Roof lantern installation in SouthamptonOrdered to your build.

On most extensions the lantern is chosen early and ordered late. The homeowner picks a style from a brochure while the drawings are being done, the builder prices the shell, and everybody agrees that the lantern can be sorted once the walls are up. Then the walls go up, the joists go on, and somebody asks the question that should have been settled weeks before: who is ordering the glass?

A made-to-measure aluminium lantern is not a stock item. The maker builds it to the size of your kerb, powder coats the frame in the colour you chose, and has the sealed units made to fit. That takes time, and the clock only starts when the order is confirmed with final sizes. If the order waits until the roof is ready, the roof waits for the lantern.

What happens next is familiar to anyone who has run a build. The roofer covers the opening with timber and a tarpaulin and moves on to another job. The plasterer cannot finish the ceiling around a hole. The kitchen fitter is booked, but the room is dark and damp. When the lantern finally arrives, the roofer has to come back, open up the temporary cover and dress the membrane again.

A lantern is several trades in one opening

Part of the problem is that a lantern sits where three jobs meet. Your builder frames the opening and builds the kerb. The roofer lays the deck and dresses the membrane up the upstand. We set the lantern on top and seal it. The plasterer lines the inside of the kerb afterwards. Each of those people needs the same size, and each of them needs to know when the others will be done.

Nobody in that chain is doing anything wrong, but until somebody takes charge of the lantern it has no owner, and that is when it runs late. We take charge of it from the first drawing: we take the size, we put it in writing, we place the order to suit your programme and we turn up when the roof is ready for us.

That is the approach behind every roof lantern installation we carry out in Southampton, whether it is a small square lantern over a utility room or a long one running the length of a kitchen extension. The rest of this guide follows the job in the order it happens on site.

A lantern is mostly glass, so the glazing decides how the room feels in January and in July. Every lantern we fit has sealed glazed units, and the choice is about how many panes, which coatings and how the units meet Building Regulations for the work you are doing.

For work to an existing house in England, which includes a new extension, Approved Document L sets a limit for rooflights of 2.2 W/m²K. A roof lantern counts as a rooflight for this purpose. Good double glazed lanterns meet that figure, and triple glazing goes further where you want a warmer room or a quieter one under heavy rain. We give you the maker's figure for the whole unit, not just the glass, because that is the number Building Control looks at.

Fitting day is the shortest part of the whole job, and it only goes smoothly if the roof is ready for it. Before we arrive, the roofer has laid the deck, fitted the insulation and dressed the membrane up the sides of the kerb, finished over or under the top edge as the lantern maker specifies. We check that detail first, because it is where water finds its way in if it is rushed.

Most aluminium lanterns arrive as a kit: a perimeter frame, a ridge, hips and glazing bars, and the sealed units packed separately. The frame is set on the kerb, squared and fixed with the brackets the maker supplies. The ridge and bars go on next, then the glass is lowered into place. On larger lanterns we lift the units with straps and work from the deck beside the kerb, never standing on the glass.

The lanterns we fit around Southampton go onto a wide range of houses. In Bassett, Highfield and Upper Shirley there are interwar semis and detached houses with long gardens, where single-storey kitchen extensions with a lantern over the island are a common choice. The brickwork there is often a warm red, and a dark grey aluminium frame sits comfortably against it.

In Portswood, Freemantle and Bitterne Park the Victorian and Edwardian terraces lend themselves to side return extensions. Those are long and narrow, and a lantern can fight for space with the party wall. A rectangular lantern set along the length of the room, or a pair of smaller ones, often works better than one big square lantern.