What is the difference between a skylight and a rooflight?
In ordinary speech there is no difference and both words mean a piece of glass in a roof. In the trade there is a real one, and it decides which product arrives on the lorry. Skylight is the general public word: it covers everything glazed set into a roof and tells an installer nothing about the job. Rooflight is a supplier’s word, and in the way manufacturers and roofers use it, it means a unit for a flat or nearly flat roof, sitting on a kerb built up above the roof surface. The pitched-roof product is called a roof window and it is a third thing again. Getting these straight before you ask for a figure saves a round of confusion, because the three are priced, made and fitted differently. The full range of what Severn installs is set out on the roof window installation and flat roof rooflight installation pages.
Skylight is the household word and it is not a specification
Skylight is centuries old and it originally meant light from the sky, then any opening that admitted it. It survives as the word most people reach for and the word most people type into a search box, which is why it appears in the name of the company and on the front of this site.
Its problem in a technical conversation is breadth. A skylight can be a Victorian cast iron pavement light, a plastic dome on a 1970s garage, a glazed atrium roof, a sun tunnel, a roof lantern or a modern roof window. Ask for a price for a skylight and any honest answer starts with three or four questions, all of which are about narrowing it down. It is a perfectly good word for describing what you want in a room. It is a poor word for describing what you want on an order.
What a supplier means by rooflight
In manufacturer catalogues and among roofers, rooflight has settled into a specific meaning: a glazed unit for a flat or very shallow roof, mounted on an upstand or kerb rather than sitting in the plane of a covering.
Several characteristics follow from that. The glass is usually one plane set at a shallow angle of around 5 degrees so rain sheds and dust does not sit. The frame is slim aluminium with a polyester powder coating. The unit is commonly made to order rather than picked from a grid, because a flat roof opening can be any dimension. It arrives as a finished item to be lifted onto a kerb the roofer or the builder has already constructed and weathered. And it has no flashing kit at all, because there is no tile profile to flash into: the roof membrane itself is dressed up the kerb and over. That kerb, and where the critical 150mm is measured from, is on the rooflight upstand page.
The roof window, and the plane it sits in
A roof window is the pitched-roof product, and the defining characteristic is that it sits in the plane of the covering with the tiles or slates running right up to its frame. From the garden it reads as part of the roof rather than as an object on top of it.
Everything about it follows from that. It is made to a fixed grid of manufactured sizes rather than to order. It comes with a flashing kit matched to the covering profile, which is roughly half the product. It opens on a centre pivot or a top hinge, giving a sash you can reach and rotate. Most ranges are rated for pitches from about 15 to 90 degrees, and below 15 the water stops shedding properly. VELUX, Fakro, Keylite and Roto all make them, and the size codes are how a survey turns into an order. Because the projection above the roof plane is small, an in-plane roof window is nearly always inside the 150mm permitted development limit for a roof alteration, where a kerb-mounted unit on a pitched slope may not be.
The four terms side by side
Laid out together the distinctions stop being subtle.
| Term | Roof it belongs on | How it is mounted | Sizing | How it is weathered |
|---|---|---|---|---|
| Skylight | Any. It is a category, not a product | Depends entirely | Depends entirely | Depends entirely |
| Roof window | Pitched, roughly 15 to 90 degrees | In the plane of the covering | Fixed grid of manufactured codes | Preformed flashing kit matched to the tile profile |
| Rooflight | Flat or nearly flat | On a kerb, 150mm above the finished surface | Usually made to order | Roof membrane dressed up the kerb and over |
| Roof lantern | Flat | On a kerb, rising to a ridge or apex | Made to order | Membrane up the kerb, then the frame’s own glazing system |
A fifth term belongs in any Gloucestershire version of this list, and that is the conservation rooflight. It is a pitched-roof unit despite the name, with a slim cast or pressed steel frame, one central glazing bar and a low profile designed to sit almost flush in a thick stone slate roof. It exists because of the way Cotswold roofs are built, and it is covered under conservation rooflight installation.
Why the wrong word sends the wrong product
This is not pedantry, and the consequences are concrete. Ask a merchant for a rooflight for a pitched roof and you may be quoted a made-to-size flat unit that would have to be mounted on a kerb built up off the slope, projecting well beyond the 150mm permitted development limit and reading as a box bolted onto the roof.
Ask for a roof window for a flat roof and you get a product rated to 15 degrees minimum that will sit in standing water if it is laid flat, with a drainage channel that cannot empty. It is fitted on a purpose-made pitched kerb instead, which is a legitimate installation and is described on the can you fit a roof window in a flat roof page. And ordering the wrong one carries a longer tail than the unit itself, because the flashing kit, the blind, the insulation collar and every accessory order against the product family you chose.
Building Control and planning use their own vocabulary again
The regulations do not follow trade usage, which adds a third layer. The Approved Documents use “rooflight” as a catch-all for any glazed element in a roof, including what the trade would call a roof window, and they distinguish instead by the plane the glass sits in.
That distinction has a number attached. Current guidance on conservation of fuel and power sets a limiting standard of 1.4 W/m2K for a replacement roof window, and quotes rooflights in the vertical plane at 2.2, which is a different measurement basis rather than a laxer rule. Planning language is different again: a roof alteration must not project more than 150mm beyond the plane of the existing roof slope, and the word used there is neither of ours. The Building Control notification is handled as part of the job, and the application uses the regulator’s terminology rather than the trade’s.
Dome, sun tunnel, walk-on: the words further out
Four more terms turn up and each is a specific product rather than a synonym.
- Dome. A moulded polycarbonate or acrylic unit on a kerb, common on 1970s and 1980s flat roofs and garages. Cheap, light, and noticeably louder in rain than glass. Most enquiries about one are about replacing it with a glass unit.
- Sun tunnel. A reflective tube running from a small dome on the roof down through the loft to a diffuser in the ceiling below. It brings daylight to a windowless landing, hallway or bathroom without a structural opening. See sun tunnel installation.
- Walk-on rooflight. A flat unit with structural glass rated for foot traffic, used where a terrace or a path sits over a room. Thick laminated build-up, and the loading has to be designed. See walk-on rooflight installation.
- Roof lantern. A framed structure with several sloping glass planes rising to a ridge, adding height and volume rather than just daylight. See roof lantern installation.
Which word to use when you ask for a figure
Use whichever word comes naturally and describe the roof instead. The roof answers the question by itself, and it is the information that actually shortens the conversation.
Four things do it. What the covering is, or a photograph of it if the name is not obvious. Whether the roof slopes steeply, gently or not at all. A photograph of the room from inside, so the reveal and the run of slope can be judged. And roughly where you would like the daylight to land. Send that to info@severnskylightsgloucester.co.uk and the reply will name the product family before it names a figure. On a Gloucestershire roof the covering and the conservation status are often obvious from the postcode alone: a Painswick address suggests stone slate and a conservation unit, a Quedgeley address suggests trussed rafters and a narrow in-plane window, and a Cheltenham parapet address suggests a shallow hidden slope and a kerb. Two figures follow, a range by email and then a fixed price after a survey, as set out on the costs page, and the product ranges themselves are on the VELUX window installation page.
Next on this subject.
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- Question How much does a roof window cost to fit? Fitting a roof window is priced from the roof, not from the window. Two houses a mile apart in Gloucestershire…
- Question What size roof window should I get? Start with the room and let the roof edit the answer. The useful method is to work out how much…
- Question Can a roof window be an escape window? Yes, and on most loft conversions in Gloucestershire a roof window is the escape opening. It only works if three…