The door cill is one of those parts of a door most people never think about until it fails. It sits right at the bottom of the frame, doing the quiet but vital job of keeping rain out and directing water away from your home. This guide explains what a door cill is, how it differs from a threshold, and why it matters, all in plain English.
A door cill (also spelled sill) is the horizontal section that runs along the bottom of a door frame. It forms the base of the frame and the surface the door closes down onto. Its main job is weathering: shedding rainwater away from the building rather than letting it pool or run back inside.
A well-designed cill is slightly sloped and shaped so that water hits it and drains outwards. Many have a small lip or upstand and drainage channels that carry water away. Get this right and your doorway stays dry through a proper Yorkshire downpour. Get it wrong and you end up with damp, draughts and rot.
You will see the words cill and sill used to mean the same thing. Cill is common in the window and door trade, while sill is the everyday spelling. Both are correct.
People often mix these two up, but they are not quite the same.
On many modern doors the two work together as a single system. The cill provides the weatherproof base outside, and the threshold, often with a weather seal, creates the barrier your foot crosses. Together they keep water, wind and draughts out at the most vulnerable point of any external door.
There is more than one option, and the right one depends on the door and who uses it.
If accessibility matters in your home, a low threshold is worth discussing. It trades a little weather protection for a step-free entrance, which is why the seals and drainage around it need to be spot on.
The bottom of a door takes the worst of the weather. Rain runs down the door face and gathers exactly where the cill sits, so this is where problems usually begin. A sound cill:
When a cill or its seals fail, you notice it fast: a cold draught along the floor, damp patches inside the door, or water sitting on the threshold after rain. On our composite doors and uPVC doors, the cill and threshold are matched to the door as a complete weathertight unit, which is a big part of why a properly fitted door performs so much better than a tired old one.
Keep an eye out for these:
Sometimes a fix is simple, such as replacing worn seals or clearing blocked drainage channels. Other times, especially where rot has set in, replacing the cill or the whole door is the sounder long-term answer. If you are not sure which, our door and window repair team can take a look and give you an honest steer, rather than pushing a new door you may not need.
A little care goes a long way:
The cill is the weathering base of the door frame that sheds rainwater away from the house. The threshold is the strip you step over where the door meets the floor. On modern doors they work together to keep out water and draughts.
Both are correct and mean the same thing. Cill is common in the window and door trade, while sill is the usual everyday spelling.
Yes. Low thresholds are designed to be nearly flush with the floor for easier access with wheelchairs and prams. They rely on good seals and drainage to stay weathertight, so proper fitting matters.
Start by checking the threshold seals and cill for wear, gaps or blocked drainage. Replacing worn seals often solves it. If the cill or frame has rotted, repair or replacement is the better fix.
Thinking about a new external door with a properly sealed cill? Download our latest brochure, or request a free quote and we will make sure your doorway stays dry.