Swap a fixed rooflight for an opening one: can it be done?

Because a cut already runs through the roof, the heavy structural part of the job may be done. Even so, an opening unit is not a fixed one with a handle added. Its frame can be deeper, it carries an arm or actuator, and the glass swings clear of the frame, so the unit must suit the kerb or slope detail as well as the hole.

At the survey we take the outer size of the upstand, or the opening's width and height in the slope, plus the depth of the roof layers, and set those against the new unit's dimensions. A match makes the swap straightforward. A kerb that is too small or the wrong shape is replaced as part of the swap. A skylight over a landing might be one case and a rooflight on a kitchen extension another.

Age is a further variable. A unit that has sat in the roof for a long time may stand on a covering or upstand built for a different kind of unit, and the survey is where that shows. The sizes are settled before the fixed written quote and before anything goes to the maker.

A fixed flat rooflight stands on an upstand, so the opening replacement has to sit on that upstand or on a new one. Height, the width of the top edge and the state of the covering turned up the side are all examined. On a pitched roof the fixed window sits in the slope, and an opening roof window has to fit the opening and the flashing around it, which varies with the roof covering.

Changing only the glass cannot turn a fixed frame into an opening one, so a swap means a new frame. Replacing only the glass, or a window in its existing frame, is not providing a controlled fitting, so the energy limits do not apply to that work. For a fixed to opening swap, though, the new frame is a controlled fitting, so the limits below apply.

A manual opening unit asks for nothing. An electric one needs a supply point, a cable to the frame and a switch or remote. In a room with a finished ceiling, the cable might go through a space above the plaster, inside trunking or down a wall, depending on what the room allows. A solar unit relies on a cell on the frame and a battery, which is worth weighing up where a decorated ceiling would otherwise need disturbing.

The comparison is set out in choosing between mains and solar power, and the actual route is judged on site.

When the existing rooflight already opens by hand, the question changes. Some makers sell a motor kit for certain of their own manual roof windows, and others do not. Whether yours qualifies turns on the maker, the model and its age, so ask the maker which windows its kit fits. The data plate or the maker's literature names the model, which is the starting point. Where no kit exists, the whole unit can be swapped instead.

For a rooflight, 2.2 W/m²K is the English limit on the U-value. In England, a new or replacement roof window in an existing home should have a U-value no worse than 1.4 W/m²K, or a Window Energy Rating of Band B or better. For a swap, the replacement element should also be no worse than the one it replaces. A rooflight and a roof window follow different figures, which is why the type is identified first.

Putting a different roof window into the slope is work that usually needs Building Regulations approval, so allow for that early. Where the work is notifiable, we handle the Building Regulations notification with the council's building control team or a registered building control approver. Ventilation gets looked at too: the survey checks how that room is ventilated before anything is ordered.

In England, adding a rooflight or roof window to a house is usually permitted development under Class C, so it does not need a planning application, provided it meets the Class C limits and conditions. Any rooflight or roof window on a roof slope that forms a side elevation of the house must be obscure-glazed, and must be non-opening unless the parts that open are more than 1.7 m above the floor of the room it is in. On a side slope, that condition decides whether an opening unit can take the place of a fixed one under Class C, and the planning permission guide sets out the other limits.

The survey costs nothing. To start, a snapshot of the existing frame can go to WhatsApp 07880 284133. Then comes a fixed written quote; in writing, the fitting also has a 10-year workmanship guarantee. At handover the controls, the blind options and the way the outer glass is cleaned are explained. How the figure is built up is covered by the cost answer for electric units, and the wider family sits on the electric opening rooflights page.