In this video, This Old House general contractor Tom Silva shares some tricks-of-the-trade for installing a new, long-lasting windowsill.
1. Use an oscillating multi-tool to cut through the nails holding the stool to the sill.
2. Cut through the house siding directly below the sill with the multi-tool.
3. Take a reciprocating saw and cut through the sill about 2 inches in from the side jamb. Repeat to make a second cut 2 inches from the opposite jamb.
4. Drive 3-inch-long screw into the edge of the severed sill. Position the screw close to the reciprocating saw kerf. Leave about ½ inch of screw protruding from the sill. Repeat to position a second screw at the opposite end of the sill.
5. Hook a pry bar onto the protruding screw heads and pry the sill from the window frame.
6. Chop out the remaining ends of the old sill with a hammer and ¾-inch-wide chisel.
7. Use end nippers to cut away any old nails that protrude into the sill space.
8. Measure depth, width between side jambs, and overall length for new sill.
9. Transfer sill measurements onto cedar board and cut to length with a circular saw.
10. Use horn (end) of old sill to mark the horn length and angle onto the new cedar sill.
11. Hold old horn against saw shoe and adjust bevel angle of blade to match angle on horn. Bevel-rip the sill to width, and cut the horn angle into each end of the sill.
12. Finish up horn cuts with handsaw.
13. Saw a shallow groove along the underside of the sill to act as a water break to prevent rain from running under sill. Position the groove about ½ inch from front edge of sill.
14. Plane the edge of the sill smooth.
15. Sand the sill with a random-orbit sander and 120-grit sandpaper.
16. Coat the all surfaces of the sill with spray primer.
17. Apply silicone caulk to the underside of the casings and inside the side jambs.
18. Inject minimal-expansion foam under the sill.
19. Slide the new sill into position, pushing it tightly into the wet silicone.
20. Tap the sill all the way into place with a dead-blow mallet.
21. Secure the sill to the stool with a pneumatic finishing nailer and 2-inch-long nails.
22. Run a thin bead of silicone along seam between sill and stool, around each side casing, and along the siding beneath the sill.
23. Paint the new sill to match existing window trim.