If you have ever wondered whether it is a window cill or a window sill, the short answer is that they mean the same thing. Both refer to the flat shelf at the bottom of a window. The difference is purely in the spelling, and which one you see depends a bit on the trade and the context. Here is what is going on, so you can stop second-guessing yourself.
Same part, two spellings
A window cill and a window sill are the same component: the horizontal ledge along the bottom of a window opening. There is no difference in what the part does, only in how the word is written. Both spellings are used across the UK, and neither is wrong.
The word most people learn at school is "sill". You will find it in dictionaries, and it is the everyday spelling for most readers. "Cill" is the older building-trade variant, and it still turns up on plans, in specifications and in conversation among builders and window fitters.
Why the trade uses "cill"
So why does the construction world hang on to "cill"? A couple of reasons.
- Tradition. The spelling has been used in the building trade for a very long time, and it stuck. Older technical drawings and standards often use it.
- Clarity on plans. Some argue that "cill" avoids any mix-up with other uses of "sill" and keeps building terms consistent. On a drawing packed with abbreviations, a distinct spelling can help.
None of this changes the part itself. Whether your fitter writes "cill" on the quote or "sill" in an email, they are talking about the same ledge.
What the window cill actually does
Now the spelling is out of the way, it is worth knowing what the part does, because it earns its keep. The cill sits at the bottom of the window and, on the outside, sheds rainwater away from the wall below. It usually has a slight slope and a small groove underneath, called a drip groove, that lets water fall clear rather than tracking back to the brickwork.
Inside, the cill, more often called a window board, gives you a neat shelf and covers the join between the frame and the plaster. So the same word can point to two related parts, one weatherproofing and one finishing.
If you want the full run-down of the parts and materials, we cover it in our plain-English guide to window sills.
Cill materials you will come across
Whichever spelling is on the paperwork, cills come in the same range of materials:
- uPVC. Low maintenance and easy to wipe clean, common on modern windows.
- Timber. Traditional and warm, seen on many period York properties, though it needs painting or oiling.
- Stone and concrete. Hard-wearing external cills found across older Yorkshire homes.
- Aluminium. Slim and strong, used with aluminium window systems.
If you are choosing new windows, the cill usually comes as part of the frame system, so it is matched to the rest. Our uPVC windows and aluminium windows both include cill options to suit the style of house.
Which spelling should you use?
For everyday writing, letters, emails, a note to a decorator, "sill" is the safe, widely understood choice. If you are reading a builder's quote, a set of plans or a technical spec and you see "cill", do not worry that something is different. It is the same part, spelled the trade way. Use whichever feels natural, and know that both are correct.
When your cill needs attention
Spelling aside, an external cill does a real job, and a worn one causes real problems. If the slope has flattened, the drip groove is blocked, or the cill has cracked, water can sit against the wall and lead to damp under the window. That is worth catching early, especially after a hard Yorkshire winter.
The good news is you rarely need whole new windows just because a cill has gone. If the frame and glass are sound, a repair is often enough. Our team deals with this regularly, so if your cill is crumbling or leaking it is worth asking about window repairs in York first.
FAQs
Is it window cill or window sill?
Both are correct. They refer to the same part, the ledge at the bottom of a window. "Sill" is the everyday dictionary spelling; "cill" is the older building-trade variant you often see on plans and quotes.
Why do builders spell it cill?
Mostly tradition, plus a preference for a distinct building term on technical drawings. The spelling has been used in construction for a long time and has simply stuck, even though "sill" is more common in general writing.
Does the spelling change what the part is?
No. Whether it is written cill or sill, it is the same component doing the same job: shedding water on the outside and finishing the reveal on the inside.
What is a window cill made from?
Common materials are uPVC, timber, stone or concrete for external cills, and aluminium on modern frames. The choice affects the look, the lifespan and how much upkeep it needs.
Can I repair a damaged window cill on its own?
Often, yes. If the frame and glazing are fine, a cracked or leaking cill can usually be repaired or replaced without renewing the whole window. A quick look will confirm whether a repair does the job.
Thinking about new windows or tidying up worn cills? Download our latest brochure to see the styles and finishes, or request a free quote and we will pop out for a look.