Rooflight or roof window
Rooflight or roof window: what is the difference?
The short answer
A roof window lies in the plane of a pitched roof, flush with the covering that surrounds it. A rooflight is a glazed unit that stands out of that plane, usually on a kerb or upstand above a flat roof. The difference matters because the heat-loss limit is 1.4 for a roof window and 2.2 for a rooflight, and because the two are fixed and weathered in different ways.
How to tell the two apart, which suits which roof, and what changes in the rules, the fixing and the price.
Two definitions that decide everything else
In everyday talk, skylight, rooflight and roof window are used for almost any glass in a roof. The building rules are stricter. Approved Document L calls a roof window a window that follows the orientation of the roof around it and lies in its plane. A rooflight, in the same document, is glazing that sits out of that plane, carried on a kerb or an upstand. Everything else follows from those two lines, including the heat-loss limit, the planning checks and the way each unit is sealed against the weather.
Picture the glass. If it tilts at the same angle as the roof and sits level with the covering, it is a roof window. If it sits on top of a low wall or box and rises above the roof surface, whether flat glass, a dome or a lantern, it is a rooflight. A lantern is therefore a rooflight in the rules. A sun tunnel with a raised dome falls on the rooflight side too.
Which roof suits which
Pitch is the main guide. Where the roof is pitched and covered in tile or slate, the roof window is the natural choice, because the frame is made to take the covering up to its edges and the flashing runs around it. Pitched roofs tend to have loft rooms beneath them, so roof windows are the usual choice for a bedroom, stair or landing in the roof space.
A flat roof is nearly level, so a window in its plane would hold water and collect leaves. The glass is lifted on an upstand, and the waterproof covering is dressed up and over it, which makes the unit a rooflight. Flat roof extensions, kitchens and garages are the usual places, and a rooflight over a kitchen island is a common request. Some rooflights are walk-on, with glass strong enough to stand on, and some open for ventilation. Our answer on flat roof skylights covers the range for level roofs.
How the rules treat them differently
Heat loss is the clearest split. Under Table 4.2 of Approved Document L 2021, a window, roof windows included, may lose no more than 1.4 W/m²K (Band B on its Window Energy Rating is the alternative), while a rooflight is allowed 2.2. A site-built upstand is limited to 0.35. The 2026 edition, which takes effect on 24 March 2027, keeps those figures in its Table 3.2.
Planning treats them differently in practice, though not in law. Permitted development on a house caps how far a roof alteration may protrude, at 0.15 metres beyond the slope, and stops any part of it climbing higher than the original roof's top. A roof window made to lie flat usually keeps inside that. A rooflight on a tall upstand may not, and a lantern most likely will not. Flats, listed buildings and special areas bring their own checks, which our guide to planning permission for a skylight sets out.
Fixing, weathering and where leaks start
A roof window is cut into the pitched slope. The rafters are trimmed to make an opening, the frame is fixed, and a flashing kit chosen to suit the covering seals it. Each covering, whether plain tile, interlocking tile or slate, takes its own kit. Leaks tend to start at the flashing, at a worn seal or where the covering around the opening was laid badly.
A rooflight starts lower down, with the upstand. It may be timber, built up from the roof deck and clad, or a manufactured kerb supplied with the unit. The waterproof layer wraps the upstand and tucks under the unit's lip. Leaks here usually begin where the covering meets the upstand, or where an old covering has aged around a new unit. The covering round the opening gets our attention first, because a new unit on a tired roof is a repair that will not last, and the survey tells you that plainly.
Choosing, and what each costs to fit
You rarely choose between the two on one roof, because the roof pitch makes the choice for you. On a pitched roof, ask for a roof window. On a flat roof, ask for a rooflight, and ask whether a lantern or a flat unit suits the room. A cost difference does exist, since a rooflight job includes the upstand and usually more waterproofing work, but a roof window job can need scaffold on a steep front slope. Neither has a fixed price.
Whichever type you choose, the survey photographs the roof and our quote is written, with one total at the bottom. Check that the figure covers VAT and any scaffold, and that the unit meets the limit that applies. The skylights and roof windows page describes the whole range of work, from swaps to new openings. Our answer on the U-value for a new roof window goes deeper on the heat-loss figures.
Related questions
Rooflight and roof window questions
Is a Velux a rooflight or a roof window?
Velux is a brand, and the name tells you nothing about the type. A Velux set into a sloping roof is a roof window, while a similar unit lifted onto a kerb over level roofing is a rooflight. The rules look at how the glass sits, not who made it.
Does a flat roof take a roof window?
A window that lies in the plane of a near-level roof would not drain, so flat roofs take a rooflight on an upstand, or a low-pitch unit made for the job with its own drainage. The manufacturer's minimum pitch is on the data sheet, and the survey will confirm what suits the roof.