Roof Lantern Installation in Bristol A raised glass roof that lets the warm air out at the top

The height does a second job, and it is the one this firm cares about most. Warm air from cooking, from people and from sun on the glass rises and collects in the lantern's peak. If every pane is fixed, that warmth simply sits there through the afternoon. If the lantern has vents near the top, the warm air leaves, and replacement air arrives through a window or a trickle vent lower down. A kitchen extension can then be aired with the bifolds shut and the dog kept in.

When a flat rooflight suits the roof better

A lantern is the wrong answer on some flat roofs. It needs space outside as well as in: on a roof overlooked by a bedroom window, a tall lantern can block the view down the garden and look bulky from upstairs. It needs width, too. In a narrow side return extension, a slim flat rooflight running along the party wall often gives more useful daylight than a squat lantern squeezed between the joists. And on any roof that will be walked on, a lantern is out of the question; a walk-on rooflight does that job.

Cost is part of the picture. A lantern has more glass, more frame and a taller upstand than a flat rooflight of the same footprint, so it costs more to make and more to fit. When either would work, both go on the quote, with a plain note on which we would choose for that room and why.

Lanterns come square, rectangular, octagonal and in longer ridged shapes, framed in aluminium or timber, with fixed glass or with vents. The chapters below take those decisions one at a time: size, opening, frame, fitting to a roof you already have, and the rules that apply. If you already know the room and want to get the proportions right first, the guide to roof lantern sizes works through the measuring in detail, with examples for square and long extensions.

Where it sits on the roof

Position matters as much as size. Over the island or the table is the classic place, because that is where people spend the evening. Keep the kerb clear of the wall of the original house, where the extension roof meets the brickwork and the flashing needs room to be dressed properly. Leave a margin from the roof edge as well, so the falls (the gentle slope that drains a flat roof) can carry rainwater round the kerb to the outlet instead of pooling against it.

The joists set the limits

On an existing roof, the joists decide what is easy. A lantern that runs along the joists means cutting fewer of them. One that runs across them means cutting several, then carrying the cut ends on trimmers (doubled joists fixed across the opening) or on a beam. Wide openings may need a steel or engineered timber beam, sized by a structural engineer. We look at joist size and direction at the survey, so a size that would need a beam is flagged before you fall for it.

Height follows width. Each maker builds a design to its own roof pitch, so a wider lantern stands taller than a narrow one of the same style. That extra height adds volume and gives warm air more room to rise, which helps a vent do its work. It also adds to what the neighbours see, which can matter on a terrace where upstairs windows look straight down on the extension roof.

Many makers offer a run of standard sizes, which are quicker to supply and cost less. A made-to-measure lantern fits an awkward room or an existing kerb exactly. When an old lantern is being replaced, the size is usually set by the kerb that is already there, unless the kerb is rebuilt and insulated as part of the job.

How many vents depends on the size. A small square lantern may need one. A long ridged lantern over a deep kitchen can take a vent on each slope, so air can move across the top as well as out of it. The detail on vent choices, controls and positions is in the guide to opening roof lanterns.

Air needs a way in as well

A lantern vent works best when fresh air can enter somewhere low. A trickle vent in the garden doors, a side window left on the latch or the door through to the hallway all do it. Warm air leaves at the top, cooler air comes in at the bottom, and the extension clears itself. Close every low opening and the vent has much less to pull against.

Glass for a roof that catches the sun

Extensions facing south or west take the strongest afternoon sun. Solar control glass carries a coating that reflects part of the sun's heat while letting daylight through, and it comes in neutral, blue and grey tints. A self-cleaning coating on the outer pane is worth having on a lantern you cannot easily reach. Made-to-fit blinds for lantern panes exist, though they add cost and can look busy against the bars.

Part O, the regulation that deals with overheating, applies to new dwellings only, so an extension to an existing house carries no overheating calculation. That does not stop the room getting stuffy on a July afternoon. Choosing the glass and the vents together is how a lantern room stays pleasant to sit in, and the guide to roof lantern overheating sets out the options side by side. For the wider range of powered units, see electric opening rooflights.

Timber suits a house where the extension should feel like part of the original building, such as a Georgian house in Clifton or a large Edwardian villa where painted joinery runs through the ground floor. The bars are wider than aluminium, and the inside reads as joinery rather than metalwork. Modern timber lanterns use engineered timber with a factory finish, and many have an aluminium capping on the outside, so the part that faces the rain needs very little upkeep while the room still sees wood.

How the work runs

1. Dust sheets go down, furniture is moved or covered, and a temporary support goes under the joists to be cut.
2. The covering and deck are cut back, the joists are trimmed, and any beam the engineer has specified is fixed in.
3. An insulated kerb is built on the trimmed opening, and the membrane is turned up it and sealed.
4. The lantern frame is assembled on the kerb, glazed and sealed, and any vent is wired or set up on its solar panel.
5. Inside, the reveal is boarded, insulated and plastered, ready for you to decorate.

The roof is closed up at the end of each working day, whatever stage the job has reached, with a covered and sealed opening if the lantern is not yet on, so the kitchen stays dry and usable overnight. One team does the work from start to finish, so the person who cut the opening is the person who fits the frame and finishes the reveal.

Houses we see this on

On terraces in Bishopston and Totterdown, the extension roof is often tucked below the windows of the rear bedrooms, so the lantern height and the view from upstairs are discussed early. On 1930s semis in Keynsham and Westbury-on-Trym, the rear extensions tend to be wider, which allows a long ridged lantern over a kitchen and dining space. On the newer estates of Bradley Stoke, and out towards Emersons Green, flat-roofed extensions from the builder sometimes have a plastic dome or a small flat unit that can be replaced with a lantern on a rebuilt kerb.

When an old lantern is coming out rather than a new one going in, the job is shorter. The frame is taken off, the kerb is checked for rot and insulation, and the new lantern is fitted to it or to a rebuilt kerb. Old polycarbonate lanterns are much lighter than glass, so the kerb and the timbers under it are checked before a glass replacement is ordered.

A flat or maisonette has none of these rights. A listed building needs listed building consent for a lantern, which matters across much of Clifton and Bath. Some conservation areas have Article 4 directions that remove permitted development rights, so check with Bristol City Council, Bath and North East Somerset Council or whichever council covers your house before work is booked.

Building Regulations

A new lantern in an existing roof is building work. The opening has to be trimmed properly and the finished installation must comply, Part L's 2.2 W/m²K rooflight figure included. Replacing a whole lantern, frame and glass, is notifiable building work too. Replacing only a sealed glass unit in a sound frame generally is not. Where the work is notifiable, we deal with the notification, through the council's Building Control or a registered building control approver, and you receive the completion certificate for your house file.

Two other parts come up. Under Part F, a lantern vent that opens may be counted as part of the room's purge ventilation, meaning the rapid airing used to clear cooking smells and steam. And there is no Building Regulations daylight requirement for an existing house, so the size of the lantern is your choice, not a rule. What the regulations do care about is the structure, the insulation round the kerb and the energy figure for the unit, and those are the points Building Control checks. The full picture, with the Part L figures explained, is in the guide to roof lantern Building Regulations.

What moves the price

We do not quote a price without seeing the roof, because lanterns vary so much. The size and shape, the frame material, the glass, the number of vents and how they are powered all count. So does the structure: trimming two joists costs far less than fitting a steel beam. The state of the roof covering, scaffolding or access, and the plastering inside make up the rest. Broad UK market ranges are on the costs page, and the answer on roof lantern cost explains each factor. The survey is free, the quote is written and itemised, and the fitting carries a 10-year workmanship guarantee.