A rear extension with a flat roof and one small window can be a dim place at the back of the room, however well it is finished. A rooflight fixes that quickly. It puts daylight where the plan needs it most, in the middle of a deep room where side windows do not reach. There are several kinds, and the choice is not just about the glass. Whether it opens, whether you can stand on it, what it is made of, how it sits above the roof and how the waterproofing is formed around it all affect whether it performs for years or leaks in its second winter. This guide compares the main types and explains the roofing details that decide the result.
Fixed, opening and walk-on: the three main choices
Fixed rooflights are sealed units that do not open. They are the simplest and cheapest, and the least likely to leak because there is no moving seal. They suit a hall, a landing, a bathroom with an extractor fan, or any room where the light matters more than the air. A fixed rooflight can be a flat pane of glass on an upstand, a pyramid or a lantern, which is a raised glazed structure with sloping sides.
Opening rooflights have a hinged sash that lifts, sometimes by hand with a pole and often by electric motor, with rain sensors on better models. They let hot air out of a kitchen or a bathroom without a window at wall height. They also bring more parts, including hinges, gaskets and motors, that can wear. Where the rooflight is the only escape route from a room, the opening size and the mechanism must be suitable, and in a loft room different rules apply, covered in our answer on what an escape roof window is.
Walk-on rooflights are rated to take a person's weight. They are made from thick laminated glass in several layers with a non-slip surface. They are used over basements, stairwells, courtyards and roof terraces to bring light down from a floor above, and in places where the glass must be crossed or stood on. They are heavy, specialist and expensive, and the structure beneath needs to be designed for the load. They are not ordinary rooflights, and they should never be stood on unless the maker states they are rated for it.
Glass, polycarbonate and domes
The glazing material affects light, safety and heat. Sealed double or triple glazed glass units give the best thermal performance and the clearest view. Toughened outer panes resist hail and falling branches, and laminated inner panes hold together if they crack, which matters because a person falling through a rooflight is a serious risk. Polycarbonate domes, the moulded plastic bubbles seen on garages and older extensions, are cheap and light, but they scratch, yellow, and offer weaker insulation, though multi-wall versions improve it. A dome is a sensible choice for a utility room or a store, where the budget matters more than the finish.
The energy standard also draws a line. Under Part L 2021, rooflights in existing dwellings are expected to meet 2.2 W/m²K, while roof windows are tested against a tighter 1.4 W/m²K, or a Band B energy rating as the alternative route. The distinction is that a rooflight in this sense is a glazed element set in a flat or low-pitched roof, and a roof window is a framed opening in a pitched slope. Your supplier's data sheet should show the figure, and we check it when we survey.
The kerb: where most of the work is
A flat rooflight does not sit directly on the roof surface. It is fixed to a kerb, a raised upstand around the opening, usually 150 mm or more high so that standing water cannot reach the join. The kerb is built from timber, from an insulated GRP moulding or from a manufactured upstand supplied with the rooflight. The roof covering, whether EPDM rubber, GRP fibreglass or felt, is turned up the outside of the kerb in one piece and finished under a cap or trim. The insulation inside the kerb connects the roof insulation to the rooflight frame, so that there is no cold gap.
Getting this right matters because a flat rooflight is a break in the roof. Three faults appear again and again. The kerb is too low, so ponding water reaches the join. The membrane is cut and patched at the corners instead of being formed properly. And the kerb is not insulated, so condensation forms around the opening. Our pages on EPDM rubber roofing and GRP fibreglass roofing describe how each covering is formed around upstands, and EPDM against GRP against felt helps choose between them.
New roof or existing roof
Fitting a rooflight when a flat roof is being renewed is straightforward. The deck is cut, the kerb is built, and the new covering is laid around it as a single operation, so there is one set of joints. Fitting one into an existing flat roof is more work, because the old covering has to be cut back, the opening framed with a trimmer, and the new upstand tied into the old membrane. If the existing covering is near the end of its life, the tie-in will not last, and it is better to renew the roof and the rooflight at once. Rooflights are also often added during an extension, in which case the structure and the kerbs are designed together. The choice between a rooflight and a lantern is covered in our answer on roof lantern or flat rooflight.
The opening needs structural support. A flat roof is carried by joists, and cutting one to make an opening means framing the hole with a trimmer and doubled joists. For a large lantern or a bank of rooflights the engineer's calculation may call for steel. This work is part of the Building Regulations application, which also covers the energy and ventilation sides. What applies to a replaced flat roof is covered in our flat roofing regulations page.
Heat gain, glare and privacy
A rooflight collects the sun from above, so rooms can overheat on a bright summer day. Solar control glass cuts the heat that gets through, at the cost of some tint. An opening rooflight, even a small one, lets hot air escape, and external blinds or internal blinds with reflective backing help. Orientation matters, because a north-facing rooflight gives soft constant light, and a south-facing one gives more brightness and more heat. Glare on a television screen is another surprise, solved by siting the rooflight carefully. Our answer on whether roof windows make rooms hot goes into the options.
Privacy is rarely a problem from the street at roof level, but a rooflight above a neighbour's upstairs window can overlook. Obscured glass is available if needed. Night time is the opposite problem: lit rooms make glass reflect, so a blind helps.
Choosing size and position
A common mistake is to buy the biggest rooflight that fits the opening. Size is a trade-off between light, heat loss and structure. A rule of thumb used by designers is to compare the glazed area with the floor area of the room below, but the right amount depends on orientation, other windows and how the room is used. A single large unit gives an even wash of light, whereas two smaller ones spread it across a deeper room. Position matters as much as size. Place a rooflight over the work surface in a kitchen and the light falls on the task, but set directly above a cooker it will take the steam and grease, so extraction is needed. Over a dining table it creates a strong pool of light that makes the room feel bigger. In a bathroom or a hall a fixed unit is fine. Keep it clear of the edge of the roof and of any parapet or wall that will cast a shadow most of the day, and keep it away from overhanging branches that drop leaves and sap on the glass.
Safety on and around a flat roof rooflight
Rooflights are a classic fall hazard. Plastic domes and old wired glass become brittle with age and can give way under a person's weight, and a rooflight surrounded by roof covering is easy to step on without noticing, particularly when covered in leaves or snow. Anyone working on a roof with rooflights must treat them as fragile. For a commercial building this is a serious issue and our pages on fragile roof safety explain the practice. At home, keep ladders and tools off the glass and never use a rooflight as a step. Ask us to fit a guard or a safety mesh under a dome that is accessible.
Keep the glass clean with water and a soft brush, and clear the debris from the upstand annually so that leaves do not hold water against the join. After a storm, check that the seals, the cap and the edge of the membrane are intact.
What it costs
Prices vary widely with size, glass and mechanism. The only sourced figures for roof windows generally, which are pitched-roof products and not flat rooflights, are a typical UK range of £680 to £1,300 supplied and fitted for a centre-pivot window in one trade guide, and manufacturer figures of £1,623 to replace and £1,906 to add, including VAT. Flat rooflights, lanterns and walk-on glass are priced differently, usually higher for the larger and structural units. We keep no price list for these. When we quote, the sheet itemises the unit, the kerb, the roof covering, the structure and any building control fee, and as always your fixed quote follows the survey.
How we help
We look at the roof, the deck and the structure before suggesting a rooflight, and we photograph the area where it would go. If the flat roof is tired, we say that renewing it with the rooflight makes more sense than cutting into it now. Where the current rooflight leaks, we find out whether it is the seal, the kerb or the membrane. Our roof windows page lists the full service, roof window types covers the pitched options, and a call to 01256 213956 books a visit to your roof.
