Centre-pivot or top-hung: choosing how a roof window opens

With a centre-pivot window, the sash rotates around a horizontal axis close to the middle of the frame. Open it and the upper half goes out over the roof while the lower half comes in towards the room. The top-hung window has its hinge along the upper edge instead. Open that one and the lower edge lifts clear of the frame, travelling out and up over the slope.

That single difference in geometry shapes everything else. It sets where the glass sits when the window is open, how much clear space is left to look or climb through, and how much of the sash you can reach from the floor of the room.

Approved Document L (Wales) defines a roof window as a window installed in the same orientation as the surrounding roof and in plane with it. Both of these fit that description, so the energy figures and the planning conditions are the same for both. The hinge matters for purge air and escape, which the sections below cover.

Words worth knowing before the survey

A few terms come up in every conversation about opening styles. They are short, and knowing them makes a quote easier to read.

Start by standing where you will use the window. A bed head, a desk, a shelf of books and a stairwell all sit differently under a sloping ceiling, and the lower half of a centre-pivot sash has to swing into the space above them. A top-hung sash keeps its lower edge outside when open, so the room below stays clear but you reach out and up to the sash.

Height matters as much as furniture. A tall person standing at the sill can find the swinging edge of a pivot sash at head level. A shorter person at a desk may be more bothered by how far they have to stretch to a handle. Each is a reason to choose with the room in mind.

Think about the view as well. A centre-pivot window opened wide can frame the sky in a different way from a top-hung one, because the glass tilts instead of lifting. If the room looks across rooftops towards the bay, check the view with the sash open as well as shut.

A walk round the room

These four checks take ten minutes with a tape and a pencil, and they tell us a good deal at the survey.

Each habitable room has to be capable of rapid airing, which Approved Document F (Wales) calls purge ventilation. An opening window can supply it, and the size of opening that the document asks for depends on how far the sash swings. For a hinged or pivot window, 30 degrees or more calls for an opening area of one twentieth of the floor area. A swing of 15 to 30 degrees calls for one tenth. Anything under 15 degrees is ignored.

That puts the opening angle of the model on the checklist, next to its looks. A top-hung sash that stops at a modest angle may need a bigger window than a pivot sash that swings further, and the reverse can be true for another model. The angle and the opening area come from the manufacturer's data for the exact window, so ask for them before choosing on style alone.

A replacement carries its own ventilation rule. Where trickle vents were fitted to the old window, background vents on the new one should be at least as large, adjustable by hand or working automatically. We look at what is in the roof before we agree what goes in.

Rain is the test every roof window faces sooner or later, and the way a sash opens decides what an open window meets. A window left ajar for a steamy bathroom can be caught by a squall within minutes, whichever way it swings.

That is where electric operation earns its keep. On the solar and electric roof windows VELUX sells, a built-in rain sensor closes the sash automatically when it rains. Our page on how rooflight rain sensors work covers what the sensor does and what it cannot see.

Noise belongs in the same conversation. Glass and slate answer a shower with different sounds, and the room under the roof hears whichever it has. Reduced rain noise is one of the glazing variants VELUX offers, subject to the model, so ask about it for the exact window you like.

A roof window collects dirt and pollen on the outside of the glass, and the outer face is hard to reach from a ladder on a slope. The hinge matters here. A centre-pivot sash can be built to turn so that the outer pane faces into the room, which puts both faces within reach. Whether a given model does this is a question for its data sheet, and we are happy to look it up with you.

A top-hung sash opens outward, so the outer face is reached from the outside or by reaching through the opening. That suits a window low enough to reach from a stair or a flat roof, and it suits a house where someone is practised at working on a slope safely.

Glazing can help. VELUX offers a dirt-repellent glazing finish intended to make the glass easier to clean. If cleaning is the thing you dislike most about the window you have, ask which coating suits each model you are considering.

In a loft bedroom the roof window may have a second job. Approved Document B (Wales) describes an emergency escape window as one with an unobstructed opening of at least 0.33 square metres, at least 450 mm high and 450 mm wide, with the bottom of the opening no more than 1,100 mm above the floor. The sash has to hold itself open, though a stay with a release catch is permitted.

The wording makes the sash style matter. The opening is measured with the sash open, so the swing of the sash, its stay and the position of the pivot axis all affect how much clear space is left. Whether a roof window counts for escape depends on its size and position meeting the escape-window test, and one that looks large from below can still fall short.

Escape also depends on the house. An escape window, or direct access to a protected stairway, serves upper-floor habitable rooms where the floor is not more than 4.5 m above ground level. Converting the loft of a house that already has two storeys normally brings fire-resisting doors and partitions around the stairway from the top to the bottom of the house.

Putting the room, the air, the rain and the escape rules together gives a short list of questions. Where does the sash travel? How far does it open? Who cleans it? Does it have to serve as the way out? The survey looks at both the slope and the room, and once it has, those questions have plain answers.

If you already have a roof window and want a different style, the opening in the roof is the constraint. Windows are listed by size code, and the page on reading VELUX size codes shows where to find the one on yours. Whether a different opening style fits an existing opening is settled at the survey, while a like-for-like VELUX swap is covered on its own page.

The rules that follow are the ones for any new or replacement roof window. In Wales it should have a U-value no worse than 1.4 W/m²K, or a Window Energy Rating of Band B. On a home of architectural or historic interest where that cannot be reached, the fallback is 1.2 W/m²K centre-pane or low-e secondary glazing. Where the work is notifiable, we handle the Building Regulations notification with local authority building control or a registered building control approver.

Planning conditions are written about the slope, how far the frame stands proud and the protected land around it, and none of them names top-hung or centre-pivot. The one that touches opening is the side-slope rule: within 10.5 m of the boundary opposite a side-facing slope, the window must be obscure-glazed and fixed shut unless its opening part is at least 1.7 m above the floor or stair directly below it. The Welsh rules are laid out in skylight planning permission in Wales, and roof windows shows the range.