What a VELUX unit will and will not fix

VELUX is a brand name that has become the ordinary word for a thing, the way people say hoover for a vacuum cleaner. When someone says they want “a VELUX”, they almost always mean a roof window: a factory-made, sealed double or triple-glazed unit designed to sit flush in the slope of a pitched roof, between two rafters, with a flashing kit that weathers it into the surrounding tiles or slates.

That last part matters more than people expect. A roof window is engineered for a pitched plane, not a flat one. The standard centre-pivot and top-hung units are rated for roof pitches from about 15 degrees up to 90 degrees. Below 15 degrees they are out of specification, and a genuinely flat roof needs an entirely different product built to shed water off a low deck. So the first question is never which VELUX, it is whether a roof window is even the right shape of answer for your roof. If it is, our VELUX window installation work covers the survey, the structural opening, the flashing and the Building Control notification as one job.

A roof window is also not a roof lantern and not a flat rooflight. Those are separate products with separate geometry, and confusing them is where a lot of quotes go wrong. Keep the picture simple: a VELUX unit is a window that lies in the pitch of a sloping roof.

Daylight is the thing a roof window does better than almost anything else, and it is the reason to fit one. A pane set into a roof slope faces a large slice of open sky, so it collects far more daylight per square metre of glass than the same area of glass in a vertical wall. A loft with a single gable-end window is bright by the window and gloomy everywhere else, because that one vertical pane is fighting the depth of the room. Put two roof windows into the slope and the light falls across the whole floor instead of stopping a metre and a half in.

This is why a roof window earns its place in the darkest parts of a house: a converted loft, a top landing with no external wall, a rear room tucked under a catslide slope where a wall window would look straight into next door’s fence. If your problem is that a room is dim in the daytime and you keep flicking a light on, a roof window will answer that squarely. It is the one promise the product keeps without argument.

The second real benefit is air. Warm, humid and stale air rises and sits at the top of a room, which is exactly where a roof window opens. Cracking one on a summer evening pulls that layer straight out through the highest point in the space, far more effectively than a window at head height ever manages. In a loft bathroom or a kitchen under a slope, that daily clearing of moisture and cooking air does real work against the surface condensation that otherwise streaks the glass and the reveals.

There is a regulatory benefit as well. A habitable room in a loft needs a means of escape, and a correctly sized roof window can serve as that escape under the fire safety guidance in Part B. The clear opening has to meet a minimum: broadly an unobstructed area of at least 0.33 square metres, with no dimension smaller than 450mm, and the opening within a set height range above the floor. Top-hung units, which swing outwards from a top hinge, give a clear escape gap more easily than a centre-pivot, where the sash sits in the middle of the opening. If the window is out of easy reach on a high ceiling, an electric opening unit earns its keep, because a window nobody can reach is a window nobody ventilates.

Southend faces south across the Thames Estuary, which is unusual for an English seaside city and changes what a roof window does once it is in. On a south-facing slope the daylight you asked for arrives with heat and glare you did not. The window itself has no opinion about this. Standard glass lets a large share of the sun’s energy straight through, and on a bright afternoon a south-facing loft bedroom with ordinary glazing can climb to a temperature nobody wants to sleep in, hours after you last saw direct sun.

A roof window will not moderate that on its own. What controls it is the glass specification, specifically the G-value: the proportion of solar energy the glazing lets through. Standard roof-window glass sits around a G-value of 0.5, meaning roughly half the sun’s energy comes into the room. Solar-control glass on the same unit can bring that down towards 0.3 or lower while keeping the room bright, because a good coating blocks the invisible near-infrared heat rather than dimming the visible light. On a south or west-facing slope in this city that difference is the line between a loft you use all summer and one you avoid from lunchtime onwards.

So the honest version is this: a roof window solves your daylight problem, and on a sunny aspect it hands you a heat problem in the same breath. The heat problem is solved by the glass, not the window. Specifying solar-control glazing for south and west slopes, and clear glass for north slopes where every bit of light is welcome, is the part most quotes skip. Ask which way your slope faces before you agree to a single glass specification across the whole roof.

Here is the other half of the honest picture. These are the jobs people quietly hope a roof window will do, and it will not.

It will not give you headroom

A roof window adds light, not height. If you cannot stand up under a slope, glazing that slope changes nothing about the space you can use. Standing headroom in a loft is a question of ridge height and floor level, and the answers to that are a dormer, a raised roof or a different room, not a window. A bright low space is still a low space.

It will not sit in a flat roof

If your rear extension has a flat or very shallow roof, a pitched-roof window is the wrong product, because it needs at least 15 degrees of pitch to weather correctly. That job wants a flat-roof skylight built for a low deck, or a roof lantern raised on an upstand. Trying to force a roof window into a flat structure is where leaks are designed in from the start. If your slope is genuinely pitched, a pitched-roof installation is the right route.

It will not cure damp from another cause

Ventilation through an opening roof window helps with surface condensation, the water that forms on cold glass and cold reveals in a humid room. It does nothing for rising damp, penetrating damp from a failed roof covering, or a slow plumbing leak. Those have their own sources and their own remedies. Adding a window to a room with a damp problem elsewhere just gives you a well-lit damp problem.

It will not warm a cold room

Glass is still the weakest thermal point in any roof. A modern roof window with a good whole-unit U-value around 1.2 W/m²K, or nearer 1.0 on a triple-glazed unit, loses heat far more slowly than an old rooflight, but it still loses heat faster than the insulated roof around it. Putting one into a cold room adds a small area of extra heat loss. A well-specified unit keeps that loss modest, but no window heats a room. If a loft is cold, the answer is insulation and heating, with the glazing chosen to lose as little as the budget allows.

It will not answer a leaking roof

If your roof is letting water in around old coverings, that is a roofer’s job on the slates or tiles, not a window question. Where a genuinely failed old rooflight or roof window is the source, replacing the whole unit with a new sealed one and a correct flashing kit is a replacement, and that is our work. Chasing a leak that runs through the surrounding roof covering is not something a new window addresses.

VELUX units come in a coded range of standard sizes, which is why an installer will reel off letters and numbers rather than centimetres. The code tells you the width band and the height. These are the common ones, with approximate dimensions and where each tends to land. Exact figures vary slightly by model generation, so treat this as the shape of the range rather than a cutting list.

On pitch: a centre-pivot unit, where the sash rotates about a middle hinge, is rated across the full 15 to 90 degree range and suits most loft slopes. A top-hung unit, hinged at the top so the whole sash lifts outwards, is generally offered for pitches around 15 to 55 degrees and gives an unobstructed view and an easier escape opening. Below 15 degrees you are outside roof-window territory and into flat-rooflight territory. Getting the pitch right first narrows the product choice before size ever comes up.

The housing stock around Southend decides how often a roof window is the right tool. The very large belt of 1930s semis across south Essex typically has a pitched main roof over the loft, which is ideal roof-window country: a bedroom or study in the loft, one or two units in the rear slope, escape sizing and glass specification chosen for the aspect. The Victorian and Edwardian terraces and villas along the cliff and the seafront carry steep slate slopes that take roof windows cleanly, and their lofts are often deep enough to convert.

The trap is the rear extension. A great many of those same houses have gained a single-storey rear extension with a flat or shallow roof over a knocked-through kitchen, and a pitched-roof window simply does not belong there. That space wants a flat rooflight or a lantern instead. So on one house you can easily need a roof window in the loft slope and a different product entirely over the kitchen, and a quote that offers the same unit for both has not looked at the roof.

Planning is the other local wrinkle. Roof windows are usually permitted development on a house when they project no more than 150mm beyond the roof plane and sit below the ridge, with obscure glazing and restricted opening where they face a side elevation below 1.7 metres. The exceptions bite here. The Leigh and Leigh Cliff conservation areas carry an Article 4 Direction that removes those permitted-development rights for changes to roofing and windows, so a planning application is needed where it normally would not be. Flats, maisonettes and listed buildings sit outside permitted development too. Treat all of this as general guidance and check your own address with the local authority before ordering anything.

You can settle most of this yourself, at ground level, before anyone quotes. Run through it in order.

1. Is the roof pitched or flat? Below about 15 degrees, a roof window is the wrong product and you want a flat rooflight or a lantern. This decision comes first and rules out half the confusion.
2. Is it a light problem, a heat problem or a space problem? A roof window answers light. Heat is answered by the glass specification, not the window. Space, meaning headroom, is answered by a dormer or a roof alteration, not by glazing at all.
3. Which way does the slope face? North slopes take clear glass and all the daylight you can give them. South and west slopes take solar-control glass, or the room you brighten in the morning becomes the room you abandon in the afternoon.
4. Does the room need a means of escape? A loft bedroom does. That sets a minimum opening size and points you towards a top-hung unit or a taller centre-pivot.
5. Is there damp or a leak first? Sort the cause of that before you glaze anything, or you will simply light up an existing problem.

Answer those five and you will know whether a roof window is your product, and roughly which one, before a surveyor arrives. A roof window is an excellent answer to the right question and a waste of money against the wrong one. If you want the specification worked out properly for your own roof, with the glass chosen slope by slope, you can see indicative figures on our costs page or ask us for a quote that names the size code, the pitch and the G-value in writing.