How is an old skylight replaced?

The first job is to know what is already there. Width, height and the type of unit all matter, and so does the way it is fixed into the roof. A roof window sits in the plane of the slope, while a flat glass rooflight or a lantern sits on an upstand above a flat roof, so the two are replaced differently.

If the old window is a VELUX made after 1968, it carries a metal type sign. You find it by opening the window and looking along the upper edge of the sash. VELUX lists each size code with its width and height, and the code on the sign tells you both. Our page on VELUX size codes explains how to read it. A matching size is the simplest replacement, because the opening in the roof can stay as it is.

A photograph of the old unit from inside, with the sash open and the type sign in view, is a useful starting point. Send it to WhatsApp on 07880 284133 or to info@supremeskylightsnewport.co.uk. The survey still checks the opening, because a code on a sign says nothing about the timbers or the covering around the frame.

With the size known, the old unit comes out. The covering beside the opening is lifted or cut back so the frame can be freed from the roof, and the unit is lifted out. Which parts are disturbed depends on the roof: tiles, slates and a flat roof covering are each handled differently.

This is the point at which the roof stops being a guess and becomes a surface you can see. Anything the survey could only suggest, such as the state of the timbers around the frame, is now in plain view.

If the old unit is a lantern or a walk-on panel, the lifting can be heavier and the route to the roof matters more. Our page on replacing a roof lantern looks at that case. A roof window over a loft bedroom is the lighter case, and the main question there is whether the size code matches, so that the opening can stay as it is.

Three parts decide whether the new unit simply drops in. The first is the opening itself: it must be sound, square and the right size for the new unit. The second is the flashing, the material that carries water from the roof covering over the frame. The third is the lining inside, the boards and plaster that close the gap between frame and ceiling.

A new unit of the same size can call for the flashing to be renewed with it, since it is shaped for the frame it surrounds. A lining that has marked, cracked or come away is made good before the room is decorated. If the new unit is bigger or smaller, the opening has to be altered, and that moves the job towards the larger work described in our rooflight replacement cost answer.

The new unit is fixed into the opening, levelled and tied into the roof covering, and the flashing is dressed back over the surround. Inside, the lining is closed up and finished ready for paint. A blackout blind can be chosen with the unit if the room is a bedroom, since the glass reads black at night but does nothing about first light.

Large lanterns and walk-on panels can be heavier to handle and take more effort to get into place. Sun tunnels have their own steps, as the tube runs through the loft to a ceiling diffuser rather than into a room opening.

A Welsh replacement of this kind is covered by the Building Regulations. The regulations treat windows, roof windows and rooflights as controlled fittings. Where the work is notifiable, we handle the Building Regulations notification with local authority building control or a registered building control approver.

Approved Document L (Wales) is guidance. It says a new or replacement roof window should have a U-value no worse than 1.4 W/m²K or Window Energy Rating Band B, and a rooflight no worse than 2.2 W/m²K. A listed house needs listed building consent where the work would affect its character. The Building Regulations guide for Wales and the planning guide go through both, and the free survey tells you which applies. Book it through the form, and the price follows as a fixed written quote.