Cill

A cill (sometimes spelled "sill") is the horizontal element that forms the bottom of a door or window opening. At an external door, it sits beneath the threshold, bridging the gap between the internal floor finish and the outside ground or paving level.

It's shaped to shed water outward and downward, away from the building, rather than letting it pool against the frame or run back into the structure.

The cill's main job is weathering: it throws rainwater running down the face of the door clear of the wall below, rather than letting it track back underneath the frame. Most cills are formed with a slight fall (slope) away from the building and a "throating" or drip groove on the underside to stop water creeping back along it by capillary action.

It also carries the transition in level and material between the inside floor finish and the outside ground, so it has to bridge that gap while staying weathertight.

• Beneath external door frames
• Beneath window openings, in the same role

The cill has to be specified to suit the door set it pairs with (metal doors will often be supplied with a matched metal cill) and the sill height and outside ground level shown on the drawings, since the cill is what actually resolves that level change. Where the cill is stone or reconstituted stone, it can often be selected from a manufacturer's range of standard cill profiles rather than specified from scratch.

A cill that doesn't shed water properly is a common source of damp at door thresholds, so the fall and drip detail are important. Where accessibility requires a low-threshold (flush) door, it's advisable to use a threshold drain recessed into the ground instead of relying on the cill's own fall.

• Door / Window: the cill sits beneath the door or window frame
• Threshold strip: often sits over or alongside the cill at the internal face
• DPC: continues under the cill to maintain the damp-proof line across the opening
• Screed / floor finish: meets the cill at the internal floor level