How to replace a rotten window sill

Based on a detailed walk-through by Sumeray Builds. This guide is drawn from "Need a New Window Sill? WATCH THIS!!!" by Sumeray Builds, a UK building and renovation channel run by Jack Sumeray. The video covers the full replacement process from removal to finishing, and is especially useful for showing how to cut back the existing sill without disturbing the window frame.

1. Assess the damage before you start cutting

Poke the sill with a screwdriver or a bradawl in a few places to find out how far the rot has gone. Surface paint can disguise a sill that has turned almost completely to soft wood underneath. If the rot is confined to the outer face and the sill is otherwise solid, filler repair or epoxy consolidant might do the job. If the screwdriver goes in easily or the wood is visibly collapsed, replacement is the right call.

Also check the wall below the sill and the reveal around the window. If the rot has spread into the frame itself or the brickwork below is saturated and crumbling, you may have a larger damp problem to deal with. It is worth understanding the full picture before you commit to just swapping the sill.

2. Source the replacement sill timber

Window sills traditionally use a profile called a “splayed sill” or “window board,” with a slope on the top face and a drip groove moulded into the underside to prevent water running back under the frame. You can buy this ready-profiled from a timber merchant in softwood (redwood) or hardwood (oak or accoya, which resist rot far better). Choose a width that matches your existing sill, usually 125mm to 175mm, and leave yourself a bit of extra length to trim.

Treat softwood sills with an end-grain preservative before fitting - the cut ends are where moisture gets in first. Pressure-treated timber is even better if you can source it to the right profile.

3. Remove the old sill

Score around the sill with a Stanley knife to break any paint bond and caulk joints. Use a multi-tool or an oscillating saw to cut through any fixings or caulk along the back edge where the sill meets the window frame. Work carefully here - you want to avoid marking the frame.

Lever the sill out from below using a wide bolster chisel and a mallet. It will likely be nailed or screwed into the sill plate beneath, and possibly into the window frame above. Work along the length gradually rather than trying to pop one end free first - old, dry timber can split unexpectedly. Once the sill is out, clean up any remaining mortar or old fixings from the surface below.

4. Prepare the surface and measure for the new sill

The surface the sill sits on - the sill plate or the top of the masonry below the window - needs to be clean, flat, and treated for damp if there’s any evidence of moisture. Apply a coat of timber preservative to any bare wood in the reveal. If the brickwork is crumbly or stained, brush on a stabilising primer before you fit the new sill over it.

Measure carefully for the new sill. Width is straightforward, but the length matters more: the new sill needs to sit into the reveal on both sides by at least 25mm, with the ends cut at the correct angle to fit flush against the reveals. Take your measurements from the old sill if it has come out cleanly, or measure the opening directly with a combination square to mark the angles.

5. Cut and treat the new sill

Cut the new sill to length, angling the ends to match the reveal. Use a hand saw or circular saw with a fine blade - sill profiles are not thick, and a coarse blade will splinter the face. Check the fit dry before you commit to fixing. The sill should slide into the reveal with a small gap at the back to allow for movement, and the front edge should overhang the wall face by around 25 to 40mm.

Once the cut is confirmed, brush two or three coats of timber preservative onto all cut faces and the underside, especially the end grain. Allow to dry fully. Then prime all faces before fitting - it is much easier to get paint into the edges and underside before the sill is in position than afterwards.

6. Fit the new sill

Apply a bead of frame sealant or exterior-grade mastic to the back edge of the sill and to the reveal surfaces it will sit against. Slide the sill into position and check it is level across the width and has a consistent small fall toward the front for water run-off - around 5 to 8 degrees is typical. Screw through the sill into the frame or sill plate below using stainless steel screws (galvanised at minimum). Countersink the screws so the heads sit below the surface and can be filled.

Fill the screw holes with an exterior wood filler and allow to dry. Then apply a neat bead of flexible exterior mastic around all the joints - back edge, side reveals, and the junction with the frame above. Smooth with a wetted finger. This seal is what keeps water out, so do not rush it or leave gaps.

7. Finish with primer and paint

Once the mastic has skinned over (follow the product instructions - usually a few hours), apply a further coat of exterior primer over any bare areas, then two coats of exterior gloss or satinwood to match the existing windows. Take the paint a few millimetres onto the frame above and onto the render or cill below to seal the joint completely. A poorly painted sill lasts a few years; a well-painted one, with the joints properly sealed, lasts a decade.

When to call a handyman

Replacing a single sill is within most people’s abilities with a bit of care. Where it makes sense to bring Richard in is when the rot has spread into the frame or the wall, when several sills need doing at the same time, or when working at height makes the job unsafe to tackle alone. Window repairs across Sandwich and East Kent are a regular part of the work.