Typically, sills for exterior doors are made from oak, and windowsills are made from redwood. Both types are installed at a downward angle so water runs off the sill instead of inside a house. In addition, a flashing must be included to seal below the sill. Installing sills takes some experience with power tools and attention to detail. A novice do-it-yourself enthusiast should have the assistance of an experienced installer.
Preparation and Flashing
Use a utility brush to remove dust and dirt from the subfloor area at the doorway or the framing sill at a window opening.
Measure the width of the opening between the framing at each side. Add 12 inches. Refer to the measurement and cut piece of 8-inch-wide, self-adhesive elastomeric flashing to length with a utility knife.
Position the flashing across the doorway or window opening with the paper backing facing down. Center it each way so 6 inches extends up the framing at each side. Align the inner edge at the inner edges of the framing at each side. Peel off the backing and press the piece into place. Cut a 1/2-inch slit that aligns with the outer edge of the framing at each side. Press the outer edge of the flashing down and against the outer edge of the subfloor or windowsill framing.
Installing the Sill
Refer to the measurement for the doorway or window opening width, and add 4 inches to allow two inches of the sill to extend past the edges of the opening at each side. Mark a piece of 2-by-8-inch oak or redwood to length. Square the mark across a wide face of the piece with a builder’s square.
Put on safety glasses. Cut the wood sill at the mark with a circular saw.
Mark the underside of the sill for notches that cut out at each end so the outer edge extends two inches past each side of the opening. Measure in from each end and make a 2-inch mark. Measure in 5 1/2 inches from the edge of the sill at each end. Use the builder’s square to mark the notches.
Cut the lines for each notch at the back of the piece. Stop the sawblade when it reaches the corresponding line instead of cutting until the notched piece is cut. Finish each cut with a finish handsaw.
Turn the sill over and place it in the opening. Position the 2-inch extended portions at each end against the outer face of the building. Install wood shims at four-inch intervals from end to end below the inner edge of the sill to elevate the inner edge 1/4 inch. Place a a 24-inch level atop the sill along its long axis, and adjust the shims as necessary, so the sill is level from end to end and elevated 1/4 inch at the inner edge.
Set up a power or cordless drill with a 1/16-inch drill bit. Drill pilot holes for screws at six-inch intervals along the center-line axis of the sill from end to end. Install a 1/4-inch countersink bit in the drill, and drill 1/8-inch-deep countersinks at each pilot hole.
Install a screw-tip attachment in the drill. Secure the wood sill to the subfloor or window framing sill with 3-inch wood screws at each countersink pilot hole. The wood sill is ready for wood filler at the screws, primer and paint, or stain and sealer.