Top-hung or centre-pivot roof windows

The pivot points sit a little above the middle of the sash, so the window is balanced and moves with a light touch. You lift a handle along the top rail, and the sash rotates about the pivot. The lower half goes outwards and upwards over the roof covering, and the upper half drops inward, towards the room. Opened fully, it can turn all the way round, with its outer pane facing into the room, which is how the glass is cleaned without going on the roof.

Because the handle is at the top, you reach up to open it, which suits a window set low on a sloping ceiling and is awkward when the window is set high. In a loft room the top rail often falls near head height, so the handle is within easy reach. The same geometry means the top of the open sash comes down into the room, so a tall person standing right under the glass can find the sash edge at forehead level.

The VELUX range uses the codes GGL and GGU for its centre-pivot windows, with the L finished in white-painted pine and the U in white polyurethane. Other makers have equivalents, and the principle is the same.

A top-hung roof window has its hinges at the head of the frame. The handle is on the bottom rail, and you push the sash outwards, usually into a stay that holds it open at an angle. The window then acts as a small canopy over the opening, with the lower edge raised well clear of the room. Most top-hung windows are stopped part-way open by the stay rather than swinging all the way round.

That arrangement has a real advantage in a loft room. The sash lifts away from your head instead of dropping towards it, so you can stand directly beneath an open window, and you can lean out with your elbows on the sill and look out over the lower edge of the roof. The handle is at the bottom, which is comfortable to use when the window sits high on the slope.

The trade is cleaning, though it is a smaller one than people expect. A VELUX top-hung sash can be turned into a cleaning position: you release the bottom handle and the top ventilation bar, rotate the sash through half a turn and hold it with the bolt provided, so the outer pane can be wiped from inside. It takes more steps than a centre-pivot window, the position is meant for cleaning only, and other makes differ, so check the instructions for yours. In the VELUX range the top-hung codes are GPL, in white-painted pine, and GPU, in white polyurethane. GGU is not a top-hung code, so check the first letters when you read a quote.

The slope of the roof matters more than any other factor. The two types are designed for different ranges, and the ranges overlap in the middle rather than matching.

A centre-pivot window works across a wide span of pitches, from quite shallow roofs up to steep ones. Top-hung windows are generally recommended for a narrower, shallower to moderate range. Beyond that range a top-hung sash, swinging up and out from a steep slope, has to travel a long way and sits awkwardly against the roof. Check the manufacturer's stated pitch limits for the exact model you are choosing, because they differ between ranges and from one product generation to the next. Our guide to the minimum roof pitch for a roof window covers the shallow end.

In practice, Devon roofs fall into recognisable groups. Many Victorian and Edwardian terraces and villas around Exeter and Torbay have steep slate roofs, where a centre-pivot window is the natural fit. A 1930s semi in Heavitree or a postwar house with a concrete tile roof often sits at a moderate pitch where either type works. Extension roofs and lean-tos can be shallow, and there the pitch decides the matter: some shallow roofs fall outside what a centre-pivot window can be fitted to without a different product or a low-pitch kit, and the surveyor will measure the angle on site rather than guess it.

The two types answer different needs. This table puts the main points together, and the sections that follow take the more important ones in turn.

Where you put the window on the slope decides which handle you will be glad of. A window whose bottom edge is at knee height or a little higher, which is common over a stair or a landing, has its top rail at about the height of a raised arm. A centre-pivot handle at the top is handy there. A top-hung handle would be down at your ankles.

Turn that round for a window high on a steep pitch with a deep sill above the floor. The top rail is out of reach, and a bottom handle on a top-hung window is the sensible one. Some manufacturers offer a control bar or an electric opener for high windows, and electric opening is covered on our page about electric opening rooflights. If a window will be out of reach, decide that at the survey, not after the plaster is dry.

It is also the moment to think about furniture. A bed or a desk right under the glass wants a top-hung window if you want to open it fully, or a centre-pivot placed so its swing falls over the foot of the bed, not the pillow.

Both types open to ventilate, and a roof window is a useful source of fresh air in a loft room because warm air rises and leaves through the top. Approved Document F recognises opening windows as a way to provide purge ventilation, which means flushing a room quickly, for example after painting or cooking. Most roof windows also have a ventilation flap or trickle slot at the head that lets air in with the sash closed.

The two opening styles give different amounts of clear opening. A top-hung window, held open on its stays, gives a wide gap along the bottom that you can reach through. A centre-pivot window gives two gaps, one at the top and one at the bottom, and the airflow between them is good, though the clear opening through the middle is cut by the sash itself.

If the window is meant to serve as an escape route from an upper room, as is often needed in a loft conversion, Approved Document B sets the size of the clear opening and the height of the sill above the floor. Whether a given window meets that depends on the model, the opening angle and the position of the sash, and the building control body decides whether it is acceptable for your house. We explain the position in can a roof window be a fire escape. Our guide to roof windows for loft conversions carries on with the rest of the loft-specific points.

The sash does not stop at the frame. When it opens, it swings through the space of the lining or beyond it, and the lining has to allow for that.

With a centre-pivot window, the inner edge of the sash drops down into the room as it turns. A deeply splayed lining makes this easier, because the angled sides move away from the sash path. A lining that is straight and tight to the frame can leave the sash a very small margin, so the plaster edge at the head is the part most at risk of a knock. A tidy reveal here is also more than a nicety: a sash that rubs the plaster will chip the paint line within weeks.

With a top-hung window the sash moves outwards, away from the lining, so the clearance inside is simpler. The sill is the part to watch. A bottom handle needs a gap to be gripped, so the sill returns must not be built up so thick that the handle is hard to use. A good plaster finish and a clean paint line against the frame matter as much on a top-hung window as on any other, since the whole of the frame edge is on show once the sash is open.

On a shallow loft slope the lining is short and either type is easy to finish. Where the lining runs through a deeper roof or a lightwell, we form the splay around the way the window opens, and the choice is written into the finish schedule so it is agreed before any roof is opened. The finish schedule records how the lining is formed, so you see in writing how the reveal will meet the sash.

Both types are available with double or triple glazing, and a laminated inner pane is a sensible choice for glass overhead because it holds together if it breaks. That is good practice and industry guidance, not a requirement of Part K, and we say so plainly when we quote it. Warm-edge spacers and low-emissivity coatings reduce heat loss, and the window must meet the limiting U-values in Part L of the Building Regulations, so it is more than a choice of taste.

Opening type affects comfort in small ways. A centre-pivot window turned fully round for cleaning is well placed to be wiped from indoors, and it is also more likely to be left open in a shower of rain by someone in a rush, so check the rain sensor option if you want one. Neither window is more weathertight than the other when closed, since both are made to the same standards and fitted with the same flashing.

The flashing is the kit of metal pieces that seals the frame to the roof covering. Its type depends on the roof, not on the window, and its codes start with ED: EDW for profiled roofing such as interlocking tiles, EDZ for flat roofing such as plain tiles or thicker slate, and EDL for thin slate. Our guide to roof window flashing for slate and tile roofs goes through the choice.

Choosing top-hung or centre-pivot makes no difference to the rules. On a house, a roof window normally falls within permitted development, Class C of Part 1 in Schedule 2 of the General Permitted Development Order. The window must stand no more than 150mm out from the roof plane and finish below the roof's highest point. In a side-facing slope it must be obscure-glazed and fixed, unless its opening section is higher than 1.7m above floor level. Conservation areas, listed buildings, flats and the national parks have their own rules, so ask the local planning authority before you order.

Fitting a new roof window is building work that needs Building Control approval, since the rafters are trimmed and Part L applies. A complete replacement, frame and glass, is notifiable too, while a fresh sealed glass unit in the existing frame is not. Where the work needs notifying, we do it.

The type matters when you swap one window for another. Changing from top-hung to centre-pivot, or the other way round, is usually possible when the frame size matches, but the handle position, the sash swing and the lining all change, so it is better decided with the room in view. Our roof window or dormer guide is the one to read if you are still weighing whether a window is the right answer at all.

A few questions settle it for most homes.

• What is the pitch of the roof, and does it fall inside the range for the model you like?
• How high is the bottom edge above the floor, and can you reach the handle comfortably?
• Is there furniture or a bed beneath, and which way will the sash swing over it?
• Do you want to be able to clean the outer glass from inside?
• Will anyone want to lean out and look, or is it mainly for air?

If the roof is steep, the window sits low and cleaning matters, centre-pivot is usually the answer. If the roof is moderate, the window sits high and you want clear headroom beneath, top-hung deserves a look. Either way the surveyor will check the pitch, the ceiling and the position of the rafters before anything is ordered.

When you are ready, ask for a quote and say which room the window is for. Every quote comes with a finish schedule, so how the sash opens and how the lining meets it are in writing before the work starts. The full picture of the work is on our roof window installation page.