A flat roof light can turn a dark kitchen, hallway or extension into a bright room without changing the walls. It can be a simple double-glazed pane set on a low upstand, a plastic dome, an opening unit that vents hot air or a full glass lantern that becomes the feature of an extension. The terms are used loosely, though, and the details around the glass, the upstand and the waterproofing decide whether the roof light performs well or leaks. This guide explains the types of flat roof light and lantern, how they are built into the roof, typical UK costs, the rules and the problems to watch for.
Roof Light, Skylight and Lantern: What Is the Difference?
- Flat roof light (flat rooflight): a flat glass or plastic panel in a frame, set on an upstand above a flat roof.
- Roof lantern: a raised glazed structure with sloping sides and a ridge, usually on a flat roof, often in aluminium, timber or uPVC frames.
- Dome: a curved acrylic or polycarbonate light, usually the lowest-cost option.
- Skylight: a general term, often used for any roof glazing, and for roof windows on pitched roofs. Our guide to what a skylight is explains the terms for pitched roofs.
Types of Flat Roof Light
| Type | Description | Best for | Points to note |
|---|---|---|---|
| Fixed framed flat rooflight | Double or triple glazed unit in a visible frame | Most extensions and kitchens | Lower cost, neat and simple |
| Frameless flat rooflight | Glass appears to float above a slim upstand | Modern designs, maximum light | Costs more, details matter |
| Opening rooflight | Manual or electric opening panel | Kitchens, bathrooms, stairwells | Adds ventilation and cooling, costs more |
| Roof lantern | Raised glazed lantern on a kerb | Extensions, kitchen diners, orangeries | Heavy, needs structural support, often bespoke |
| Dome rooflight | Plastic dome on an upstand | Garages, utility rooms, budget projects | Lowest cost, less insulation and clarity |
| Walk-on rooflight | Heavy-duty glass rated for foot traffic | Terraces, roof gardens | Specialist product with load ratings |
Glazing Options
- Double or triple glazing: triple glazing gives better insulation, while double is more common and cheaper. Check the manufacturer’s declared U-value, where lower means better insulation.
- Laminated inner pane: often used as a safety feature, because glass overhead should hold together if it breaks.
- Solar control coatings: reduce heat gain in south-facing or heavily glazed rooms.
- Self-cleaning and anti-glare coatings: reduce maintenance and brightness, though results vary.
- Tints and opal panes: soften light and cut glare.
Upstands and Kerbs
A flat roof light normally sits on an upstand, also called a kerb, which raises it above the roof surface so that water cannot run in. The upstand is typically timber or insulated uPVC, and the roof membrane is turned up the outside and sealed to it. A common minimum height is around 150 mm above the finished roof level, but check the manufacturer’s detail. The kerb should be insulated and the inside should be lined, otherwise it can create a cold spot and condensation. Our guide to warm roof construction explains how insulation and the roof deck work together, and the EPDM rubber roofing guide shows how upstands are detailed on a rubber roof.
Size, Weight and Structure
A roof light needs a framed opening in the roof, with extra timbers around it to carry the load. Glass is heavy, and a large flat roof light or a glass lantern can weigh a significant amount, so the roof structure needs to be designed for it. For big openings, lanterns and walk-on glass, ask a structural engineer or the manufacturer to confirm the support. The roof also needs a fall to drain water away from the upstand, and poor drainage around the kerb is a common source of leaks.
Light, Heat and Condensation
- Orientation: north-facing roof lights give soft, even light, while south-facing ones can overheat unless they have solar control glass and blinds.
- Ventilation: opening units let hot air escape and can help in kitchens and bathrooms.
- Condensation: a poorly insulated kerb or very cold glass can cause condensation. Good insulation and ventilation reduce the risk.
- Blinds: integral blinds can control heat and glare on larger glazed areas.
Flat Roof Light and Lantern Cost in the UK
Prices vary widely with size, glazing, frame material and structural work, and sources differ, so treat the figures below as general guides based on 2026 UK price guides. They usually exclude scaffolding.
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| Type | Typical installed cost | Notes |
|---|---|---|
| Fixed framed flat rooflight | Roughly £1,000 to £2,600 | Rises steeply with pane size |
| Fixed frameless rooflight | Roughly £1,300 to £2,600 | Premium look and finish |
| Opening rooflight (manual) | Roughly £1,600 to £2,400 | Adds ventilation |
| Opening rooflight (electric) | Roughly £2,600 to £3,600 or more | Motor and controls add cost |
| Roof lantern (about 1.5 m by 1 m) | Roughly £1,800 to £3,500 | Bespoke and larger lanterns cost much more |
| Large bespoke aluminium lantern | Often £3,600 to £9,000 or more | Includes structural support and finishing |
| Small uPVC lantern or dome (supply only) | Roughly £400 to £900 | Lowest-cost options |
Replacing an existing roof light in an existing opening is usually cheaper than cutting a new opening, which brings structural work, new waterproofing and internal finishing. Many people replace the roof light when they renew the flat roof, which avoids paying for access twice. Our flat roof replacement cost guide sets out typical prices for the roof covering.
How a Flat Roof Light Is Installed
- Plan the opening. Check the structure, joist layout, size and position, and the fall around the upstand.
- Cut the opening. Frame it with timbers to carry the load, and protect the interior.
- Build or fit the upstand. Fix a kerb, or fit the pre-made upstand, and insulate it.
- Waterproof the roof. Turn the membrane up the kerb and seal it with the correct trims and flashings.
- Fit the unit. Place the roof light, secure it as instructed and seal the joint.
- Finish inside. Line the opening, usually in plasterboard, and add trim and blinds if needed.
- Test and check. Look for leaks, fixings and condensation after rain.
Common Problems
- Leaks at the kerb caused by poor membrane detailing, failed sealant or too low an upstand. A roofer can repair these, as explained in our flat roof repair guide.
- Ponding around the roof light from a blocked outlet or insufficient fall. See our guide to the roof gully.
- Condensation between panes, usually from failed seals in the glazing unit.
- Cracked glass from impact, thermal stress or settlement.
- Overheating in south-facing rooms without solar control glass.
- Dirt and algae on the glass, which reduce light.
Planning Permission and Building Regulations
Roof lights on a flat roof are often allowed without planning permission, but those that project more than about 150 mm from the roof surface, large lanterns and any work on listed buildings or in conservation areas can need it. The Planning Portal explains the rules in England. Building regulations apply to energy performance, structure, ventilation and safe glazing, so a new opening or extension should be checked with building control. Scotland, Wales and Northern Ireland have their own systems. For wider ideas on how roof extensions are classified, see our guide to roof extension types.
Safety
Working at height is dangerous. Never walk on a roof light unless it is specifically rated for foot traffic, because falls through roof lights are a well-known cause of serious injury. Glass units are heavy and should be lifted by a trained team.
Asbestos warning. Older flat roofs and outbuildings may contain asbestos cement sheets or boards around the opening. Do not drill, cut or break them, and get them identified by a competent surveyor before any work.
Frequently Asked Questions
How much does a flat roof light cost?
As a general guide, roughly £1,000 to £2,600 installed for a fixed unit, more for opening and frameless units, and £1,800 to £3,500 or more for a roof lantern.
What is the difference between a roof lantern and a flat roof light?
A flat roof light is a flat panel on an upstand, while a roof lantern is a raised glazed structure with sloping sides and a ridge.
Do I need planning permission for a roof light?
Often not on a flat roof, but it depends on projection, size and whether the building is listed or in a conservation area. Check with your local planning authority.
Can I walk on a flat roof light?
Only if it is a specifically rated walk-on unit. Standard roof lights are fragile and must not be stood on.
How do I stop a flat roof light leaking?
Make sure the upstand is high enough, the membrane is properly dressed and sealed, the fall drains away from the kerb and outlets are clear.
Can a roof light be added to an existing flat roof?
Yes, but it needs a new framed opening, waterproofing and internal finishing, so it costs more than replacing an existing unit. Get a roofer or builder to assess the structure.
Final Thoughts
A flat roof light or lantern brings daylight into a room, but the details decide the result. Choose glazing and a size that suit the space and structure, make sure the upstand is insulated and correctly waterproofed, and combine the work with a roof replacement if the covering is due. Check planning and building rules, never walk on standard glazing, and treat any old cement sheets as asbestos until proven otherwise.
Get a professional assessment
If you are unsure how much life your current roof has left, a local roofer can give you an honest assessment as part of a free inspection. Browse vetted roofers through our roofer directory, or get a quick replacement estimate using our free cost calculator.