The short answer
A roof window is a glazed window set into a pitched roof slope, usually opening on a pivot or hinge so you can reach it and ventilate through it. A skylight in UK trade use more often means a fixed or low-profile glazed unit set into a flat or very shallow roof, often called a rooflight. In everyday speech the words overlap, but the difference decides which product, kerb and flashing you need.
The words and what they cover
In the United Kingdom, a roof window normally refers to a window installed in the plane of a pitched roof. It has a frame, an opening sash that pivots or tilts, a flashing kit that seals it to the tiles or slates, and a glazed unit built to take weather from above. A skylight is a looser word. Americans use it for almost any glazing in a roof, but in this country it more often describes a fixed or domed unit on a flat roof, which the trade calls a rooflight, or a lantern or a sun tunnel. Many manufacturers and sellers use all these terms interchangeably, so check the product description rather than the name.
This matters because a pitched-roof window will not work on a flat roof, and a flat rooflight will not seal on a steep slope. Our page on the minimum pitch for a roof window explains the angle limit.
How they are built and fixed
A roof window is fixed directly to the rafters through the roof slope, with the flashing kit overlapping the surrounding covering. The sash is usually centre-pivot, turning around a bar halfway up so you can clean the outside from inside, or a top-hung or electric design. There are also versions that combine a top-hung sash with a pivot, giving a larger opening for loft rooms.
A rooflight on a flat roof normally sits on a raised kerb, a timber or insulated upstand built around the opening and covered with the roof membrane. The glass sits on the kerb with a low slope so rain runs off. Some open on a hinge or with an electric actuator; many are fixed. A sun tunnel is a different product again, a small domed collector on the roof and a reflective tube that carries daylight into a dark room. Our guide on sun tunnels explains how they work.
Which suits which room
If the room is in the loft, a roof window gives light, ventilation and a view, and can serve as an escape route if the opening is big enough and the room is low enough. It also sits closer to the line of the slope, so it can be less visible than a dormer. For a kitchen extension or a hallway under a flat roof, a rooflight or lantern brings in light from above without needing a pitched slope. If you cannot decide between those two, see roof lantern or flat rooflight.
Thermal performance differs too. For an existing dwelling in England, Part L sets 1.4 W/m²K or Window Energy Rating Band B for a replacement roof window and 2.2 W/m²K for a rooflight. A glass product that does not meet the right figure for its category may not be acceptable to building control.
What to ask before you order
Start with the roof: is the slope pitched or flat, and at what angle? Then the room: does it need opening ventilation, an escape route, or just daylight? Then the covering: tile, slate, felt, EPDM or GRP each needs a matching flashing or upstand. A supplier who asks for these details before quoting is doing the job properly. Our wider roof window types guide goes through the opening styles.
Replacing one with the other
Swapping a flat-roof skylight for a pitched roof window is not a like-for-like change, even if the opening looks similar. It usually means altering the roof structure, because a flat roof has a different frame, covering and fall from a pitched one. The reverse is also true: you cannot take a roof window out of a slope and put a flat unit in its place without rebuilding the area. If you are replacing an old unit, the safest route is to identify what you have from its label and keep to the same product type, unless the survey shows the roof itself needs to change.
Planning and building control come in too. Many roof windows on rear slopes are permitted development, subject to limits, while a lantern may need planning consent because it rises above the roof. We handle the Building Regulations side where work is notifiable. We survey on site, photograph the roof and structure so you can see the options, and put the price in a fixed written quote. See roof windows in Basingstoke for the full range.
Every roof is different. A free survey with photos settles it for yours.
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