Velux Loft Windows

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Which Velux Window for a Loft Conversion, GZL or GLU?

Both ranges fit the same openings, use the same double glazing at a Uw of 1.3 W/m²K, and carry the same 10-year Velux guarantee. The choice comes down to what the room is going to be.

The GZL 1051 has a clear-lacquered pine interior. It suits a loft bedroom, a study or a living space, anywhere that stays dry. Prices start at £235.

The GLU 0051 has a white polyurethane finish over the timber core. It wipes clean and copes with condensation, which matters if the conversion includes an en-suite or the window sits above a shower. It also has a laminated inner pane. Laminated glass holds together when broken, so it counts as safety glazing, and that is the one to fit above a stairwell or anywhere out of easy reach. Prices start at £300.

For most loft conversions the GZL is the sensible pick. Add an en-suite and you want a GLU over the wet area, even if the bedroom windows are GZLs.

What Size Velux Do I Need for a Loft Conversion?

Two things decide the size: your rafters, and how much daylight you want. Rafters come first, because they are structural and the daylight calculation is flexible.

Measure the clear gap between your rafters before you settle on a width. A window that fits inside an existing gap needs no structural work at all. Strip the covering, cut the battens, fit the frame. Choose one wider than the gap and a rafter has to be cut, trimmed and doubled either side. That is structural work under Building Regulations, it needs building control involvement, and it adds a day or more to the fit. That single decision changes the cost of the installation more than the price difference between two window sizes does, so it is the first measurement to take.

For daylight, a reasonable rule of thumb is glazing of around 10 to 15% of the room's floor area. A loft room of 3m by 4m is 12 square metres, so you are looking for roughly 1.2 to 1.8 square metres of glass. That is one large window or two smaller ones, and on a long loft two windows spread along the slope light the room far more evenly than a single big one in the middle.

78x98 and 78x118 are the two sizes we sell most for loft conversions. They light a decent-sized room properly, and they are common enough that any roofer will have fitted dozens of them.

Velux loft window sizes we stock

Size (cm)	GZL 1051 (pine)	GLU 0051 (white)	Typical use
55x78	Yes	Yes	Small loft room, landing or en-suite. Our narrowest frame.
55x98	Yes	Yes	Same width, more glass. Useful on a shallow pitch.
78x98	Yes	Yes	The standard loft bedroom window.
78x118	Yes	Yes	Larger bedroom or open-plan loft.
78x140	Yes	Yes	Full-height glazing, best on a steeper pitch.
114x118	Yes	-	The widest we stock. Plan for rafter work.

Building Regulations for Loft Conversion Windows

This is where loft conversions catch people out. Settle it before you order anything.

The mistake we see most often is assuming a roof window handles the fire escape route on its own. Whether it does depends on the finished house, and in particular on how many storeys it ends up with. A conversion that adds a storey is treated differently from one in a lower building, and in some cases the escape route has to be the staircase itself, properly enclosed and fire-rated, with the roof window contributing nothing to it. Your building control officer will tell you which applies to your project. Ask before you order, because the answer changes what you need to buy.

Where an escape window is required, it has to give a clear opening large enough to climb out of, and building control will confirm the minimum area and dimensions that apply. Get those figures first and then pick the window, rather than the other way round. It is a short conversation and it avoids buying a window that turns out to be too small to sign off.

Thermal performance and ventilation come into it too. Both our ranges are double glazed at a Uw of 1.3 W/m²K, which is a low figure for a standard roof window, and both have ventilation positions built into the top control bar. Your builder or building control officer will confirm what the room needs overall, since ventilation and insulation are assessed across the whole conversion rather than window by window.

If you do need a dedicated escape window, we stock the Velux VLT Access Escape Roof Window in 45x73. It is side-hung and swings fully open to one side, which is why it works as an exit where a centre-pivot does not.

Where to Position a Velux in a Loft

In a loft you are fitting into a slope, so the position on that slope decides what you can see out of it and how much headroom you keep.

Set the window low and you get a view sitting down but lose it standing up, and the open sash ends up at head height. Set it high and the room feels lighter but you are looking at sky rather than out. Most people want both, which usually means the bottom of the frame at around seated eye level and the top clear of anyone standing under it. On a shallow pitch the window has to sit further up the slope to reach the same head height, which is one reason a taller window often works better on a low-pitch roof.

The control bar on both ranges sits at the top of the frame, so a bed or a desk can go directly underneath and you can still open the window. One thing to raise with your builder before the opening is cut: if the cill ends up low, guarding may be needed in front of it. Easier to plan for than to retrofit.

Roof Pitch and Flashing

Both the GZL 1051 and the GLU 0051 fit roof pitches from 15° to 90°, which covers effectively every pitched roof in the UK. Below 15° you are into flat roof territory and a standard centre-pivot will not work.

The flashing is what makes the window watertight, and it has to match your roof covering rather than the window. A tile flashing for profiled coverings such as concrete interlocking tiles or clay pantiles, a slate flashing for flat coverings such as slate or plain tile. Get this wrong and the window will leak no matter how well the frame is fitted. A badly chosen or badly fitted flashing is behind more roof window problems than the windows themselves.

We include a free flashing with many of our window purchases. If you are not certain which one you need, our roof window flashings range covers all the common roof coverings, and we are happy to check against a photo of your roof.

Blackout Blinds for Loft Bedrooms

A loft bedroom takes sun from directly overhead, which is a different problem to a vertical window. A south-facing Velux will have you awake at five in the morning through the summer, and it will push the room temperature up through the afternoon.

A blackout blind fitted inside the frame deals with both. Order it at the same time as the window. Blinds are sized to the window code rather than the opening, and buying them together avoids the usual mistake of measuring the hole in the roof. Our roof window blinds range covers the sizes we stock.