Easy Steps to Replace a Windowsill

A rotting windowsill can make your whole house look like it is in disrepair. Luckily replacing a damaged or rotting windowsill is an inexpensive task that should take only a few hours. This job requires only a few basic tools and some beginning construction knowledge, making it an excellent repair for do it yourselfers.

Before beginning this repair, you must pick out your replacement windowsill. Take some measurements of the existing windowsill (thickness, length, width) so you will know the appropriate size to purchase.

In general, it is best to replace it with the same type of wood the original windowsill was made of. However if the windowsill will be painted, you do not need to match the wood. Also, if your original windowsill was made of softwood, consider replacing it with a hardwood for longevity.

Once you have the piece of wood for your new windowsill, you can begin replacing the old sill. You should begin by removing any trim around the area you are working using a small pry bar. This often includes casing trim and the sash channel trim.

Next remove any nails or other fasteners in the sill piece. Then carefully remove the sill piece. You should try to keep it in one piece so that you can use it as a template to cut your new windowsill.

Trace the old windowsill onto the piece of lumber you will be using for the replacement sill. You can also measure the old sill and transfer those measurements to the new sill if you prefer.

Cut the new sill using a saber saw with a finish cut blade to make a smooth, even cut. Sand the edges carefully until they are as smooth as possible.

Now you should place the windowsill in the opening and nail it into place using 6d or 8d finish nails. Start with one or two nails only partially driven to hold the sill into place. Then double check the placement before driving the rest of the nails into place.

To finish the installation, countersink all of the nails with a nail set. Using a nail set will allow you to cover each nail hole with wood putty, creating a seamless look. After the wood putty has dried, sand down the area and paint or stain to match the rest of your windowsills.

Finally, reapply any trim your removed during the previous steps. You should use a nail set to reattach the trim if needed and patch any holes.