A drip sill is a protruding edge or profile at the bottom of a window frame, window, or sill that diverts rainwater away from the facade. Without a drip sill, water runs along the frame and into the wall, leading to moisture damage, algae growth, and rot in the timber frame.
How does it work?
The drip sill protrudes a few centimetres beyond the facade. On the underside there is a drip groove (drip nose): a small channel that causes water to detach and drip downward rather than running back along the wall.
The principle: water always follows the surface (capillary action). The drip groove breaks this — the water “drips off” at the lowest point.
Where are drip sills found?
- Under the window frame — The bottom rail or a separate drip sill profile
- On the window sill — The external sill slopes downward and protrudes
- Above windows and doors — Sometimes a drip sill in the upper masonry
- On the fascia board — A drip edge at the top of the fascia board (on flat roofs)
Materials
| Material | Application |
|---|---|
| Aluminium profile | Under window frames, on fascia boards |
| Natural stone | Window sills |
| Concrete | Cast sills |
| Timber (profiled) | Part of a timber window frame |
| uPVC | With uPVC window frames |
Common mistakes
- No drip sill on the frame — Water enters the cavity or runs down the facade
- Drip sill mounted flat — Must always slope slightly downward (away from the building)
- No drip groove — Without a drip nose, water runs back along the wall
- Sealant over the drip sill — The sealant must not bridge the drip sill, otherwise it won’t work
Related terms
- Window/door frame
- Plinth window
- Fascia board
- Joint