Roof lantern installation in Leicester

The kerb is the part nobody photographs and the part that decides whether a lantern stays dry. It is a box frame around the opening, either a factory-made insulated upstand supplied with the lantern or a timber kerb built on site and insulated. The roof covering, whether that is felt, EPDM rubber or GRP fibreglass, is dressed up the outside of the kerb and over its top edge. The lantern then sits on the kerb and is sealed and fixed to it. Water running across the roof meets a wall it cannot climb, and the glass stays well above it.

Height matters. Every lantern maker sets a minimum height for the glass above the finished roof, and the kerb is built to meet it once the covering has gone on. On a roof with a poor fall, where water sits after rain, the kerb goes higher again.

A kerb built on site has a thermal job too. Part L of the Building Regulations says an upstand built on site should have a U-value no higher than 0.35 W/m²K, which means real insulation inside it, not a hollow box of timber. A factory upstand supplied with the lantern as one assembly is judged together with the lantern instead. Either way, the kerb is where cold creeps in if it is skimped, and the first sign is a dark line of condensation round the ceiling opening on a January morning.

Trimming the joists

A flat roof is held up by joists: timbers running from wall to wall, or from a wall to a steel beam. Cutting an opening means cutting some of them. The cut ends are picked up by trimmers, timbers fixed across the opening at each end, and the joists either side of the hole are doubled so they can carry the extra load the trimmers hand on. This is trimming, and it is what lets a lantern sit in a roof without the roof sagging around it.

A small lantern between two joists needs little more than that. A wider one cuts more joists and carries more weight, because glass is heavy, so the trimming is sized to the span, and on larger openings an engineer's calculation sets the timber or steel. If the extension has a steel beam where the old back wall was, the lantern is placed to clear it rather than lean on it. The survey finds the joist direction and the steel before the lantern size is fixed.

Measuring a lantern is where mistakes are cheapest to make and dearest to discover. Lantern sizes are often quoted as the external size of the kerb, but the room sees the internal opening, and the two differ by the thickness of the kerb walls and their insulation. We measure both, along with the full roof build-up: the deck, the insulation, the covering and the height of the finish above the joists. Diagonals are checked too, because an opening that is a few millimetres out of square shows up as a gap at one corner once the lantern goes down. The written quote states the external and internal dimensions, so there is no argument later about which size was meant.

Position is part of size. The lantern should sit where the room needs light: over the island, centred on the table, or on the line between kitchen and dining. It should clear the joists that are hardest to trim, leave space round it for downlights and an extractor, and stay a sensible distance from the walls so the plasterboard reveal looks deliberate. Sometimes the right answer is two smaller lanterns in a line rather than one long one, which keeps each span modest and puts light over two zones.

Lantern or flat rooflight?

On a flat roof, the lantern's rival is the flat rooflight, a single sheet of glass on a kerb, laid with a slight pitch so the rain runs off. The flat rooflight sits lower, so it hides behind a parapet and draws less attention from the upstairs windows next door. It has a crisper, more modern look from inside, with no bars overhead. The lantern gives more height and a stronger sense of sky, and it catches low winter light from the sides. Our guide on roof lantern or flat rooflight weighs one against the other, and the flat roof rooflights page covers the flat option in full.

A lantern answers to two rulebooks. Planning cares about what the neighbours and the street see, which for a lantern mostly means its height. The Building Regulations care about what holds it up, what keeps the heat in and what happens if the glass breaks. Passing one says nothing about the other. Which of them your lantern triggers is part of the survey report, so you know before you commit.

Does a roof lantern need planning permission? Not always, but a lantern needs a closer look than a flat rooflight. Rooflights on a house usually fall within permitted development, under Class C in Part 1 of Schedule 2 to the GPDO 2015, as long as they stand no more than 150mm proud of the roof slope and stay below its highest point. A flat rooflight passes that easily. A lantern, by design, stands taller. Where it goes on with a new single-storey extension, it is part of that extension, and government guidance caps a single-storey extension under permitted development at 4 metres in total height.

The usual exceptions apply. Listed buildings need listed building consent. Flats and maisonettes sit outside householder permitted development altogether. And a council can withdraw the rights for a street with an Article 4 direction. The full statutory detail is in our planning permission and Building Regulations guide.

The Building Regulations apply as soon as a new opening is cut, because the roof structure changes and the lantern becomes part of the home's insulated shell. If the lantern counts as notifiable work, we take care of the Building Control notification for you, and the lantern, the kerb and the trimming are built to pass inspection. The lantern itself comes with the maker's product warranty. Our own work on it is covered by a 10-year workmanship guarantee, which our guarantee page sets out clause by clause.

On price, the honest answer is that it follows the roof. The size, stock or made to measure, the frame material, the glass specification, any opening vents and their controls, the amount of trimming, the kerb, the access and the inside finish each shift the figure. The survey pins each one down, and the costs page explains how they add up.

Aftercare and maintenance

A lantern asks for little, but it does ask. Clean the glass a few times a year, or more under trees, with water and a soft brush on a pole from a safe position. Self-cleaning glass helps but does not rule out a wash after a dry, dusty summer. Keep leaves out of the joint where the lantern meets the kerb, and look at the roof covering round the kerb after a hard winter. If there is an electric vent, keep the rain sensor clear of moss and droppings, or it will think every day is dry.

Over a kitchen, run the extractor when cooking. Steam rising into a cold lantern on a winter morning will form condensation on the glass however good the unit is. How long a lantern lasts depends on the system, the glass and that kind of care. A sealed unit that eventually mists can be replaced without replacing the lantern, which is work for our misted rooflight replacement service.

Are roof lanterns secure? A lantern on a single-storey roof can be reached by anyone willing to climb, so security is part of the specification, not an extra. The glass is fixed from inside or held by cappings that cannot be removed from outside without tools and time. A laminated inner pane stays in one piece when struck. Opening vents can lock, and an electric vent closes on its own when told to. The quote lists the security features of the system we propose, so you can weigh them against the room below.