Flush sash windows in London, sitting flat in the frame.
A flush sash closes so the opening sash finishes level with the outer frame, the way a joiner would have hung a timber casement a century ago. Modern versions keep that flat face and quietly add A-rated glass, multipoint locking and a frame that never needs painting.
Flush against stormproof, in one line
Almost every uPVC casement sold in Britain is stormproof, where the sash overlaps the frame and stands proud of it by around 8mm. A flush sash sits inside the frame instead, so the outside face is one continuous plane. That is the whole difference, and it changes how a period elevation reads more than any other single choice.
| Flush sash | Stormproof | |
|---|---|---|
| Outside face | Sash finishes level with the frame, one flat plane | Sash overlaps and stands proud of the frame |
| Reads as | Timber joinery, correct on period elevations | Obviously modern, fine on postwar stock |
| Weather | Relies on the gasket line and drainage, needs good manufacture | The overlap itself sheds water, more forgiving |
| Cost | Roughly 10 to 15 percent more for the same opening | The industry default, cheapest per opening |
| Sightline | Slimmer visible frame, more glass | Chunkier, more frame on show |
Three ways to get a flat face
uPVC flush sash
The volume choice. Woodgrain foils in cream, chartwell green, anthracite and painted-timber whites hold up well in London air. No repainting, A-rated as standard.
Timber flush casement
The real thing, and the only option some conservation officers will pass. Engineered redwood or accoya, factory finished, on a repaint cycle of roughly eight to ten years.
Aluminium flush
Slimmest sightline of the three and the best choice for wide openings, where a timber or uPVC sash would need a transom to stay rigid.
All three take the same A-rated sealed unit, so the thermal performance is a frame decision rather than a glass one. If you are matching an existing run of windows, bring us a photo and we will tell you which of the three will actually blend.
The London stock this suits
Flush sash earns its premium on anything built before the war, and on the interwar semis that fill Enfield, Barnet and the outer North London boroughs. On those elevations a stormproof sash reads as a replacement window from across the street. A flush one does not.
It is also the usual compromise in conservation areas, where timber is preferred but a well made flush uPVC or aluminium casement will often pass where a stormproof one will not. We survey with that in mind and tell you before you order, not after.