External sills and window boards
The finishing of an opening. The sill outside carries water away from the façade, the window board inside closes off the reveal. Without them even the best window lets water into the wall.
Why the sill is a serious item
The sill is a metal tray below the window, sloped away from the wall. Its job is to take the rain running down the glass and carry it clear of the façade, instead of letting it sit in the joint between window and wall.
When the sill is missing, when the fall is too shallow or when the ends are badly closed, water gets into the reveal. The result only shows months later: damp on the inside of the wall below the window, paint flaking, sometimes mould. Replacing the window then does not help, because the window was never the problem.
Three details most often skipped
- Fall — the sill must slope away from the wall. A sill laid flat holds water and snow right against the joint with the window.
- End caps — they close the ends of the sill. Without them water runs off sideways straight into the reveal.
- Anti-drumming tape — a strip under the metal that damps the sound of rain. Without it an aluminium sill drums in heavy rain, and it is heard in the room.
Internal window boards
A PVC board closes the reveal from inside and gives a shelf below the window. It is cut to the depth of the reveal and the width of the opening, with an overhang at the sides. The ends are closed with caps in the same finish.
Finishes run from plain white through marble effects to wood decors matched to the joinery. PVC is chosen because it does not absorb moisture — a window board regularly meets condensation, a plant pot or spilt water.
Cut to size on site
Openings in older houses are rarely truly square. That is why sills and boards are finished on site, to the actual reveal, rather than cut in advance to a drawing and then forced to fit.
Specification
| External sill | Aluminium, powder coated |
|---|---|
| Sill colours | White, brown, anthracite, RAL |
| Board | PVC |
| Board finishes | White, marble, wood |
| Ends | Sealing caps in the same shade |
| Damping | Anti-drumming tape under the metal |
| Sizing | Cut to the reveal on site |
Frequently asked questions
Can old sills be kept when replacing windows?
Rarely. New joinery has a different installation depth from the old, so the old sill either does not reach far enough out or does not seat against the profile. Keeping the old sill is the most common cause of damp after a window replacement.
How far should a sill project from the façade?
Far enough for water to drip clear of the wall rather than run down the façade. The exact projection depends on the thickness of the render or cladding — it is measured on site.
Does a sill drum in the rain?
An aluminium sill without damping does. With a damping strip under the metal the sound drops to a level not noticed in the room. It is a cheap item that is often left out of a quote — with us it is standard.
Related services
Measure before you decide.
Measuring is free, and the quote comes back with every item priced separately.