Centre-pivot and top-hung roof windows compared
A centre pivot window hinges about its middle, so half the sash swings outwards over the roof while the other half swings down into the room. A top hung window hinges at the head, so the whole sash travels outwards and nothing crosses the line of the reveal. Everything else follows from where that hinge sits: the pitch range the unit is rated for, how much of the room the open sash borrows, whether a bed or a worktop can sit beneath it, what happens when it rains on an open window, and whether the opening counts as a means of escape. The range codes for each are on the VELUX window installation page. What follows is the mechanism underneath them, and how the choice actually falls on the roofs around Gloucester.
Two hinge positions, and the different job each one asks of the hardware
On a centre pivot the sash turns on a pair of pivot hinges fixed to the side jambs, usually set marginally above the true midpoint so the sash sits slightly bottom heavy and stays where it is put. Friction blocks in the hinge hold it at any angle. The important consequence is that the sash weight is divided either side of the pivot line, so whatever the pitch of the roof, the hinge is only ever resisting friction and a small out of balance moment. That is why the design is rated from about 15 degrees all the way to 90.
A top hung sash has no counterbalance at all. It hangs off two hinges at the head and is pushed out against gravity by the person opening it, then held there by a spring cassette or a stay. The steeper the roof, the more the sash hangs vertically and the more of its own weight the springs have to carry. That is a hardware limit, not a caution, and it is the reason the top hung ranges stop at around 55 degrees while the centre pivot keeps going.
The arc a rotating sash sweeps, and the space it borrows from the room
Rotate a centre pivot to its normal ventilating position and the lower half of the sash swings inwards by roughly a third of the sash length. On a 78 by 140cm unit that is a sheet of glass and timber arriving somewhere around 400mm into the room, measured perpendicular to the slope. On a steep 50 degree Painswick roof that arc happens high, above head height, and nobody ever notices it. On a 24 degree lean-to over a Gloucester terrace kitchen, the same arc puts the bottom edge of the sash at forehead level for anybody standing at the sink.
A top hung sash sweeps nothing inside. The reveal can be finished tight to the frame at the cill and the sides, and the room below the window is usable to within the thickness of the plaster. Where a loft bedroom has the bed under the window, or a study has a desk against the knee wall, that is the whole argument, and it usually settles the choice before pitch is even discussed.
Why the top hung ranges stop at about 55 degrees
Push a top hung sash open on a shallow roof and you are lifting it. Push the same sash open on a steep roof and you are swinging it, because most of its weight now acts along the plane of the glass rather than across it. That sounds easier. It is not, because the same geometry means gravity is trying to slam the sash shut against its stops, and the spring pack has to hold it open against a moment that grows as the pitch rises.
Beyond about 55 degrees the springs either cannot hold the sash reliably or have to be wound so hard that the window becomes heavy to close. Manufacturers set the ceiling there and it is worth taking seriously. Cotswold roofs through Painswick, Bibury and Chipping Campden regularly measure 50 to 55 degrees, and the steep visible face of a Cheltenham mansard can run to 70. On those slopes the centre pivot is not the preferred option, it is the only rated one.
Where the handle sits, and how far up the slope you can reach it
A centre pivot carries a control bar across the top of the sash. A top hung carries a handle at the bottom edge. That difference matters as soon as the window is set high in the slope, which is what a steep roof and a sensible cill height produce together.
On a 45 degree Welsh slate roof in Coleford or Mitcheldean, with the cill of an MK08 landing at a normal height above the finished floor, the top of the frame is close to two and a half metres up the slope. A control bar there is out of reach from standing. The bottom handle of a top hung unit on the same roof is comfortably reachable, which quietly makes the top hung the more practical unit on steep slopes right up to the point where the pitch rules it out. Where neither is reachable the answer is a rod operator, or an electric unit with a rain sensor, covered at electric rooflight installation.
Ventilation behaviour: two slots against one wide mouth
Open a centre pivot and you get two apertures rather than one. Warm air leaves through the gap at the top, outside the sash, and cooler air enters through the gap at the bottom, inside it. The sash itself acts as a baffle between them. In a still room that pair of openings drives a small but genuine convective loop, which is why a centre pivot cracked open a few centimetres shifts more stale air than the same gap in a vertical window would.
A top hung gives a single opening at the cill edge, all of it, with no baffle. For clearing a room fast after a shower or a fry-up it is the more effective of the two, because the free area is one continuous mouth and the sash above deflects wind into it. For trickling air through a bedroom overnight in February, the centre pivot’s split slots are gentler and less draughty. Background ventilation with the sash closed is the vent flap’s job, and that is dealt with on roof window ventilation flaps and filters.
Rain on an open window, and which design forgives it
A centre pivot open at 30 or 40 degrees puts the upper half of the sash out over the roof at an angle, and that projecting half acts as a small canopy above the lower opening. In drizzle, and in rain falling near vertically, very little comes through. Householders work this out for themselves within a fortnight and leave the window ajar in weather they would never leave a top hung open in.
A top hung sash open wide is a scoop. The glass is lifted away from the roof plane and the whole opening is presented upwards. Rain arriving on the wind from the south west, which is the direction it usually arrives from in the Severn Vale and with real force in the Forest of Dean, goes straight in. Neither design should be left open in weather, and both are available with a rain sensor that closes them automatically, but the centre pivot is the more forgiving of the two on an ordinary showery day.
Snow, and the morning neither window opens well
Snow lying on the outer pane is the one condition where both designs behave badly, and differently. A top hung sash has to push the snow up the slope to open, and on a 25 degree roof with 100mm of wet snow on the glass it simply will not move. A centre pivot will rotate, because the top half goes out from under the snow while the bottom half comes in, but the snow that was on the outer pane arrives on the floor of the bedroom.
The practical answer on either is to wait, which is easy to say and worth knowing in advance if the window is the only ventilation in the room. It also argues for the vent flap being usable independently of the sash, which on current units it is.
The two designs against the things people actually notice
| What you notice | Centre pivot | Top hung |
|---|---|---|
| Rated pitch range | About 15 to 90 degrees | About 15 to 55 degrees |
| Handle position | Control bar across the head of the sash | Handle at the bottom edge of the sash |
| Room borrowed when open | Roughly a third of the sash length swings inwards | None. The sash stays outside the reveal line |
| Furniture under the cill | Has to stay clear of the swept arc | Can sit tight to the wall below the window |
| Behaviour in light rain, open | Upper sash shelters the lower gap | Opening faces upwards and takes the weather |
| Fast purge of a steamy room | Two moderate slots | One continuous opening, the larger free area |
| View out standing back from it | Sash frame crosses the sightline when open | Uninterrupted through the whole aperture |
| Cleaning the outer pane | Rotates fully and locks for cleaning | Secondary pivot function rotates it for cleaning |
| Serving as an escape opening | Cannot. The sash halves the clear area | The escape variants are top and side hung |
| Wind on the open sash | Balanced about the pivot, holds position | Stay and springs take the full gust load |
| Hardware to go wrong | Two pivot hinges with friction blocks | Hinges, stays, spring cassette, catch |
| Where it costs more | The simpler and cheaper of the two | More hardware in the sash, so more to buy |
The view out, and the bar that crosses it
Closed, the two look nearly identical from inside: same glass, same frame section, same sightline. Open, they part company completely. A rotated centre pivot puts a horizontal band of timber or polyurethane straight across the middle of the aperture at roughly seated eye level, so the view splits into a strip of sky above and a strip of garden below. A top hung leaves the whole opening clear.
On a roof where the window is high and the view is sky, it makes little difference. On a Stroud valley cottage where the rear slope looks down over the town, or a Cotswold rear elevation with a hill behind it, the top hung is the one people prefer once they have stood under both. How high the cill is set changes what there is to see at all, and that is worked through on roof window cill height and the view out.
Blinds, screens and awnings on each design
Internal blinds run in side channels fixed to the sash, so they travel with it and work identically on both. Insect screens are different. The screen cassette mounts to the frame rather than the sash and pulls down across the aperture, which suits a top hung perfectly because the sash is out of the way. On a centre pivot the screen has to be released before the sash can rotate for cleaning, so it becomes a small ritual rather than a fitting you forget about.
External awnings clip into the outer frame grooves on both designs and are the only shading that stops solar heat before it reaches the glass. Ordering either at the same time as the unit is worth doing, since the brackets are far easier to fit before the covering is dressed tight against the frame. The range and the fixing detail are on roof window blinds and when to specify them.
Escape openings, and the design that cannot serve as one
Where a loft room is a bedroom, the roof window frequently has to be the means of escape, and a centre pivot cannot do it. The rotating sash sits across the middle of the opening and roughly halves the clear area, so a unit that looks generous from the floor fails the dimensional test with the sash in its open position. The escape variants hinge at the top and side and swing right out of the aperture.
This gets discovered late more often than it should, usually when a building inspector looks at a finished loft conversion. The dimensions, the cill height band and which size codes actually clear them are set out on roof window escape openings and the rules, and it is a question to settle before the opening is cut rather than after the plaster is on.
Wear points after fifteen years, and what stays adjustable
A centre pivot ages slowly because there is so little in it. The friction blocks in the hinges lose some grip, so the sash starts drifting shut on its own, and the fix is a screw adjustment at each hinge. The perimeter gasket takes a compression set eventually and lets a draught through on a windy night, and gaskets are a replaceable item.
A top hung has more to go: the spring cassette weakens so the sash no longer holds at full opening and has to be propped, the stay wears at its pin and develops a rattle, and the catch that pulls the sash down onto the gasket loses its bite so the window feels loose when shut. All of it is serviceable, and most of it responds to cleaning and lubricating the hinges once a year. Neither design is likely to be the reason a unit is replaced. The glass usually goes first, which is covered at skylight replacement.
Vale lean-tos at twenty two degrees, and the sash that arrives at eye level
The two-storey rear addition on a Gloucester or Tewkesbury terrace carries a lean-to roof at 22 to 30 degrees over a kitchen, and it is the case where the centre pivot’s arc is genuinely awkward. The slope is shallow, so the sash swings in almost horizontally rather than up out of the way, and the room below is usually the one with the sink, the hob and somebody standing at both.
Add the short run of slope on those roofs, often only 1.8 to 2.4 metres from gutter to abutment, and the window has to sit fairly low in the slope to leave a run of tile above the head for the flashing to lap under. Low in the slope on a shallow pitch is the worst combination for a rotating sash. A top hung unit at the same size code solves it outright, and it is comfortably inside its pitch rating at 24 degrees.
Interwar hips, purlins, and the reason both designs turn up on the same house
The hipped concrete tile roofs at Longlevens, Hucclecote, Warden Hill and Charlton Kings sit at 30 to 35 degrees with a purlin crossing at about mid-height. The available rectangle is short, so units go in below the purlin, low in the slope, in pairs across two rafter bays.
At 32 degrees a centre pivot sash still swings low enough to be a nuisance over a bed, but the pitch is nowhere near the top hung limit, so either will work and the room decides. It is common to end up with a top hung over the bed on the rear slope and a centre pivot on a landing or a box room where nothing sits underneath and the cheaper unit is the sensible one. Mixing them on one house is fine, though matching the frame finish across an elevation is worth doing so the two read as a pair from the garden. Where they sit side by side, see roof window pairs and triples on one slope.
How the choice is actually made at survey, and what to send first
The order of questions is fixed. Pitch first, off a digital level, because above about 55 degrees the decision is already made. Then what sits under the window, because a bed, a desk, a worktop or a bath rules out the swept arc. Then whether the room is a bedroom needing an escape opening, which sends the specification to the side hung variants regardless of preference. Then reach, which on steep Forest of Dean and Cotswold slopes often decides between a bottom handle, a rod and an electric opener. Only after those four does the view out, the ventilation pattern and the difference in unit cost come into it.
To get a useful first answer by email, send a photograph of the roof from the garden, a photograph of the room from inside showing what is under the proposed position, the roof covering if you know it, and the address. Email info@severnskylightsgloucester.co.uk. What moves the figure afterwards is set out on the costs page, and the wider fitting detail sits at roof window installation.
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