To replace a window sill, you remove the old sill by cutting through caulk and prying it free, treat or cut back any rot in the surrounding wood, fit a new sill to the same dimensions, and seal all edges with exterior-grade caulk. The job takes two to four hours and the hardest part is making sure the new sill slopes outward so rainwater drains away from the frame.
- A window sill has two parts: the exterior sill (the sloped ledge outside that drains rain) and the interior stool (the flat shelf you see from inside).
- Rot in a timber sill is the most common cause of moisture tracking into the wall cavity; catching it early saves thousands in structural repairs.
- Exterior sills should slope outward at a minimum 5-degree angle to drain water clear of the frame and wall.
- The best materials for a replacement exterior sill are composite PVC, solid hardwood, and cellular PVC trim board, in that order for longevity in a Canadian climate.
- After replacing a sill, always bed the new unit in caulk at the back edge and seal both ends to stop water wicking in from the sides.
How Do You Know Your Window Sill Needs Replacing?
Your sill needs replacing if you can press a screwdriver into the wood and it sinks in easily (soft rot), if paint is peeling in a pattern that follows the wood grain (early rot), if you see daylight or feel a draught at the sill-to-frame joint, or if water stains appear on the interior wall below the window. Left unaddressed, a rotted sill allows moisture into the wall cavity, where it can damage the rough framing and even the interior drywall. Catching the problem at the sill is far less expensive than repairing the structure behind it.
Tools and Materials You’ll Need
Tools: oscillating multi-tool or circular saw, pry bar, hammer, chisel, tape measure, combination square, miter saw or hand saw, cordless drill, caulking gun, sandpaper (80 and 120 grit), safety glasses, work gloves.
Materials: replacement sill board (species or profile to match existing; see material options below), exterior-grade primer and paint or stain, polyurethane or silicone-hybrid exterior caulk, rot-repair epoxy filler (if treating adjacent rot rather than removing), stainless steel or galvanised screws (exterior grade), wood filler for interior stool repairs.
If you’re replacing the interior stool at the same time, add paint-grade MDF or solid pine to your list. Interior stools rarely rot but can be damaged by condensation over years if the original window had poor thermal performance.
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Safety Notes
Wear safety glasses and gloves throughout. If the existing sill has multiple paint layers and your home was built before 1990, test for lead paint before sanding or cutting; lead dust is a serious health hazard. Use a dust mask rated P100 in that case. On exterior work above the ground floor, secure a stable ladder at the base and have someone hold it, or use scaffolding for anything more than a one-storey reach.
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Step-by-Step Instructions
Allow two to four hours for a straightforward exterior sill replacement on a standard window. Interior stool replacement is usually quicker, about one to two hours.
Step 1: Assess the Damage
Before ordering materials, probe the full width of the sill and at least 50 mm into the frame on each side. Soft spots mean rot has spread. If the rot extends into the rough framing or window frame itself, a full window replacement may be more cost-effective than patching the sill alone. If only the sill is affected, proceed.
Step 2: Remove the Old Exterior Sill
Score the caulk line along the back of the sill (where it meets the window frame) and along both ends with a utility knife. Use an oscillating tool to cut any hidden fixings or nails along the sill length. Pry the sill forward and down; it usually lifts off in one piece if the rot hasn’t bonded it to the wall. Remove all old caulk residue and sand the contact surfaces smooth.
Step 3: Treat or Cut Back Rot
If the rough sill plate below shows soft spots, cut them back to solid wood with a chisel. Apply a penetrating epoxy consolidant to any remaining soft wood and let it cure per the manufacturer’s instructions before applying a two-part epoxy filler to build the surface back to level. This is preferable to replacing structural members if the damage is minor.
Step 4: Cut and Fit the New Sill
Measure the existing sill dimensions: length, width, depth, and the slope angle. Replicate these exactly. The critical measurement is the slope: most exterior sills pitch forward at 5 to 10 degrees. A flat sill will pool water and rot again within a few years. Cut your replacement sill to length with a miter saw. Test-fit it dry before applying any caulk or fasteners.
Step 5: Prime All Faces Before Installation
If using wood, prime all six faces of the sill before it goes in, including the underside and the cut ends. Exposed end grain is where moisture enters first. Use an exterior primer and allow it to dry fully. Cellular PVC and composite sills don’t absorb moisture the same way, but painting them still protects from UV degradation.
Step 6: Bed and Fix the New Sill
Apply a continuous bead of exterior caulk along the back edge of the rough sill plate. Set the new sill in place, press it firmly into the caulk bed, and fasten with stainless or galvanised screws driven at an angle into the wall framing below. Countersink screw heads and fill with exterior-grade wood filler or matching caulk.
Step 7: Seal All Joints
Apply caulk along the back joint (sill-to-frame), both end returns, and the underside front edge if there’s a gap against the siding. Tool smooth and wipe clean. Allow 24 hours before painting over the caulk.
Step 8: Paint and Final Check
Apply two coats of exterior paint or stain to the finished sill. Once dry, check the slope by pouring a small cup of water onto the sill surface: it should run forward and off the front edge within seconds. If it puddles at the back, re-assess the slope before the next rain.
Common Mistakes to Avoid
| Mistake | How to avoid it |
| Installing a flat sill with no drainage slope | Cut the sill with a 5 to 10 degree forward slope |
| Skipping primer on exposed end grain | Prime all six faces before installation |
| Using indoor wood for an exterior sill | Use rot-resistant hardwood, cellular PVC, or composite trim |
| Not treating adjacent rot in the framing | Probe and treat or cut back all soft wood before fitting |
| Caulking over damp surfaces | Wait for dry weather; moisture under caulk causes early failure |
| Using low-grade galvanised nails | Use stainless steel or hot-dipped galvanised fasteners |
Cost Breakdown
| Item | DIY cost | Professional (supply and install) |
| Cellular PVC sill board | $30 to $80 | $150 to $350 per window |
| Hardwood sill (oak, meranti) | $25 to $60 | $130 to $300 per window |
| Composite trim sill | $20 to $50 | $120 to $280 per window |
| Epoxy rot repair kit | $30 to $50 | Included if needed |
| Caulk, primer, paint | $25 to $40 | Included |
If the window frame itself needs replacing at the same time, get a quote that covers both: combining the jobs saves on labour.
DIY vs Professional
| DIY | Professional | |
| Risks | Missed rot, poor slope angle, failed caulk seal, moisture intrusion | Minimal; a certified installer will probe surrounding structure |
| Benefits | Cost saving on a straightforward job | Structural assessment included; joinery fitted to correct drainage angle |
Next Steps
If you’re finding rot at the sill, it’s worth checking the window frame at the same time. Use our cost estimator to compare sill repair against a full window replacement before you decide.
Frequently Asked Questions
What is the difference between a window sill and a window stool?
The sill is the exterior sloped ledge that drains rainwater away from the wall. The stool is the flat interior shelf you rest things on. Both are often called “the sill” in casual conversation, but they’re different components with different replacement methods.
Can I replace a window sill without replacing the window?
Yes, if the window frame is in sound condition. Remove only the sill, treat any adjacent rot, and fit a new sill to the same dimensions with correct slope and sealing.
What is the best wood for an exterior window sill in Canada?
Cellular PVC and composite trim board last longest with zero rot risk. If you prefer natural wood, use a rot-resistant species: meranti, sapele, or treated pine are common choices. Avoid MDF and standard pine for exterior use.
How do I stop my new window sill from rotting again?
Prime all faces before installation, use exterior-grade caulk at every joint, maintain the drainage slope, and inspect and re-caulk the joints every two to three years. Cellular PVC eliminates rot risk entirely.
Is a cracked window sill a serious problem?
A crack that runs through the full depth of the sill allows water behind it and should be replaced, not filled. A surface crack in paint or filler can be repaired if the underlying sill is structurally sound.
How long does a replacement window sill last?
Cellular PVC sills can last 30 or more years with minimal maintenance. Painted hardwood sills last 10 to 20 years if primed and sealed correctly and repainted every five to seven years.
