What is the difference between centre-pivot and top-hung?

Working out which one is in your roof already

You can tell without a data plate, and it takes ten seconds.

Look at where the bar handle sits. A handle across the top of the sash is a centre-pivot: it is at the top so you can reach it from the room and then rotate the sash by pulling the top towards you. A handle across the bottom of the sash is top hung: it is at the bottom because that is the edge that travels furthest as the sash swings out.

If you can open it, the confirmation is immediate. Rotate a centre-pivot and the top of the sash comes into the room and passes your face. Push a top hung and the whole thing goes away from you. A third possibility is a combination unit, which offers both actions from one sash with a separate catch to switch between them, and those are identified by having two operating points rather than one.

The combination sash, which is the option people miss

Both major manufacturers now make a unit that does both. Fakro’s preSelect and the equivalent VELUX arrangement let the sash rotate about its middle for cleaning and, with a different catch engaged, swing from the head as a top hung. One window, one opening, two actions.

That resolves the common dilemma directly. A loft bedroom on a 40 degree slope where somebody wants the unobstructed view and the clear escape opening of a top hung, but also wants to be able to clean the outer glass without a ladder, has been choosing between two compromises for years. The combination unit is more expensive and it has more hardware in it, which is the honest trade. It is worth asking about at survey rather than discovering afterwards.

Availability, ranges and what is actually stocked

The two designs are not equally represented on a price list. Centre-pivot is the volume product: it comes in the widest range of sizes, the most finishes, the most glazing options, and it is what merchants hold in stock, so lead times are shorter. Top hung ranges are narrower in every sense, with fewer sizes, fewer glass options and more items that come to order.

That has three practical effects. A top hung unit in an unusual size may add weeks to a programme, which matters if a scaffold date is already booked. A top hung unit in the exact size the roof will take may simply not exist, in which case the choice is made for you. And on a replacement, the size code you already have may exist in centre-pivot and not in top hung, which is worth checking before anyone gets attached to a design.

Swapping one design for the other at replacement

This comes up often, usually because a loft room has been given a use it did not have before, and the answer is generally yes.

Both designs use the same structural opening for a given size code, so a like-for-like swap from centre-pivot to top hung, or the other way, needs no structural work at all. The old unit comes out, the perimeter is brought up to current detail, a new flashing kit ties the covering back in, and the internal plaster usually survives untouched because the work is done from outside. What does not carry across is the accessories: blinds, insect screens and awnings are specific to the frame and often to the action, so those get re-ordered.

Two checks before assuming it. Frame profiles changed across product generations, so a current unit of the same nominal size does not always land on the same fixing line as a 1990s one, and that is verified at survey against the code stamped on the old sash. And a switch to top hung has to respect the pitch limit: a house at 55 degrees or steeper that currently has centre-pivot units has them for a reason. Replacement work in general is at replacing a failed or misted rooflight, and enlarging at the same time at replacing a roof window with a bigger one.

The two places the choice is forced rather than made

Most of the time this is a preference. Twice it is not.

The first is escape. Where a roof window is the escape opening from a loft room, the sash has to clear the aperture, and a standard centre-pivot sitting halfway across its own opening does not. A top hung clears it naturally, and a centre-pivot can be specified with an escape hinge that lets it swing right up to around 90 degrees. Either satisfies the requirement. A standard centre-pivot ordered without that hinge does not, and the hinge is not something added to a sash afterwards. The direct answer is at can a roof window be an escape window and the full requirements at roof window escape openings and the rules.

The second is pitch. Above about 55 degrees the top hung ranges run out, and on the steepest Gloucestershire roofs, the Cotswold stone slate above the wolds villages and the Forest terraces around Coleford and Mitcheldean, that band is reached regularly. Below 15 degrees neither design applies and the unit becomes a kerb-mounted rooflight instead, which is a different product covered at flat roof rooflight installation.

How the decision gets recorded, and what to send

At survey the pitch is taken off a digital level, the rafter or truss centres and depth are measured, the run of slope is recorded, and the finished floor level is noted so the head of the sash and the handle position can be worked out in the room rather than on a drawing. Those numbers decide the action, the size code and whether a rod or an electric opener is wanted, and they go into the order together.

Severn Skylights Gloucester fits new roof windows and replaces units that have reached the end of their service life, across Gloucester and thirty miles by road. To get a view before a survey, email info@severnskylightsgloucester.co.uk with a photograph of the roof from the garden or the street, a photograph of the room from inside, a photograph of the data plate if there is an existing window, and the address. An indicative range follows within a day or two and a fixed price after the visit, as set out on costs.