What is an overlay window sill?
An overlay window sill is a sill fitted over an existing internal sill, without the old one being removed. It is a renovation solution for the inside: you replace the appearance without the demolition and making-good that removing a built-in sill involves.
When to use one
- The old sill is built deep into the masonry. A stone or granite sill bedded on three sides cannot be removed without damaging the plaster and sometimes the brickwork of the reveals. Making that good costs more than the new sill.
- The sill is damaged or dated. Scratches, burn marks, or a colour that no longer suits.
- In rented accommodation, where structural or irreversible work is generally not permitted.
- After a window replacement, where the new frame depth no longer matches the existing sill.
Materials
Overlay sills are available in plastic, timber and composite — usually an MDF or board core with a plastic or melamine top surface. When choosing, look at:
- Scratch resistance of the surface; a window sill has to take plants, candles and cleaning products.
- Moisture resistance of the core. This is the critical point: if the core is MDF or chipboard and water reaches the cut edges or the rear junction, the core swells and the surface layer lifts. Every cut edge must be sealed or closed with edging tape.
- Thickness and projection. Allow for several millimetres up to a good centimetre of extra height; check that the window and any opening lights still clear it.
Fitting — practical points
- Clean and degrease the old sill and check that it is flat and firmly bedded. If it is loose, fix it first; an overlay sill stiffens nothing.
- Pack out any hollows with a levelling layer or packers, so the new sill is supported everywhere and does not flex or creak.
- Bond it, do not screw it. Run beads of adhesive on the old sill; screws through the surface leave visible holes and let moisture in.
- Seal the junction with the frame using a flexible sealant — that is where condensation from the glass ends up.
- Finish the ends and front edge with a matching end cap or edging, so the core is nowhere left open.
Related terms
- Window sill
- Door sill
- Glazing
- Primer standing time
- Wood rot
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