A window sill is the flat shelf that sits at the bottom of a window opening, and it does more than hold your plant pots. It sheds rainwater away from the wall, finishes off the join between the frame and the brickwork, and gives the whole window a tidy edge. Here is what a window sill actually is, the bits that make it up, and why the internal and external versions do quite different jobs.
Every window sits in a hole in your wall. The sill is the horizontal part along the bottom of that hole. There is usually one on the outside and one on the inside, and although people lump them together, they are built for different reasons.
On the outside, the sill takes the weather. On the inside, it gives you a neat ledge and hides the gap where the frame meets the plaster. Get either one wrong and you tend to find out about it through draughts, damp patches or peeling paint.
A sill is simple, but a few features do the heavy lifting:
Internal sills are usually flat, since they only need to look neat and give you somewhere to rest the odd mug of tea.
This is where most of the confusion comes from, so let us be clear about the two.
The external sill is a working part of the building. In older York terraces you often see stone or concrete sills, sometimes painted timber, all designed to throw water off the wall. On modern uPVC and aluminium windows the sill is usually part of the frame system or a separate profile clipped underneath. Whatever the material, its job is to keep water moving away from the house.
If your external sill is cracked, crumbling or has lost its slope, water can sit against the wall and find its way in. That is a common cause of damp under windows, and it is worth sorting sooner rather than later.
The internal sill, which many people call a window board, is the flat surface inside the room. It is more about looks and finishing than weatherproofing. It covers the gap between the frame and the wall, gives you a shelf, and rounds off the reveal. These are often made from laminated board, timber or uPVC.
The material you choose affects how the sill looks, how long it lasts and how much upkeep it needs:
If you are updating your windows and want the sills to match, our uPVC window range and aluminium windows both come with sill options that suit the frame.
A sill is easy to ignore until something goes wrong. A tired external sill lets water sit where it should not, which leads to damp, mould and even timber rot in older frames. A poorly fitted internal board leaves a gap that draughts sneak through. Neither is dramatic on day one, but both cost more to put right the longer they are left.
If your windows are otherwise sound and it is only the sill that has gone, a repair is often the sensible route rather than a full replacement. Our team handles this sort of thing regularly, so if you have a crumbling or leaking sill it is worth a chat about window repairs across York before you assume you need whole new windows.
A little care goes a long way. Wipe internal boards down so condensation does not sit and mark them. Check external sills each year for cracks, missing sealant or a blocked drip groove, especially after a hard Yorkshire winter. Clear any moss or debris that stops water running off. Keep the slope clear and the front edge intact, and a sill will quietly do its job for decades.
The window board is the internal shelf inside the room, mainly there to look neat and finish the reveal. The window sill often refers to the external part that sheds rainwater, though people use both terms loosely for either.
Usually because the slope has worn flat, the drip groove is blocked, or the sill has cracked. Water then sits against the wall instead of running off, which can lead to damp. It is worth getting it checked before it spreads.
Often, yes. If the frame and glass are sound, a damaged sill can frequently be repaired or swapped on its own. A quick look will tell you whether a repair is enough or a fuller fix is needed.
It depends on the look and the location. uPVC is low maintenance and popular, timber suits period homes, and stone or concrete are hard-wearing choices for external sills. Match the material to the window and the style of the house.
Costs vary a lot with material, size and access, so a small internal board is very different from replacing a stone sill. Prices change, so the honest answer is to get a proper look and a quote for your specific window.
Thinking about updating your windows or tidying up tired sills? Download our latest brochure to see the styles and finishes, or request a free quote and we will pop out for a look.