What is a door sill?
A door sill is the horizontal timber or bar at the bottom of a door or window opening, forming the transition between the floor and the frame. The sill protects the bottom of the frame from wear, water and draughts, and forms a threshold barrier at external doors.
Types of door sill
| Type | Material | Application |
|---|---|---|
| Timber sill | Hardwood or pressure-treated pine | Internal frames, dry areas |
| Stone sill | Concrete, natural stone or composite | External entrances, front door |
| Aluminium sill | Aluminium profile | Modern frames, sliding doors |
| PVC sill | Plastic | Low-threshold access, level entry |
Part of the frame
The sill is the bottom element of the frame. In a complete frame you have:
- Head (top rail) — horizontal member at the top
- Sill (bottom rail) — horizontal member at the bottom
- Stiles — the vertical sides
At an external door, the sill continues outside as a drip edge: an angled nose that diverts rainwater away from the building.
Fitting a door sill at an external door
- Allow a setting gap — 5–10 mm between frame and structural opening
- Set the sill level — use a spirit level
- Fix in place — with frame screws or plugs and bolts into the concrete floor
- Seal — expanding foam on the inside, sealant on the outside
- Finish — fit a threshold bar or connect the floor covering
Level-threshold installation
At level access (often required in new build), the sill is made as low as possible or omitted entirely, with a special channel drain or sub-sill profile that keeps water out without a raised threshold.
Common mistakes
- Sill not level — door closes at an angle
- No drip edge outside — water runs down the wall
- Poorly sealed joint — water infiltration and timber rot
Related terms
- Window / door frame
- Drip edge
- Structural opening size
- Frame stile
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