- A flat or inward-sloping sill on which water lies
- An external sill without a drip edge, so water runs down the facade
- A missing or interrupted water barrier under the sill
- Unfinished ends of the external sill, letting water get behind it
Building detail · Window frame
Sill and drip: keeping water off the facade
Most water collects at the bottom of a window frame. A well-designed bottom rail and external sill carry that water away from the facade and the cavity. Here you can see the build-up and the points that make the difference.
- Outward fall on the sill and drip
- A drip edge that keeps water off the facade
- A water barrier that drains leaking water into the cavity
Principle sketch
Build-up of the detail
Vertical section through the bottom rail of a window frame in a cavity wall. The numbers in the sketch refer to these parts:
- Outer leaf
- Cavity insulation
- Inner leaf
- Timber bottom rail falling outwards
- External sill with drip edge
- Water barrier under the sill, draining into the cavity
- Insulating glass
- Window board
Common mistakes
Where it often goes wrong
Leaks, condensation and cracks usually start at the junctions, not in the middle of a wall or roof.
In the drawing
What we look out for
We work out the detail for your home, based on a site survey and the chosen materials.
- The fall and projection of the sill and external sill
- The finish at the ends of the external sill
- A water barrier that continues into the cavity
- The connection of the glass and glazing gaskets to the sill
- A window board that limits thermal bridges on the inside
In existing frames, rot is often found in the bottom rail. We assess whether repair is possible or whether a new sill and external sill are the better choice.
Detail FAQ
Frequently asked questions about this detail
Why does a bottom rail rot faster than the rest of the frame?
Because that is where most water collects. If water lies on it or gets behind the external sill, the timber stays wet for longer.
Which material suits an external sill?
Often aluminium, natural stone or concrete. The choice depends on the facade, the frame and the look that suits the house.
Is a water barrier under the sill always needed?
In a cavity wall a water barrier under the sill is usual, so that any water that does get in drains into the cavity. The exact detail depends on the facade build-up.
Can I use this principle sketch for my own home?
The principle sketch shows the build-up, but it is not a working drawing. Dimensions, materials and fixings depend on your home. We work out the detail for your situation.
Detail library