The difference is where the hinge sits, and everything else follows from that one fact. A centre-pivot window turns about a horizontal axis roughly halfway up the sash, so as the bottom swings out the top swings into the room. A top-hung window is hinged at the head and the whole sash swings outward and upward, leaving the full opening clear. One gives you a window you can clean from the floor. The other gives you a window you can lean out of.
The axis, and what it costs you
On a centre-pivot unit the pivot bearings sit in the side frames at around the midpoint of the sash. When you open it, the sash rotates. The upper half comes down into the room and hangs across the opening at head height. That is why a centre-pivot window can be turned right round for cleaning and why it cannot give you a clear opening.
On a top-hung unit the hinges are at the top and the sash swings out on an arm. Nothing comes into the room. The aperture below the sash is completely open, which is why this is the arrangement used where escape matters and where you want to put your head and shoulders out.
| Centre-pivot | Top-hung | |
|---|---|---|
| Hinge position | Midway up the side frames | At the head of the frame |
| Sash intrudes into room | Yes, upper half | No |
| Clear opening | Restricted by the rotated sash | Full frame aperture |
| Cleaning the outer face | Rotate 180 degrees, clean from the floor | Reach out, or use a pole |
| Control position | Bar across the head of the sash | Handle at the bottom of the sash |
| Suits a low cill | Less well | Well |
| Escape duty | Only with a secondary escape hinge | Yes on most sizes |
Standing at the opening, or standing back from it
This is the difference you feel every day. Open a top-hung window and you can walk up to the frame and look down at the garden with nothing between you and the outside. Open a centre-pivot and there is a pane of glass at your forehead. You can still see out, but you look under it and past it rather than through the opening.
If the window is in a wall of a loft room where you would naturally stand and look out, top-hung is the better experience by some distance. If it is high in the slope over a stairwell or a landing where nobody is going to stand at it, that advantage is worth nothing.
Reaching the control
The control bar on a centre-pivot unit runs across the top of the sash. It is comfortable to reach on a tall window whose head is at normal ceiling height, and it lets you operate the window without leaning over whatever is underneath it. That is the reason centre-pivot units work over a desk, a bath, a low run of storage or a bed.
The handle on a top-hung unit is at the bottom of the sash. That is easy if the cill is at waist height and awkward if the window is mounted high on a steep slope. If you specify top-hung, check where the bottom of the sash lands. A handle at two metres is a handle you will not use.
Cleaning without leaving the room
A centre-pivot sash rotates through 180 degrees and locks with a barrel bolt so the outer face presents into the room at chest height. You clean it standing on the floor. That is a real advantage on a second floor rear slope where nothing short of a pole or a scaffold reaches the outside.
A top-hung sash does not rotate, so the outer face stays outside. On a low single storey roof you can lean out and reach most of it. Higher up you need a telescopic pole or a window cleaner. Some manufacturers make a dual-function unit that works top-hung for everyday use and pivots for cleaning, and on a high window that is money well spent. The method is set out on cleaning the outside of a roof window.
Escape openings, and where the pivot needs help
Where the window is the designated escape route from a loft conversion, the clear openable area has to be at least 0.33 square metres with both clear dimensions at least 450mm. A rotated centre-pivot sash sits across the opening and rarely leaves enough clear height to satisfy that, however big the glass is.
Top-hung units generally do satisfy it in the larger sizes, and manufacturers make dedicated escape versions where the sash swings up through a much wider angle on a strengthened arm. There are also combination units with a secondary escape hinge that releases a centre-pivot sash to swing from the head. Check the published clear opening figure, not the glass size. The requirements are covered on whether a roof window can be an escape window.
Pitch, sash weight and how each behaves
Roof pitch changes the feel of both. On a shallow roof, below about 30 degrees, a top-hung sash has to be pushed a long way to open much, and gravity is pulling it shut the whole time. A centre-pivot on the same roof rotates easily.
On a steep roof, above about 55 degrees, the position reverses. A centre-pivot sash swings fast and is heavy to bring back through the vertical, and a top-hung unit opens with much less effort and stays where you put it. Most manufacturers publish a pitch range for each mechanism. Check yours with a level rather than by eye.
Ventilation on a windy day
A top-hung sash acts as an awning over the opening, so it can be left open in light rain without water coming straight in, and it scoops wind across the aperture. A centre-pivot in the open position leaves a gap at the top and a gap at the bottom, which gives a good cross-flow but lets rain in at the lower gap.
Both have the same background answer for winter: the ventilation flap in the head of the sash, which flows air with the window locked shut and the filter doing its work. That is the setting to use overnight rather than leaving a sash open.
Which one we fit, and when
Centre-pivot for windows mounted high in the slope, for anything above a bath, a desk or a bed, for shallow pitches, and for any position where the outer face would otherwise be unreachable. Top-hung for a loft room where somebody will stand at the window, for steep pitches, for low cills, and for anything designated as an escape opening.
Where the roof takes more than one unit, mixing the two is entirely normal: a top-hung escape window where the room is used, a centre-pivot higher up for light and ventilation. We will set that out at survey against your actual room rather than picking one for the whole house. The comparison sits alongside the broader product question on roof window or rooflight.
We install and replace roof windows and rooflights across Oxford and up to twenty five miles by road. Fifteen years in the trade, a ten-year workmanship guarantee, VELUX, Fakro, Keylite and Roto units fitted, and Building Control notified as part of the job.
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