What is a roof window?
A window that sits in the plane of a pitched roof, weathered by a flashing kit rather than a kerb.
Part of our Roof Window Installation guide
A roof window sits in the plane of a pitched roof, in line with the covering, rather than on a kerb raised above it. That single difference decides how it is weathered, what it costs and what can go wrong.
How it stays watertight
By a flashing kit: an apron below, soakers interleaved with the covering up each side, and a head flashing above, plus an underlay collar connecting the roofing membrane to the frame.
The kit is specific to the covering it has to lap, and using the wrong profile is the most common defect in this trade.
Roof window, rooflight, skylight
In everyday use the words are interchangeable. In the trade they are not.
Roof window: in the plane of a pitched roof, flashed into the covering, usually opening.
Rooflight: on a kerb above a flat or near flat roof, weathered by the covering being dressed up the upstand.
Skylight: what most homeowners call either of them, and a perfectly good word.
What it needs
A pitch of at least about 15 degrees for a standard unit, an opening that either fits between rafters or is properly trimmed, and the correct flashing kit for the covering.
Detail on the roof window installation page.
What the three words mean in practice
Roof window, rooflight and skylight are used interchangeably and there is no legal distinction between them. In the trade there is a rough convention: roof window tends to mean a unit that opens and sits in the plane of a pitched roof; rooflight is the broader term and is used more for flat roof and fixed units; skylight is the everyday word for all of it.
Building Regulations use rooflight as the general term and treat all of them the same way.
The components that matter
A roof window is a frame, usually laminated timber for stability and on better units clad externally in aluminium so weather never reaches the wood; a sash carrying the glazing; and a sealed glazing unit of two panes with a gas filled cavity, a low emissivity coating and a spacer bar around the perimeter.
Around it sit three external components that are just as important and considerably less visible.
The flashing kit weathers the junction between window and roof covering. It comes in different profiles for slate, plain tile and profiled tile, and it is not interchangeable.
The underlay collar carries water running down the roof membrane around the opening rather than into it.
The insulation collar closes the gap between frame and structure, and its absence shows up as condensation on the plaster reveal months later.
All three are invisible when the job is finished, and they are where most rooflight problems originate.
How they open
Centre pivot. The sash rotates about its middle. Works at any height with a pole and the outer face turns inward for cleaning, which matters on a high level unit. The sash sits in the opening when open, which obstructs the view and frequently disqualifies it as an escape window.
Top hung. Swings outward from a hinge at the top, giving a clear opening and an unobstructed view. Usually what gets specified where a means of escape is required. Cleaning the outer face needs planning on a high unit.
Fixed. No mechanism, so cheaper, with slightly better thermal performance and a slimmer sightline. Suits a position that would never be opened anyway.
Some designs do both, opening top hung for use and pivoting for cleaning, which on a high level window is worth the premium.
What decides whether it works
Not the brand. The installation: the right flashing kit for your covering, the underlay collar lapped correctly, the covering reinstated over the flashing rather than under it, the frame set square in both directions, and the insulation collar fitted.
Get those right and a roof window runs for twenty five years with a gasket clean once a year.
What to ask when you are quoted for one
Which manufacturer, model and size code. Which flashing kit, by name, for your covering. Whether the insulation collar is included. What the whole unit U-value is rather than the centre pane figure. And whether the window carries a means of escape duty in the room it is going into.
Those five questions take a minute and they separate a quotation written from a survey from one written from a price list.
Ask us
If you are replacing an existing unit, photograph the data plate on the sash and we can usually tell you what it is, whether the size is still made and what the options are, without anyone visiting.
How long one lasts
The sealed unit fifteen to twenty five years, the frame and flashings considerably longer, and the actuator on a motorised window ten to fifteen. Fitted properly, the only routine care it asks for is a gasket clean once a year and a filter clean, both of which take minutes.
One last point
The brand matters less than the installation. The right flashing kit, the underlay collar lapped correctly, the covering over the flashing rather than under it, the frame square, and the insulation collar fitted. Get those right and almost any decent unit will run for twenty five years.
The definition
A window installed in the plane of a pitched roof, flashed into the covering, and generally openable. It sits within the roof slope rather than standing on it, and it is the most common form of rooflight in this country.
The term is used loosely against rooflight and skylight. In practice, roof window usually means a pitched roof unit with an opening sash; rooflight usually means a fixed or flat unit; skylight is the general public word for all of them.
How it is built
A frame fixed into a trimmed structural opening, with a sash carrying the sealed unit. Timber or timber with a moisture resistant finish is common, clad externally in aluminium to protect it from the weather.
Operation is centre pivot, which rotates on an axis, or top hung, which swings outward from the head.
What always comes with it
A flashing kit matched to the covering profile, an underlay collar dressed and lapped below it, and ideally an insulation collar filling the gap between frame and timber.
The unit is identified by a size code on a data plate, which describes the structural opening rather than the glass, and that code is what a replacement is ordered against.
More on Roof Window Installation
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