Window sills die first. They sit at a shallow angle, catch every drop of rain that runs down the glass, and bake in direct sun — so it is no surprise that the sill rots out years before the rest of the window shows trouble. Replacing exterior window sill boards is a genuinely achievable weekend repair, typically $30 to $80 in materials versus $300 to $700 for a carpenter, and doing it promptly matters more than most homeowners realize: a spongy sill is an open funnel routing water into the wall framing below. Catch it early and you replace one board. Ignore it for three years and you are opening siding to sister rotted studs.
Confirm It Is the Sill — and Only the Sill
Start with a screwdriver, not a saw. Press the tip into the sill every few inches, especially at the ends where the sill meets the side casings and under the drip edge. Sound wood resists; rot lets the tip sink in with light pressure. Now probe upward into the bottoms of the side casings and jambs, and downward into the siding or trim beneath the sill. If softness extends past the sill itself into jambs or wall sheathing, budget for a bigger repair before you commit — swapping a sill on top of a rotted subsill or wet sheathing just hides an active leak. Paint that is peeling in a horizontal line across the sill, black staining, or moss growing in end-grain checks are all early confirmation even where the wood still feels firm.
Also identify what kind of sill you have. Older double-hung windows (pre-1960s especially) have a true structural sill that is part of the window frame, sloped 5 to 15 degrees and often 1-1/2 inches thick. Newer construction frequently has a sill nosing or a simple piece of exterior trim below a self-flashing window unit. The trim version is an easy swap; a true frame sill takes more care because the jambs bear on it.
Tools and Materials
- Oscillating multi-tool (the hero tool here) or reciprocating saw with a short blade
- Flat pry bar, stiff putty knife, hammer, chisel
- Replacement stock: cellular PVC sill stock, or preservative-treated pine/fir sill nosing, or clear cedar — a 5/4 x 8 x 6-foot piece runs $15 to $45 depending on material
- Circular saw or table saw to cut the 10- to 14-degree bevel if your stock is not pre-milled
- Exterior construction adhesive, stainless or hot-dip galvanized trim screws (2-1/2 inch), high-quality sealant (polyurethane or hybrid such as OSI Quad Max)
- Self-adhering flashing tape, exterior primer and paint (skip on PVC, optional but wise on cedar)
Removing the Old Sill
Score the paint and caulk joints where the sill meets the casings and the wall with a utility knife so nothing splinters when you pry. For a trim-style sill, work the pry bar under the outer edge and lift gradually along its length, protecting the siding below with a shim block. A frame-type sill rarely comes out whole: cut it in half mid-span with the oscillating tool, then wiggle and pry each half outward from under the jambs. Old sills are usually nailed up into the jamb ends and toenailed into the rough framing, so expect to cut hidden nails flush with the multi-tool. Pull every remaining fastener, then vacuum the exposed rough sill and let it dry if it is damp — a moisture meter reading under 15 to 18 percent is where you want the framing before closing it up.
This is your one chance to inspect the rough opening. Treat any minor surface punk in the rough sill with a borate solution (Bora-Care or Tim-bor, about $25) and fill small voids with two-part epoxy consolidant. Structural softness deeper than 1/4 inch means replacing framing — stop and reassess scope.
Cutting and Fitting the New Sill
Use the old sill as a template if it came out mostly intact; otherwise measure the opening at both ends, since old houses are rarely square. Three details make the difference between a 20-year sill and a 5-year one. First, the bevel: the top surface must slope away from the window at roughly 10 degrees so water sheds — rip this on a table saw or buy pre-milled sill stock. Second, the overhang and drip kerf: the sill should project 3/4 to 1 inch past the siding face, with a 1/4-inch-deep saw kerf cut along the underside about 1/2 inch back from the front edge. That little groove breaks surface tension so water drips off rather than crawling back to the wall. Third, end “horns” on traditional sills extend past the casings; notch the new piece to match the original profile.
Dry-fit until the piece slides snugly under the jambs with a gentle tap. Aim for a fit tight enough to need persuasion but not so tight you lift the jambs.
Flashing, Fastening, and Sealing
Lay self-adhering flashing tape over the rough sill, running it 4 inches up the jack studs at each end, before the new sill goes in — this is the safety net for any future leak. Bed the sill in two continuous beads of construction adhesive, tap it home, and fasten with stainless trim screws driven at an angle through pre-drilled, countersunk holes into the framing — screws, not nails, so seasonal movement cannot work the sill loose. On a frame sill, also screw up through the sill into the jamb ends where the originals were nailed.
Seal the joints where the sill meets casings and window frame with polyurethane sealant, tooled concave. Leave the underside drip kerf and the bottom gap above the siding unsealed — water that ever gets behind must be able to drain out. Prime all six faces of a wood sill before installation if you can (end grain twice), and paint within a couple of weeks. PVC needs no coating but takes paint fine if you want color.
Why Sills Rot in the First Place
Understanding the failure keeps the replacement from repeating it. Sills rot from three directions. From above, the joint where the sill meets the window frame opens as caulk ages, letting condensation and wind-driven rain seep under the bottom rail — the reason rot so often starts in the middle of the sill’s back edge. From the ends, the sill-to-casing joints wick water into exposed end grain, which absorbs moisture up to 15 times faster than face grain; this is why the horns and ends go soft first on most houses. From below, splash-back off decks, patios, and shrubbery keeps the underside damp, and a missing drip kerf lets water cling to the bottom face for hours after every rain. Sun finishes the job: south- and west-facing sills cycle through heat and UV that crack paint years before shaded elevations, opening the door for all three water paths. Your new sill counters each route — tooled sealant at the frame joint, primed end grain, a real kerf, and paint maintained on the sunny sides — which is the whole reason a careful replacement outlives the original by decades.
Material Choice: Wood, PVC, or Repair-in-Place
Cellular PVC (Azek, Versatex) has become my default for full replacements: it will never rot, mills like clear pine, and costs about the same as good cedar. Its drawbacks are higher thermal movement — leave 1/8-inch gaps at the ends and use adhesive plus screws — and a hollow feel some people dislike on historic homes. Clear or preservative-treated wood remains the right call for visible historic profiles. And where rot is limited to a few square inches of an otherwise sound sill, a $40 two-part epoxy system (Abatron WoodEpox with LiquidWood consolidant) rebuilds the damaged zone in an afternoon and postpones full replacement for years. Whichever route you take, the sill you install with a proper bevel, drip kerf, flashing, and tooled sealant joints will outlast the one the house came with.
