A threshold is the bottom horizontal part of a door or window frame, where the door or window rests. The threshold forms the transition between inside and outside (for exterior frames) or between two rooms (for interior frames). It’s one of the most stressed parts of a frame, as it’s constantly exposed to moisture, dirt and mechanical wear.
Types of thresholds
| Type | Material | Application |
|---|---|---|
| Natural stone threshold | Bluestone, granite | Exterior doors, traditional construction |
| Concrete threshold | Reinforced concrete | New builds, commercial buildings |
| Wooden threshold | Hardwood (meranti, iroko) | Wooden frames, renovation |
| Plastic threshold | PVC, composite | PVC window frames |
| Aluminium threshold | Aluminium | Aluminium frames, sliding doors |
Slope and water drainage
A threshold must always have a slope outward: at least 3-5% gradient. This prevents rainwater from running inside. Stone thresholds often have a drip groove on the underside — a small channel that lets water drip off before reaching the facade.
Installing a threshold — what to watch for
- Waterproof connection — Use sealant or an EPDM profile between threshold and frame
- Check the slope — Level isn’t good enough; the threshold must slope outward
- Expansion — Leave 5-10 mm space on the sides for stone thresholds (thermal expansion)
- Full support — The threshold must rest on masonry or mortar over its entire length — no hollow spaces
Common mistakes
- No slope — Water sits on the threshold and seeps inside
- Forgetting to seal the underside — Moisture creeps between threshold and masonry through capillary action
- Wrong order — The threshold should be placed before the frame, not after
Related terms
- Door frame
- Reveal
- Masonry
- Slope/fall
- EPDM