How it works
The parts of a door threshold are easiest to see in section. This drawing cuts through the bottom of an exterior door and the adjustable sill it closes on, with the outside on the left and the room on the right.
Read the full guide
Where the door seals
The door carries a door bottom, or sweep, on its lower edge. It closes onto the threshold cap, which has a weather seal on top. Adjusting screws through the cap raise or lower it until the two meet.
The sill and what is under it
The cap stands on the sill. The nosing is the sill's outer edge, and a sill extender can widen it. A dam stands at the inner edge, and in a metal sill a thermal break divides outside from inside. The sill rests on a substrate, over a sill pan, on the subfloor.
- Threshold and sill are often used for the same thing. Makers call the whole base the sill and the raised strip the cap.
Questions and answers
What are the parts of a door threshold called?
On an adjustable sill: the sill itself, its nosing at the front, the threshold cap the door closes on, the adjusting screws through the cap, the weather seal on top and the dam behind. The door bottom, or sweep, is the part on the door.
What is the difference between a door sill and a threshold?
In makers' sheets the sill is the whole base of the doorway, and the threshold cap is the raised strip on it that the door closes over. In everyday use both words are used for either.
What are the screws in a door threshold for?
They are adjusting screws. Turning them raises or lowers the threshold cap, so the cap and the door bottom keep a tight seal.
What goes under a door sill?
A sill pan, also called pan flashing: a waterproof tray that sends any water under the sill back outside. Under the pan is the subfloor, which has to be flat, level and solid.