Shops and catalogues use "skylight", "rooflight" and "roof window" as if they were the same thing. In everyday talk they nearly are, and nobody minds. On a building job the words have separate meanings, because the Building Regulations draw a line between them and put different numbers on each side of it.
The line is about how the unit is installed, not how it looks. A roof window is set into the slope and lies level with the covering, much like a window in a wall. A rooflight rises above the covering on a raised frame, called a kerb or an upstand, with its glass clear of the surrounding surface. That single difference affects the heat-loss limit the glazing must meet, the way the joint is made watertight, how far the unit projects from the roof, and which kinds of roof can take it at all.
This guide sets out the official definitions first, then looks at the pitched roof and the flat roof in turn, and finishes with the decisions that settle which product you need. Where a point belongs to another page, we link to it rather than repeat it.
How Approved Document L defines each
The cleanest definitions come from Approved Document L, volume 1, the government guidance on the conservation of fuel and power in dwellings. In its terms, a rooflight is a glazed unit sitting proud of the roof surface, mounted on a kerb or upstand. By contrast, a roof window is a unit oriented like the roof surrounding it and lying in the same plane.
Both definitions turn on one word, plane. Picture a flat sheet laid across the surface of the tiles. A roof window lies within that sheet. A rooflight pokes through it. If you ran a straight edge down the slope, a roof window would leave it sitting level with the covering, while a rooflight would lift it.
What the words do not tell you
Neither definition mentions shape, size, opening method or frame material. A small hinged window set in a slope is a roof window. A large fixed pane on a timber kerb is a rooflight. A glass pyramid on a flat roof is a rooflight. A dome on a plastic base is a rooflight. A tall roof lantern is a larger version of the same idea, and our guide to roof lanterns on flat roofs and extensions goes through it.
The word "skylight" has no standing in either definition. It may mean either product, and some suppliers use it for both. When a quote says "skylight", ask which of the two is meant, because it controls the paperwork that follows. We write "roof window" or "rooflight" on every quote and drawing for that reason.
Since the terms trip up customers, builders and sometimes the listing sites that sell the units, a one-line check helps: if the glass sits in line with the roof covering, it is a roof window, and if it sits on a raised box, it is a rooflight.
Pitched roofs and the roof window
A pitched roof, the usual sloping kind with tiles or slates, is the natural home of the roof window. The unit is framed between rafters, the sloping timbers underneath, and the covering is cut and dressed around it with a flashing kit designed for the tile or slate in use. The glass follows the angle of the slope, so the window tilts to the sky and admits light from above as well as a view of the sky.
A roof window suits a loft room, a landing, a stair or a dark upper bathroom. It also suits the many houses along this coast that are bungalows with hipped roofs, where a window in the slope lights a back bedroom or a roof void that has been boarded and insulated. On a bungalow the same unit can often be reached from the floor, so a person standing in the room can open and close it.
Why a kerb is rarely used on a slope
A rooflight on a pitched roof is possible, but it is the less common choice. The kerb has to be built up from the slope, usually as a framed box, and the covering has to be dressed up the sides and tucked underneath on the high side. The raised frame also stands out from the roof by more than a roof window would, which has planning consequences covered below.
On a listed building or in a conservation area, the slim, flush style associated with conservation rooflights is the usual answer. The name is confusing, since those units sit in plane like a roof window, and our page on conservation rooflights explains where the label comes from.
The pitch of the slope limits what can be fitted at all, because every product has a minimum angle for which its flashing is designed. The manufacturer's data sheet gives it, and we check it at survey before the quote is written.
Flat roofs and the rooflight
A flat roof is the other home. A flat roof has no slope for a roof window to sit in, so the glazing goes on a kerb, which lifts it out of the waterproof layer. That is the standard arrangement for a rooflight, whether the roof is covered in felt, GRP or a single-ply membrane such as EPDM, the synthetic rubber sheet widely used on extensions.
The kerb is the weak point and the strong point at once. Water that lies on the roof surface cannot reach the glass joint if the kerb is high enough and the membrane is turned up the side of it. The covering is dressed up the outside and tucked beneath a cap or flashing, which keeps the join dry without relying on sealant alone.
Falls and ponding around the kerb
A flat roof must drain, so it is laid with a slight tilt towards the outlet. IKO, citing the 2018 edition of BS 6229 (since revised), gives the recommended minimum fall as 1:80, with the roof designed at 1:40 so that building tolerances still leave the finished minimum. A kerb sits across that fall and can stop water draining past it, leaving a pool behind it on the high side. A good installer shapes the surrounding fall, sometimes with a small tapered pad called a cricket, so the water splits and runs around the kerb.
Flat roofs are where fixed or opening rooflights, walk-on glass and larger units arise, and we have separate pages on flat rooflights, both walk-on and fixed and on sun tunnels, a lower-cost way to bring daylight into a room with no loft above it.
When the roof covering is due for renewal at the same time, it makes sense to fit the new kerb with the new covering, because the membrane can then be turned up into place as one piece.
How the rules differ for each
The practical effect of the two definitions shows up in the regulations. Table 4.2 of Approved Document L sets heat-loss limits for new or replacement parts of existing homes. A roof window must reach Window Energy Rating Band B at least, or a U-value of 1.4 W/m²K. A rooflight is allowed 2.2 W/m²K. Where an upstand is built on site, the table sets a limit of 0.35 W/m²K for the upstand itself. Our guide to Building Regulations for roof windows and rooflights goes through each figure.
The thermal limit is not the only difference. Planning law, in the class that deals with roof alterations, says nothing may stand more than 0.15 metres off the original slope. A roof window can meet that easily. A rooflight raised on a kerb is more likely to break the limit on a pitched roof, which is a further reason roof windows dominate on slopes. The limits on planning are set out in planning permission for a skylight.
Escape, glazing and opening
Escape rules refer to windows that people climb through, which in a loft means a roof window that opens far enough and sits low enough on the slope. A rooflight on a flat roof is not normally a route out of a room. The sizes that count are in the escape roof window size guide.
Opening differs too. A roof window can be hinged or pivoted, and can be motorised. Rooflights are often fixed, because the aim is light rather than ventilation, though opening versions exist. If ventilation matters, a roof window in the slope or an opening unit on the kerb is the answer, and the choice should be written into the specification before the order.
Light, heat and comfort in use
The two products feel different in a room. A roof window in a slope sets the glass at an angle, so you can see sky and treetops from a bed or a desk, and daylight falls into the room at a low angle that reaches far across the floor. A rooflight on a flat roof looks straight up, which gives more overhead light and less view, and suits a kitchen extension, a hall or a bathroom with no outside wall.
Both bring solar gain, the heat from sunshine through glass. South and west faces collect the most, and the coast gives plenty of bright light, so a room with a large glazed area can overheat in summer. Blinds, solar control glass and the ability to open the unit for ventilation all help, and they are worth choosing at the start. A fixed rooflight cannot be opened to let hot air out, so it needs a plan for ventilation elsewhere.
Condensation and noise
Glass in a roof can attract condensation in winter when the room is humid, particularly in a bathroom or kitchen. A unit meeting the current thermal limits is less prone to it because the inner pane is warmer, and extractor fans do the rest. Rain on glass can also be heard from below, an effect covered in rain noise from roof windows.
Security and misting are further matters. A roof window that opens must lock properly, and a sealed double-glazed unit can mist if the seal fails, which is a fault in the glass rather than the roof.
None of these points favours one type absolutely. They are the questions to settle with the room in mind: is it a bedroom or a corridor, a south-facing loft or a north-facing hall, and who will open and clean the glass?
Choosing between them for your roof
Four questions settle most decisions, and the answers usually point to one product.
What is the roof? A sloping tiled or slated roof points to a roof window. A flat roof, or a low-pitched extension roof, points to a rooflight on a kerb. If the roof is something in between, such as a shallow pitch over a rear extension, the survey decides, because the minimum angle on each product controls what is possible.
What is the room for? A loft bedroom wants a window you can open, see out of and escape through. A dark hall wants light from above and rarely needs a view. A kitchen extension may want a large glazed area, and for that a rooflight or a lantern is usual.
Can the building take it?
A conservation area, listed status or an Article 4 direction can narrow the options. Flush units sit better on historic roofs, and some streets in Shoreham and Worthing have had the permitted development right for roof alterations removed. Check before you order.
What will it cost? Price follows size, glazing, covering and access, and scaffolding can be a large share of it. Typical UK figures for a standard roof window installation are in our guide to alternatives, which also covers what to compare between makes. The written quote, with photographs of your roof, gives one total price, and the photographs show the existing opening and kerb.
Not certain which you need? Photos of the room and of the slope from outside let us say which type fits and what the survey should look at first. The rest of the range is under skylights and roof windows.
