sealer on top of the stain where the window sill is exposed to the elements outside the house.