Roof window or rooflight, and the difference

Two words the trade uses differently from the brochures

The clean distinction is fixing method. A roof window is framed into the rafters and the roof covering is dressed around it with a flashing kit, so it sits within the plane of the tiles or slates. A rooflight lands on an upstand built off the roof structure, and the covering is dressed up the sides of that upstand before the unit goes on top.

That difference produces everything else. A roof window can be pushed open like a window because it has a slope to swing on. A rooflight generally cannot, or opens on a hinge or a ram, because it is close to horizontal. A roof window relies on gravity and a flashing kit. A rooflight relies on the height of its kerb.

Where the word rooflight came from

Rooflight is the older and broader term, and it originally described any glazed element admitting daylight through a roof, including the cast iron and wired glass lights on Victorian workshops and the patent glazing over factory floors.

It carried through into the domed plastic units fitted across British housing in the sixties and seventies, and it is still the word used in flat roofing. In current usage a rooflight is a flat or gently sloped glazed unit on a kerb, though the same word is quite properly used as an umbrella for the whole subject, which is where the ambiguity comes from. Conservation rooflights are the exception that keeps the old meaning alive: they sit in pitched slopes but they inherited the name from the cast iron originals.

Where the word roof window came from

Roof window is a much more recent term and it is essentially a product category rather than a piece of building vocabulary. It arrived with the mass-produced centre pivot units that made loft rooms habitable from the middle of the twentieth century onwards.

The word window is deliberate. These units are designed to be operated like windows, with a handle, an opening sash, a fitted blind and in many cases a view out. That is what separates them from the older idea of a rooflight, which was about admitting daylight to a space rather than giving anybody something to look through. A roof window is a window that happens to be in a roof.

How each one is fixed into the roof

A roof window is fixed to the rafters. The opening is framed with doubled rafters at each side and trimmers above and below, the frame is screwed or bracketed to that timber, and the unit ends up sitting in the same plane as the roof covering with only its outer frame proud.

A rooflight is fixed to a kerb, which is a timber or insulated upstand built off the deck. The kerb is a piece of joinery in its own right and its accuracy governs the fit: it must be square, level and built to the manufacturer’s stated dimension, with the diagonals checked before the unit is ordered. A kerb out of square shows as an uneven shadow line and, more seriously, as unequal compression on the perimeter seal.

Weathering: a flashing kit against a kerb

This is the difference that decides whether either of them stays dry, and the two approaches have nothing in common.

A roof window is weathered by a flashing kit specific to the covering. A plain clay tile, a concrete interlocking tile, a natural slate and a low profile pantile all take different kits, because the depth and profile of the covering change how the apron and the soakers have to sit. Using the nearest kit to hand rather than the correct one is how a roof window ends up with a proud flashing and a slow leak three winters later.

A rooflight has no kit. The roof covering itself, single ply, felt or GRP, is dressed up the face of the kerb and terminated under the unit, and the height it is dressed to is the whole defence. The figure quoted across the industry is one hundred and fifty millimetres from the finished surface to the top of the kerb, derived from the depth of water that can stand on a flat roof plus the height wind driven rain can be pushed up a vertical face.

Opening actions compared

Roof windows open in ways rooflights cannot, because a sloping sash has somewhere to go.

Centre pivot units rotate about a horizontal axis roughly halfway up, which keeps the handle in reach and allows the outer face to be turned into the room for cleaning. Top hung units swing outwards from the head, giving an unobstructed view and better headroom where the window is over a low cill, and many units combine both actions.

A rooflight in a flat roof opens differently. It lifts on rams or a scissor mechanism, usually electrically, opening the whole unit a modest amount all round rather than swinging a sash. Some hinge on one edge and lift on the opposite one. The travel is short compared with a roof window, but the position at the top of the room means the air movement is efficient anyway.

Glass and thermal behaviour compared

Both are double or triple glazed sealed units and both are governed by the same two numbers, but the way those numbers behave differs with the angle.

A roof window in a pitched slope faces one compass direction and receives direct sun for part of the day. A flat rooflight faces the entire dome of sky, so it collects light from every direction and takes the worst of the overhead summer sun at midday. The consequence is that solar control glass is a considered option on a roof window depending on which way the slope faces, and close to standard on a flat rooflight regardless. Both take laminated inner panes as a matter of course, because the glass is overhead.

Sizes, and how each one is ordered

Roof windows are ordered from a manufacturer’s size code, a short alphanumeric that fixes the frame dimensions exactly, and the range is genuinely standardised. The structural opening is derived from the code, and the widths are chosen to suit rafter centres of four hundred and six hundred millimetres.

Rooflights are ordered to a kerb size, which may come off a standard grid or be made to measure. Three separate numbers are in play: the kerb size the manufacturer builds to, the structural opening cut in the deck, and the daylight opening you actually see through. Confusing those three is the most expensive mistake available on a small job, and it is a mistake that roof windows largely avoid by using a single code.

What each one costs to fit, in relative terms

Neither is reliably cheaper, and the difference between two jobs of the same type is usually larger than the difference between the types.

A roof window into a sound pitched slope is a predictable day: the carpentry is routine, the flashing kit does the weathering, and the main variable is access. A flat rooflight involves building or preparing a kerb, which is joinery on a roof, and it interacts with the condition of the existing deck. Where that deck is old, laid to no useful fall, or holding water, work on the roof itself comes before any glass. That is why a flat rooflight has a wider spread of outcomes. The components that make up either number are set out under costs.

Which one the roof has already decided

In most houses the choice has been made by whoever built the roof, and the conversation is shorter than people expect.

A tiled or slated slope over a loft or a bedroom takes a roof window. A flat roof over a rear extension, a side return or a garage takes a flat rooflight or a lantern. Genuine choice only exists on a new extension, where the roof has not been built yet and a pitched or flat roof is still open, or on a house that has both and the question is which room to light first.

Skylight, the word that means both

Skylight is the term the public uses and it covers both products. It is not wrong, and nobody in the trade will misunderstand you.

Where it causes trouble is on a written quotation. A document that says supply and fit skylight, with a number beside it, has not told you whether you are getting a roof window in a slope with a flashing kit or a flat unit on a kerb. Those are different jobs with different weathering, different structural implications and different lead times. Asking the supplier to name the product family, the manufacturer and the model turns an ambiguous line into a comparable one.

What the words mean on a quotation and on a drawing

On an architect’s drawing the distinction is usually implicit in the detail rather than the label, and it is worth reading the section rather than the plan.

A section through a roof window shows the unit within the depth of the rafters with the covering running up to it. A section through a rooflight shows the kerb standing above the deck with the membrane dressed up it, and the upstand height should be dimensioned. If it is not dimensioned, ask, because that number is the one carrying the weathering. Comparing quotations line by line is covered in reading a rooflight quotation properly.

Choosing the right word when you enquire

The fastest way to get a useful answer is to describe the roof rather than the product.

Say what the covering is, roughly what the pitch looks like, which room is below and which way it faces. That gives an installer everything needed for a sensible range, and it removes the risk of being quoted for the wrong family entirely. If you already know the roof is flat, say rooflight. If it is a tiled slope, say roof window. If you are not sure, a photograph from the garden settles it in seconds, and both words will do until it does.