How to Build an Interior Window Sill

Measure the length, width and depth of the block of wood you will be using for your interior sill. On each side of the window frame, mark where the wall meets the casing’s outside edge.

Use a tape measure to determine the distance between the two markings you made in Step 1. Add 2 inches to this measurement and cut the sill to match this length. Allot additional length for the stool if the edges of the ledge will be rounded out or cut at an angle.

Cut notches into both back ends of the interior sill using a chisel, a jigsaw or sandpaper. These grooves will ensure the sill fits securely against the wall surface, the inside jamb and the sill trim.

Predrill holes approximately 16 inches apart along the front and back edges of the board. Squeeze a line of adhesive along the window frame and push the new sill into place.

Drill 4d finishing nails into the face of the sill, attaching it to the interior sill trim with the nailer. Firmly hammer in all nail heads and cover with wood putty. Leave the putty to dry on the sill overnight.

Sand the sill thoroughly until the surface is smooth and even. Wipe away dust and shavings before applying a coat of wood primer and paint or stain and varnish.

Add decorative molding below the windowsill. Use a scrap piece of apron to trace its profile with a pencil onto the face of the block. Use a jigsaw to cut along the markings. Apply adhesive along the top edge of the apron and press the edge against the bottom of the stool. Drive 4d finishing nails along the bottom edge of the apron and through the top of the sill into the apron to secure it.