What is the difference between centre-pivot and top-hung?
The hinge. A centre-pivot sash turns about a pair of pivots roughly halfway up its sides, so the top swings into the room and the bottom swings out. A top-hung sash hinges at the head and swings outwards and upwards as one piece, like a hatch. Everything else that separates the two, the view, the head clearance, the cleaning method, whether it can serve as an escape route, follows from that one decision.
Both are made in the same size range, the same frame materials and the same glazing specifications, so this is not a question of quality. It is a question of what the room is doing and where the window sits on the slope. Fitting either is the same day’s work, covered under VELUX window installation.
Where the hinge sits, and what follows from it
On a centre-pivot the pivots are set above the midpoint of the sash, typically around sixty per cent of the way up, which is what makes the sash self-balancing. Push the top rail and it rotates with almost no effort at any pitch. That balance is why the handle sits at the top: you are pushing, not lifting.
On a top-hung unit the entire weight of the sash hangs off two hinges at the head and is pushed out against gravity on a control arm or a friction stay. It takes more effort, particularly on a shallow pitch where you are lifting more of the sash weight, and it is why top-hung units are more often specified with a spring assist or an electric actuator.
The arc each sash sweeps inside the room
This is the practical difference people feel first. An open centre-pivot has its top half projecting into the room, and on a steep slope that lower edge can come a long way down towards head height.
A top-hung sash goes entirely outwards. Nothing enters the room at all. In a loft where the useful floor is right up against the slope, or where a bed, a desk or a basin sits under the window, that is the whole argument. Set a centre-pivot over a desk and it will be opened once and then left shut, which makes it an expensive fixed panel.
Head height, and the position on the slope
The two suit different positions. A centre-pivot is usually set higher on the slope, both because it needs clearance under it for the rotating top half and because the view through it is taken from a standing position or from further back in the room.
A top-hung is set lower, closer to eye level for somebody standing at the slope, because that is where the unobstructed view pays. On a steep Victorian rear slope in Chelmsford, a top-hung with its sill around nine hundred millimetres above the floor puts the whole opening in front of you. The same window fitted at that height with a centre-pivot action would swing into your face.
The view, and what each does to it
A top-hung window gives an uninterrupted view through the whole pane, because the sash is above you and out of the way. Lean on the sill and you are looking straight out over the garden with nothing crossing the field.
A centre-pivot in the open position is halved by its own sash, so the view is best taken with the window shut. Closed, both are identical; it is only in use that they diverge. Where the point of the window is the outlook rather than the daylight, top-hung wins without argument. Where the point is daylight into a room used from further back, the difference stops mattering.
Cleaning, and the rotation only one of them offers
A centre-pivot rotates through most of a half turn and locks there on a barrel bolt, presenting the outer face of the glass into the room at a comfortable working angle. It is the easiest overhead glass in the trade to keep clean, and it is done from the floor in five minutes.
Top-hung units get to the same place by a different route. Most have a secondary pivot: swing the sash out on the head hinge, release a catch on the side, and the sash then rotates on a lower pivot into a cleaning position. It works, and it is a step more fiddly, and on some older or escape-specification units the rotation is not offered at all. The method for both is set out under cleaning the outside of a roof window.
Escape, and the one that qualifies
Only a top-hung sash can serve as a means of escape, and this is the difference that is not a preference. Building Regulations want a clear opening of at least 0.33 square metres with neither dimension under 450mm, measured through the unobstructed hole.
A centre-pivot cannot deliver that at any size, because the sash body sits across the middle of the opening. A top-hung escape variant swings wide, typically to around forty five degrees or more, and clears the whole aperture. Where a loft conversion needs an escape window, the action is decided by the regulation before anybody looks at a catalogue. The detail is on whether a roof window can be an escape window.
Ventilation, weather and living with each
Both give purge ventilation from the highest point of the room, and both carry a ventilation flap in the handle for background trickle with the sash locked shut.
In use they behave differently in rain. A centre-pivot left slightly open on its ventilation catch has the top of the sash tilted out and sheds a light shower reasonably well. A top-hung sash open on a stay acts as a canopy, so it keeps rain out at wider openings, which is useful on the exposed slopes out towards the Blackwater and Dengie. In a strong wind the top-hung is the one to close first, because an open sash on a head hinge presents a large surface to an uplift.
Side by side, in one table
| Consideration | Centre-pivot | Top-hung |
|---|---|---|
| Hinge position | Sides, above midpoint | Head |
| Sash in the room when open | Top half swings in | Nothing swings in |
| Best position on slope | Higher | Lower, near eye level |
| View when open | Interrupted by the sash | Clear through the pane |
| Cleaning | Full rotation, bolt engaged | Secondary pivot, more steps |
| Escape rated | No | Yes, on escape variants |
| Effort to open | Balanced, very light | Lifts the sash weight |
| Pitch range | Wide, typically 15 to 90 degrees | Narrower at the top of the range |
Choosing between them in a real Essex loft
Three questions settle it. Is anything sitting directly beneath the window, in which case top-hung. Does the room need an escape opening, in which case top-hung. Is the window high on a steep slope with nothing under it and cleaning ease the priority, in which case centre-pivot.
The fourth pattern is the useful one: two windows in adjacent rafter bays, one of each. The top-hung goes low for the view and the escape provision, the centre-pivot goes high for the daylight and the easy cleaning, and the pair is weathered under one combination flashing kit. On a trussed roof at Great Notley or Beaulieu Park, where width is capped between the trusses anyway, that arrangement is very often the right answer for reasons that have nothing to do with the hinge.
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