Guide · VELUX Window Installation
Centre-pivot and top-hung roof windows compared
The whole difference is where the sash hinges. A centre-pivot roof window turns about a horizontal axis roughly halfway up the frame, so as the bottom of the sash swings out, the top swings in. A top-hung window hinges at the head and pushes the entire sash outwards, away from the roof plane, so nothing enters the room at all.
Every other difference follows from that single fact: where you stand to open it, what can sit underneath it, how the outer face gets cleaned, how much air actually moves, and whether the opening can be counted as a way out in a fire. This page works through the comparison for a tiled or slated slope on an Essex house. The wider process, from survey to notification, sits on the VELUX window installation page.
Where each one hinges
On a centre-pivot unit the hinges are set into the side jambs at, or slightly above, the midpoint of the sash. The sash is close to balanced about that line, which is why a large centre-pivot window can be moved with one finger and why it holds position wherever you leave it.
On a top-hung unit the hinges are at the head, and the sash is cantilevered off them. All of the weight hangs off two points at the top, so the mechanism carries gas struts or a friction stay to hold the sash up and to stop it dropping when you let go. That hardware is the reason top-hung units cost more and the reason they have a shorter list of available sizes.
The arc a top-hung sash swings through
A top-hung sash typically opens to around forty five degrees from the roof plane, and on a moderate pitch that puts the leading edge of the sash a long way out over the slope. It needs clear roof outside: no abutting wall within the arc, no chimney stack immediately above, no adjacent unit close enough for the two sashes to argue.
What you gain is an entirely unobstructed opening. Stand at the window and there is glass above your head and nothing across the middle of the view. On a rear extension roof looking down a garden, that is the point of choosing one.
What happens in the room when a centre-pivot opens
The upper half of the sash rotates inwards and downwards into the room. On a thirty degree slope with the window opened to its first catch position, the bottom rail of the reversed sash ends up somewhere around head height for anyone standing directly beneath it.
In a loft bedroom with the bed under the window that is a non-issue, because nobody stands there. In a kitchen with a run of wall units below, or in a landing where people walk past the window, it is a daily nuisance. This is the single most common regret with a centre-pivot unit, and it is entirely predictable at survey by standing in the room and holding a hand where the sash will land.
Stand in the room and mark where the reversed sash will sit before you choose the mechanism, not after.
Reaching the handle, and where you stand
A centre-pivot window carries its handle in the top bar of the sash, so it is opened by reaching up. A top-hung window is opened from a handle at the bottom of the sash, which has to be within comfortable reach, and that fixes how low in the slope the unit can sit.
Height in the slope is a design decision in its own right, and it is set out on the page about cill height and the view out. For this comparison the short version is that a top-hung unit wants to be lower in the roof than a centre-pivot one, because the handle travels away from you as the sash opens.
Cleaning the outer face from inside
A centre-pivot sash rotates through one hundred and eighty degrees and locks in the reversed position with a barrel bolt on the jamb. The outer face then presents itself to you at a comfortable working height and the glass is cleaned from a standing position in the room.
Most top-hung units keep this ability by offering a second, pivoting mode: release a catch and the sash swings from the centre instead of the head. Where a unit is genuinely top-hung only, the outer pane has to be reached from outside, which on a first floor slope means a ladder and on a loft slope means a tower. Coated self-cleaning outer glass reduces the frequency of that job without removing it.
Air movement compared, at the same opening angle
These behave differently, and not in the way people expect. A top-hung sash gives one clear aperture with air free to move through the whole of it, which suits purging a hot room quickly on a summer evening.
A centre-pivot sash divides the opening into two: a gap at the bottom of the frame outside the sash, and a gap at the top inside it. Cooler air enters low and warm air leaves high, and the sash itself acts as a small baffle in the middle. For steady background airflow in a bedroom that split is quietly effective. For shifting a lot of air fast, the single clear opening wins.
| Centre-pivot | Top-hung | |
|---|---|---|
| Sash in the room when open | Upper half swings in | Nothing enters the room |
| Handle position | Top of the sash | Bottom of the sash |
| Cleaning the outer pane | Rotate and bolt | Pivot mode, or from outside |
| Typical pitch range | About 15 to 90 degrees | About 15 to 55 degrees |
| Clear view when open | Sash crosses the middle | Fully unobstructed |
| Size grid | Widest available | Fewer sizes, longer lead |
Wind on an open sash
A centre-pivot sash is close to balanced about its hinge line, so a gust pushes on both halves at once and the net effect is small. It stays where it was put. A top-hung sash is a flat plate cantilevered off the head, which is to say it is a sail with a lever arm.
In an exposed position, and the Blackwater estuary villages and the open land north of Chelmsford qualify, a top-hung unit held open on a windy day works its struts and its stay hard. Both types have wind-resistant catch positions that hold the sash at a small opening, and that catch is the setting to use when the forecast is unhelpful rather than propping the sash wide and hoping.
Rain, and what gets in while it is open
Neither type is weathertight while open, but they fail differently. On a top-hung unit the sash acts as a small canopy over the opening, so light rain running down the slope is largely thrown clear and a modest opening can be left in a shower.
On a centre-pivot unit the upper part of the sash has rotated inwards, and water sitting on what is now the underside of that glass runs to the bottom rail and drips onto the floor of the room. The frames carry a drainage channel that deals with the frame itself, not with the reversed pane. In practice a centre-pivot window gets shut when it starts raining, which is one argument for a rain sensor on a unit that is high in the slope.
Pitch, and the range each mechanism accepts
Centre-pivot units are commonly rated from about fifteen degrees up to ninety, which covers everything from a shallow modern extension roof to a near-vertical dormer cheek. Top-hung units are usually rated to a narrower band, around fifteen to fifty five degrees, because above that the sash is being lifted rather than pushed and the geometry of the stay stops working.
Essex has plenty of steep roofs. Victorian rear additions in Chelmsford and Colchester often run past fifty degrees, and the timber-framed cores of Thaxted and Saffron Walden steeper still. On those slopes the mechanism choice is made by the roof before anyone gets to have an opinion about it.
Escape duty, and which mechanism can meet it
A standard centre-pivot sash cannot serve as an escape opening, because the sash sits across the middle of the aperture and halves it. A top-hung sash that swings clear can, and manufacturers also make escape-hinged units that release from centre-pivot to a wide top-hung swing when a catch is operated.
Whether the room needs one at all depends on the storey and the room, and the clear opening figures are set out in full on roof window escape openings and the rules. If a loft conversion is in the project, settle that question before the mechanism, because it removes the choice on at least one window.
Furniture, worktops and the space below the window
The rule is simple. If something permanent occupies the wall or floor immediately below the window, the sash must not need that space.
A top-hung unit is the answer over a kitchen worktop, over a fitted desk in a loft study, at the head of a staircase where somebody is climbing towards the glass, and in any circulation route. A centre-pivot unit is fine over a bed, over a bath, over a sofa, or anywhere the floor in front is left clear. Where a room genuinely cannot spare either, a fixed unit with a separate opening vent elsewhere is worth considering, and the flap options are covered on ventilation flaps and filters.
Blinds and awnings on each type
Internal blinds run in side channels fixed to the sash, so they travel with it and work identically on both mechanisms. The difference shows up outside. An external awning blind on a top-hung sash has to clear the arc, and on some units the awning is fitted to the frame rather than to the sash for exactly that reason.
Solar-powered blinds put a small panel on the top of the sash, which on a top-hung unit swings out into the open sky and on a centre-pivot unit rotates inwards when the window is opened. Neither is a problem, but the panel wants a slope that actually sees sun. Blind types and the decision timing are set out on roof window blinds and when to specify them.
Sizes, availability and lead time
Centre-pivot is the volume product. It is made in the full width and height grid, it is held in stock by merchants across Essex in the common sizes, and it can usually be on site within days. Top-hung units are made in a shorter list of sizes, are less often stocked, and generally carry a longer lead time and a higher price for the same glass.
That matters on a programme. If a roof is coming off in a fortnight and the specification is still open, the centre-pivot unit is the one that will actually arrive. The code system behind those sizes is explained on roof window sizes and how they are coded.
Combination units: a top-hung sash over fixed glass
Where a tall opening is wanted and only part of it needs to move, a top-hung sash above a fixed pane below is a neat arrangement. The moving part is at the top where warm air collects, and the fixed lower pane gives an uninterrupted view down the slope with no hardware crossing it.
Structurally this is one tall opening in the roof, so the trimming and the flashing are designed as a set rather than as two separate jobs. Where the intention is instead two units next to each other on the same slope, that is a different detail with its own gutter and soaker arrangement.
Swapping one type for the other in an existing opening
Within a single maker’s range, a centre-pivot and a top-hung unit of the same size code share the same outer frame dimensions, so the structural opening usually takes either. That makes a change of mechanism a realistic option at replacement rather than a rebuild.
Two things get checked first. The pitch has to fall inside the narrower top-hung band, and the height of the unit in the slope has to leave the new handle within reach. The flashing kit is normally renewed at the same time, because a kit that has been dressed into a covering once rarely comes out in the condition it went in.
Choosing on the roof and room you actually have
Three Essex situations, three straightforward answers. A loft bedroom in an interwar semi with the bed under the slope: centre-pivot, because the space in front of the glass is never occupied and the wider size grid gives more daylight for the money. A rear extension kitchen with a worktop below the window: top-hung, because nothing may swing into the working area.
A steep Victorian rear addition serving a stair landing: centre-pivot, because the pitch rules the alternative out and the sash swings into a space nobody stands still in. The mechanism is not a matter of taste. It is decided by the pitch, the room use and the reach, and those three are settled on the survey rather than in a brochure.
Keep reading
More on velux window installation
Roof window blinds and when to specify them
Read the guide →Roof window flashing kits by roof covering
Read the guide →Roof window sizes and how they are coded
Read the guide →Roof window ventilation flaps and filters
Read the guide →Roof window cill height and the view out
Read the guide →
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