How to Change an Interior Windowsill

Remove the window stool. The stool is the piece of trim that is nailed to the window sill. Cut the caulk between the trim and the window sill using a utility knife. Insert a putty knife in between the window sill and the trim, and pry the trim carefully away from the window sill.

Remove the apron in the same way as you removed the stool. The apron is the piece of trim underneath the stool. You will now be looking at the entire window sill with no extra trim.

Remove the trim at the side of the window using the putty knife as before. At this stage, one way to proceed is to remove the exterior trim around the window and use the reciprocating saw to cut around the whole window. The window then is ready to be removed completely. However, this becomes a far larger project that should not be undertaken lightly, so for the purposes of this guide, the window is left in place and only the interior trim and sill are removed.

Cut a 2 to 3-inch wide section out of the middle of the window sill using a reciprocating saw. Extend the cut all the way to the back of the window sill. Insert a chisel underneath the cut section and lever it out carefully. Removing this piece allows the required room to remove the rest of the sill easily, which is now in two separate pieces.

Cut all around the edges of the two remaining pieces of window sill with the reciprocating saw. Work the pieces loose with a chisel and remove them completely. Alternatively, use the reciprocating saw to cut around the very edge of the sill and remove it in one piece.

Ensure the edges of the window are neat and tidy. Remove any jagged edges with the saw and sand smooth with a medium-grade sandpaper.

Lay the new wood on a flat surface, and place the window sill or pieces of window sill you have removed on top of the new wood to use as a template. Draw around the old window sill and discard it.

Cut the new window sill to size by using a hand saw or table saw to cut along the template line. Sand the edges of the new window sill so they are smooth. Slide the new window sill into place to check that it is a snug fit. If it doesn't fit perfectly, continue to sand the window sill until it's snug and secure.

Remove the window sill and apply a line of wood glue along the back edge and sides of the sill. Insert the sill and hammer it gently into position with a rubber mallet. Place finish nails approximately every 12 inches along the back and sides of the window sill.

Reinstall the side trim, apron trim and stool trim with wood glue and nails. Apply a generous amount of caulk along the edges, seams and joints of the window sill and trim, and caulk over any visible nails. If you have placed nails a little too far in towards the center of the sill, punch the nails below the surface of the wood using a nail set tool and cover with wood filler. Allow the caulk to dry.

Apply a coat of wood primer to the window sill and trim, allow to dry and then paint or apply a coat of wood stain.