How the three sashes share one frame
The idea is simple: one outer frame, three panes of glass, and narrow mullions instead of tiles and rafters between them.
With ordinary VELUX roof windows, fitting three in a row means three frames, three openings and a combination flashing that leaves a strip of roof between each window. The 3-in-1 takes that apart. The whole unit arrives as one wide frame, split into three sashes by slim uprights, and it goes into one trimmed opening in the roof. From inside the room it reads as a single piece of glazing, closer to a picture window than to a row of skylights.
The usual arrangement has a fixed centre pane with an opening sash on each side, so you get the widest possible view straight ahead and ventilation at either end. Other configurations have been offered, and the options on sizes, operation and finish change as VELUX updates the range, so the exact current line up is something we confirm against the manufacturer's data at survey stage rather than from memory. The opening sashes work like the familiar centre-pivot VELUX: a handle at the top, the sash turning on its middle to open, and the ability to rotate it right round for cleaning from inside. Electric operation through the INTEGRA system is available on the opening sashes, which our guide to VELUX INTEGRA electric and solar windows explains.
What the room gets from a wide band of glass
Width across the slope changes how light enters a loft: instead of one bright rectangle, you get a long, even strip of sky that spreads daylight along the room.
A single tall roof window throws a concentrated shaft of light that lands in one place on the floor and moves round through the day. Spreading the same glass area sideways widens that sun patch and softens the contrast at the edges of the room, which is often the difference between a loft that feels like a bright spot with dim corners and one that feels evenly lit. Sitting or standing, you also see a horizon rather than a slot of sky, which matters in a bedroom or a study where people spend long hours.
The same width cuts both ways on a sunny slope. On a south facing roof, a 3-in-1 puts a lot of glass under the high summer sun, and a Cambridge loft under a tiled or slated roof can already run warm in July. Solar protection glazing and external awning blinds for each sash are the main controls; our answer on whether VELUX glass reduces heat from the sun compares them. On a north slope the width is almost all gain: steady, cool light across the whole room with no direct sun to manage. East and west slopes sit between, with low morning or evening sun that can glare straight into the room, so blinds that can be set sash by sash earn their place. We work through orientation and placement for every window in the written light plan, and how we plan the light sets out the method.
The roof it needs
A 3-in-1 is a wide window, so it asks more of the roof structure than a single roof window and it has to fit the width you actually have between obstacles.
Because the unit spans several rafter bays, more than one rafter is normally cut to form the opening. The load those rafters carried is taken by doubled trimming rafters at each side and trimmers across the head and sill, sized for the span. Cutting several rafters is structural work, and Building Control may ask for the timber sizes to be checked by calculation. Fitting any new roof window is normally notifiable, and we handle the Building Control notification where the work requires it. The trimming principle is the same one set out in our answer on whether you can fit a bigger VELUX window, just across a wider opening.
Width is often the limit on older Cambridge houses. On a gault brick terrace in Romsey or off Mill Road, the fire guidance in the Building Regulations generally keeps rooflights out of a strip about 1500mm either side of the party wall, and chimney stacks, purlins and hips take more space again. A 1930s semi on a hipped roof may have a generous rear slope; a narrow terrace may not have room for three sashes at all. Newer homes at Eddington or Trumpington Meadows are usually built with trussed rafters, which should not be cut without the designer's input.
The rest of the build follows the normal VELUX pattern: a flashing kit made for the 3-in-1 frame and matched to the roof covering (tile, slate or a flat profile, as VELUX flashing types explained sets out), an insulation collar and vapour barrier around the frame, and a plastered lining with the sill reveal set plumb and the head reveal level to spread the light downward. The unit is heavy and long, so it is lifted and set by more than one fitter.
3-in-1 or separate windows side by side
Three single windows in a combination flashing give a similar amount of glass, but the room sees three pictures rather than one.
| Point | 3-in-1 | Separate windows |
|---|---|---|
| Between panes | Slim mullion | Roof strip and two frames |
| Rafters | One wide trimmed opening | Can often sit between rafters |
| Flashing | One kit for the unit | Combination kit |
| Future changes | Replaced as one unit | Each window on its own |
Separate windows can suit a roof where cutting several rafters would be awkward, or where sizes need to vary across the slope. Our answer on adding a second VELUX next to an existing one covers that route. Where the view and an uninterrupted band of light are the point, the 3-in-1 is the stronger choice. Planning is the same either way: a roof window that stays within 150mm of the roof plane is normally permitted development, with the exceptions covered in do you need planning permission for VELUX windows.
Questions people also ask
Can I replace an existing VELUX with a 3-in-1?
Yes, but it is a new, wider opening rather than a swap. The old window, flashing and lining come out, more rafters are trimmed and the tiling and plaster are rebuilt around the new frame. See replacing an old VELUX window for what changes on any replacement.
Is a VELUX 3-in-1 suitable for a loft conversion?
It is one of the best fits for a loft room with a long, uninterrupted slope, especially facing a garden or an open view. It suits less well where the usable width is tight between party walls, chimneys or hips.
Does the 3-in-1 come with electric opening?
The opening sashes can be manual or electric through INTEGRA, depending on the model chosen. The centre pane, where fixed, does not open.
Where can I find out if my roof will take one?
A survey measures the rafter spacing, the clear width and the structure, then the written light plan shows how the window would light the room. Read more about VELUX window installation in Cambridge, or book a light survey.
