Restoration guide
How to Repair a Rotted Sash Window Sill
The sill is the first part of a sash window to fail: it takes the rain, holds the water and sits in shade for half the year. This is the same nine-step method we use on Regency and Victorian sashes across Cheltenham — cut back to sound timber, splice in matched wood, restore the weathering and drainage, then finish so the repair is invisible from the street.
Request a Free Restoration Survey
Tell us about your windows and we will arrange a no-obligation survey, usually within five working days.
What you need
- Bradawl or moisture meter for probing decay
- Multi-tool, tenon saw and sharp bevel-edge chisels
- Slow-grown softwood or durable hardwood to match
- Exterior gap-filling resin or PU adhesive, stainless fixings
- Oil-based primer and a microporous heritage topcoat
- Clamps, packers and a block plane for the weathering
Nine steps to a lasting sill repair
Step 1
Survey the sill and find the extent of decay
Probe the sill, bottom rail and cill horns with a bradawl every 50mm. Sound timber resists; decayed timber takes the point with no effort. Mark the soft area and extend your line 50–75mm into sound wood at each end — wet rot always runs further than it looks.
Step 2
Protect the glass and remove the beads
Take out the staff beads with a wide chisel and a thin packer, labelling each one on the back so they go back in the same place. Lift out the bottom sash if the repair reaches the rail, and tape or board the opening.
Step 3
Cut back to sound timber
Cut a clean, square-shouldered scarf or splayed housing with a multi-tool and a sharp chisel. Angled scarfs shed water better than butt joints; never leave an upward-facing shoulder that can hold rain.
Step 4
Dry the opening before you fit anything
Moisture content should be under 18% before new timber goes in. Give the frame a dry day, or use gentle warm air. Fitting into damp timber traps water behind the new section and the repair fails within a couple of winters.
Step 5
Cut and fit the matched splice
Use slow-grown, close-grained softwood or a durable hardwood with the grain running the same way as the original. Dry-fit and pare the joint until the shoulders close with hand pressure — the joint should be tight before any adhesive.
Step 6
Prime all faces, then bond
Prime every face of the splice, including the hidden ones, and the exposed original timber. Bed the piece in a gap-filling exterior resin or polyurethane adhesive and clamp or screw with stainless fixings, counterbored below the surface.
Step 7
Shape the weathering and drip groove
Plane the new section back to the original profile with the same weathering fall, then re-cut the drip groove on the underside so water breaks away from the frame instead of tracking back into the joint.
Step 8
Fill, sand and prime
Fill fixing holes and hairline shoulders with a flexible exterior filler, sand through 120 to 180 grit, and prime the whole sill in one pass so no bare timber is left exposed overnight.
Step 9
Finish, re-hang and re-bead
Two coats of a microporous heritage paint system, cutting 2mm onto the glass at the putty line. Re-hang the sash, check the cords and balance, then refit the labelled beads and check the window runs freely.
When to call a joiner instead
- Decay has reached the bearing ends or run up into the jambs
- The window is on a listed building or under an Article 4 direction
- Original crown or cylinder glass has to come out to reach the rail
- The sash cords, weights or pulleys have failed at the same time
- Work is above ground-floor level and needs access equipment
- You want a guaranteed, decorated finish rather than a holding repair
We carry out spliced sill repairs across Montpellier, Pittville, Charlton Kings, Leckhampton, Lansdown and the wider Cheltenham conservation areas, with a 10-year guarantee on structural joinery. Browse all our sash window repairs in Cheltenham.
Rotted sash sill repair FAQs
Timber versus resin, when replacement is unavoidable, and what consent you need in a conservation area.
Rather we did it for you?
Free survey and a written, per-window quote anywhere in Cheltenham and the Cotswolds.