A flat roof is the easiest place on a house to add daylight, because there are no rafters to cut around and no slope to flash against. It is also the place where a badly detailed opening does the most harm. Water sits on a flat roof for longer than it does on a pitched one, so every upstand, corner and glazing bar around a new opening has to be right. The choice usually comes down to three shapes: a flat rooflight set on a low kerb, a pyramid or hipped rooflight that rises to a point, and a lantern, a taller glazed roof with its own vertical sides. They differ in how much light they admit, how they look from the garden and inside, what they cost to build and how much they ask of the structure. Each option has its own merits, certain parts of the roof decide whether a lantern stays watertight, and regulations and consent raise their own points for homes in and around Bath that are worth settling before you order.
Flat rooflight, pyramid or lantern: what each one is
A flat rooflight is a glazed unit, usually a sealed double-glazed panel in a frame, fixed to a raised timber or insulated kerb. It sits close to the roof surface and gives straight overhead light. Some are fixed, some open on a hinge or an electric motor for ventilation, and many are sold in standard sizes that suit a quick like-for-like swap.
A pyramid or hipped rooflight does the same job with four sloping glazed faces that meet at a ridge or a point. Because the glass is angled, rain runs off it rather than lying flat, and more of the sky is visible from below. It stands higher than a flat rooflight, so it is more noticeable from the garden and from neighbouring windows.
A lantern is a different thing again. It is a small glazed roof built on the flat roof, with upright sides, sloping glazing above them and a ridge. Lanterns are common over kitchen and dining extensions, where they pull light deep into a room that has a single window wall. They add height inside as well as light, because the ceiling opens up into the lantern, and they read as a feature from outside.
The three are not rivals so much as steps on a scale. A rooflight is the lightest touch, a pyramid is a modest upgrade in drama, and a lantern is a piece of architecture that must be designed with the room beneath it. If the flat roof is only a roof over a shed or garage, a simple rooflight is usually the sensible choice. If the room is the main living space, the extra cost of a lantern can be worth weighing.
If your question is about a sloping roof rather than a flat one, our guide to the difference between a rooflight and a roof window explains where each belongs.
The flat roof underneath decides how long the glass stays dry
Before choosing the glass, look at the roof it will sit on. Every flat rooflight and lantern is mounted on a kerb, a raised upstand that lifts the frame clear of standing water. The waterproof layer of the roof, whether felt, EPDM (ethylene propylene diene monomer, a rubber laid as one sheet), GRP (a fibreglass skin laid wet and cured in place) or lead, is dressed up the outside of that kerb and finished under a cover flashing or a trim. A flashing is a strip, usually of lead or an aluminium section, that bridges the joint between kerb and membrane.
Height matters. A kerb that is too low lets splashing rain and settled water reach the joint. Most manufacturers specify a minimum upstand for their units, and it is worth asking what it is and checking it against the finished roof, not the bare deck, because a new membrane and insulation raise the surface and shorten the upstand that is left.
The fall of the roof matters as well. BS 6229:2018, as described by flat roof manufacturers, recommends a minimum fall of 1:80 on a finished flat roof, and designers allow for a fall of 1:40 on the structure so that tolerances and deflection do not leave puddles. A kerb sitting at the low edge of a poorly drained area will stay wet. We look for where the water goes on the existing roof before we settle where the new opening sits, and we can often move an opening a short way to a drier spot.
Where a roof is already tired, the opening is the moment to renew the covering around it. Cutting into an old felt roof to fit a lantern and then leaving the remaining felt to fail is a false economy, because the junctions are the weakest part of any membrane. If the roof has little life left, the glazing and the new covering belong in one job.
Glazing, heat and light
The glazed panel is a sealed unit of two or three panes, with a coating on one pane to cut heat loss and sometimes a different coating to reduce solar gain. The choice affects how the room feels more than most buyers expect. A south-facing glass roof over a kitchen can warm a room quickly on a bright day, and a roof with no way to let air out will feel it. Solar-control glass reduces that, at the cost of some brightness and, in some makes, a slight tint.
Opening vents are worth considering. Warm air rises, and a room with a lantern collects it under the glass. A vent in the roof, or a pair of them, lets that air go and draws cooler air in at low level. Electric and rain-sensing vents are available, though they add cost and a power supply. Blinds fitted to the underside can also help on the hottest days.
Approved Document L limits heat loss through new glazing. Under its (2021 edition, as amended in 2023), a new rooflight in an existing dwelling should achieve a U-value of 2.2 W/m²K, while a roof window should achieve 1.4 W/m²K or a Band B rating on the window energy scale. The two figures are different because the regulations treat the two products differently, and a flat rooflight falls into the first category. Always ask the supplier for the declared U-value of the whole unit, kerb included, not just the glass.
The inner pane is also a safety matter. Anyone standing in a room below a roof of glass should be protected if a pane breaks, and anyone who has to step on or work near the glass needs to know whether it will take weight. Ask the maker whether the inner pane is laminated and whether the unit is rated for maintenance access. Never assume a rooflight can be walked on.
Finally, think about light from the inside. A single rooflight over the middle of a room gives bright overhead light and little else. Two smaller units, placed to wash a wall, often feel better than one large one. A lantern lights the whole ceiling, and a pale internal finish to the lantern sides will carry the light further down.
Costs: what the roof covering and the structure add
The cost of a glazed flat-roof opening has three parts: the glazed unit, the structure and kerb, and the waterproofing around them. The unit prices vary so widely by size, glass and finish that a typical figure would mislead, so we price them from the survey and the chosen product rather than from a table. What we can do is show how the roof covering around the opening is priced, because the covering is a cost you cannot avoid.
The figures below come from MyBuilder's flat roof cost guide (2026). Your fixed quote follows the survey.
| Flat roof covering | Reported UK range per m² |
|---|---|
| Bitumen felt | £70 to £100 |
| EPDM | £90 to £130 |
| GRP | £95 to £150 |
| Lead | £150 to £200 and above |
The same guide puts a full flat roof replacement at £2,800 to £7,500, averaging about £75 per square metre. When a rooflight is part of a roof renewal, the extra for the opening is the framed kerb, the trimmed deck, the flashings and the units. When it is a retrofit into a sound roof, the extra includes careful cutting out, patching and a proper junction.
Scaffold or edge protection is a further item. A single-storey flat roof can often be worked from ladders and a safe access arrangement. A first-floor flat roof over an extension needs proper edge protection, and that goes on the quote as its own line.
A lantern is heavier than a rooflight and needs support. The deck around the opening is usually framed with trimmers, which are the timbers that carry the cut joists, and the lantern's weight, wind loads and snow loads should be checked. This is not a task to assume. A structural check by a designer or engineer is sensible before the opening is cut, and building control may ask for it.
Building Regulations, planning and listed buildings
A new glazed opening in a flat roof is more than a cosmetic change, and several sets of rules can apply. We outline them here, but the last say belongs to whoever signs off the work, be it a local authority team or a registered private body.
On the Building Regulations side, a replacement rooflight must not be a step backwards from the unit it takes out, and should meet the Part L limit described above. A new opening in a roof also affects the structure, so Approved Document A can come into play, and a building control body may ask to see the framing details. Fire spread across the roof, escape and ventilation are further points that a designer checks. If the flat roof forms part of a conservatory or an extension with a solid roof, a conservatory given a solid roof in place of glass has been materially altered, and it has to be put to building control as an application. Our roof window regulations guide covers the common points.
On planning, whether a particular rooflight or lantern is permitted development turns on the individual house and on how the glass sits against the roof. We do not restate those rules here. Our page on roof window planning sets out where to check, and for anything standing proud of the roof, raise it with the council's planning team before you order, if the house is in Bath.
Bath adds particular care. Heritage protection in Bath works in layers: UNESCO status for the entire city, a designated conservation zone across roughly one half, and a listing on around 5,000 buildings. On a listed house, a new rooflight or lantern is an alteration that needs listed building consent before work starts, and carrying it out without consent is a criminal offence. Planning permission is a separate matter. The flat roof over a rear extension to a listed house still belongs to the listed building, and a lantern that is invisible from the street may still be a change that needs consent. Talk to the conservation team before anything is ordered.
In a conservation area the test is different. Rooflights facing the street, or visible from public places, are more likely to attract comment than a rooflight on a rear flat roof. Our guide to conservation rooflights covers the slim-framed styles that tend to sit more comfortably on historic buildings.
Leaks, condensation and insurance
When a glazed flat-roof opening leaks, the cause is rarely the glass. The usual culprits are the junction between kerb and membrane, a seal that has dried out, a flashing that has lifted, a membrane that was dressed too low on the kerb, or water that has pooled against the upstand. The first signs are a stain at the edge of the opening, drips at the internal frame, or damp on the lining beneath.
Condensation on the inside of a lantern is a separate problem and is often mistaken for a leak. Cold glass meets damp air from cooking and washing, and water forms on the surface and runs down. Better ventilation, a thermally broken frame and glass with a warm edge spacer reduce it. If it is persistent, the cause may be too little ventilation in the room, not a fault in the roof. Our page on a roof lantern that is leaking separates the two and sets out how we trace each.
Insurance deserves a thought before a glass roof is fitted. A glazed roof can be damaged by hail, falling branches or a ball, and some policies treat glazing differently from the rest of the roof. It is sensible to tell your insurer when you add a lantern or large rooflight and to ask what is covered. Our guide to roof window insurance covers the questions to put.
Maintenance is light but real. The glass wants an occasional wash, debris should be cleared from the kerb and the gap between the glass and the roof surface, and the seals and flashings are worth looking at each year. A ladder inspection takes minutes and spots a lifted flashing before water finds its way in.
Choosing between the options for your roof
Start with the room, not the product. What do you want the opening to do: bring in light, add ventilation, make a feature, or replace something that already leaks? A bathroom or landing with no wall space usually wants a simple rooflight. A kitchen extension where the family lives wants light spread across the room, which points to a lantern or a row of rooflights. A dark inner room might do better with a different kind of daylight device, which is the subject of our guide to sun tunnels.
Then look at the roof. A sound roof with a plain deck and good falls will take a rooflight without difficulty. A sagging deck, ponding, or an old felt covering on the end of its life should be fixed first or alongside. Putting expensive glass on a failing roof only brings forward the day you have to take it off again.
Then look at the house. A modern extension at the back of a Victorian terrace can carry a bold contemporary lantern. A Georgian property in the conservation area, or a listed building, will need a more restrained approach, and a simple low-profile rooflight with slim frames is likelier to find favour than a tall lantern.
Put the same list to every contractor you invite. It should name the unit, its U-value, the kerb height, the membrane and how it is dressed to the kerb, the flashings, who is responsible for building control and what is included for scaffold and waste. A quote that names the make and size but says nothing about the junctions is half a quote. Our survey looks over your own roof and its photographs feed our written fixed price, which sets out each of those items. Ask too whether the lantern can be fitted before the inside ceiling is finished, as that order makes the lining and plaster work much neater.
For more on the services around this work, see our skylights, roofs and windows page.