How to fit a Velux roof window

Video by VELUX Installation & Advice. This guide draws on the official installation video “How to fit a VELUX window. VELUX window installation. VELUX top hung window & flashing kit install.” from the VELUX Installation & Advice channel. It covers the complete sequence for a top-hung window on a tiled roof, and the flashing kit stages are particularly well explained — worth a full watch before you open the box.

1. Measure the opening and order the right window

Velux roof windows are sold by their frame size, not their glass size, and the model codes matter. A GGL is a centre-pivot window; a GGU is top-hung. Both come in a long list of standard sizes measured in centimetres — the first number is width, the second is height.

Measure between your two flanking rafters for the width, and plan the head and sill positions carefully. The window needs enough height to be usable, and the head should sit no lower than about 1.8 m from the floor below to give proper headroom. That said, the planning phase is when to have these conversations — not once the tiles are off.

2. Prepare the roof opening

Remove the tiles in the area of the opening, working outward to give yourself room to manoeuvre. Save undamaged tiles for reinstatement later. Cut and fit trimmer joists between the two flanking rafters if the window opening spans a rafter that needs removing — this is structural work, and it needs to be done properly.

Once the opening is clear, mark and cut the roofing felt neatly. Turn it back over the trimmer rafters so water cannot track underneath once the window is in. Tidy work here makes the flashing stage much easier.

3. Fix the installation brackets to the rafters

Velux windows are hung on a pair of L-shaped installation brackets screwed to the side rafters. The brackets slide along a slot in the window frame, which is how you adjust the reveal depth to match your roof thickness.

The installation instructions give the exact positioning for each window size — follow them. Get this wrong and the sash will either hit the frame or sit proud, and you will not be able to adjust it out later. Screw the brackets securely; the window frame hangs off them and they need to hold its full weight.

4. Set the window frame in the opening

Lift the frame into position and rest it on the brackets. At this stage it is not fixed permanently — you are just checking the fit and making sure it is square in the opening. Check the reveal depth on both sides matches, and that the frame sits flush with the outer face of the rafters.

Make any adjustments now, while nothing is locked off. Once the window is square and level in both directions, tighten the bracket screws to fix it firmly in place. The frame should feel solid with no movement.

5. Fit the underfelt collar

The underfelt collar seals the gap between the roofing felt and the window frame. It stops wind-driven water and draught getting in around the sides. Staple it carefully to the rafters and to the window frame, overlapping the existing felt as the instructions show.

This is one of the less glamorous parts of the job but it is genuinely important. Water will find any gap you leave. Take time to get it flat and sealed on all four sides.

6. Apply the tile flashing kit

The flashing kit is what keeps rain out around the window frame. For a tiled roof, this typically includes the bottom apron, two side flashings, and a top flashing section. They interlock in a set sequence — always follow the Velux installation guide for your specific flashing code (EDW for interlocking tiles, BDX for flat or low-pitch tiles, and so on).

Work from the bottom up. The bottom apron goes in first, then the side flashings are tucked under the tiles and over the bottom apron. The top flashing slots under the tiles above the window. Every overlap sheds water outward. Refit the tiles around the window, making sure each one is properly bedded and not rocking on the flashing edges.

7. Hang the window sash and test the operation

Most Velux windows are supplied with the sash removed for easier installation. Once the frame is fully flashed and the tiles are back, lift the sash onto its pivots or hinges and click it into place. Open and close it a few times to check it moves freely without binding.

Check that the locking handle engages properly and that the weather seals sit against the frame all the way round when closed. Any daylight showing when it is shut needs addressing — usually a hinge or pivot adjustment will sort it.

8. Finish the internal lining

Inside, fit the reveal lining kit that comes with or alongside the window. This trims out the opening between the window frame and the surrounding plasterboard or sloping ceiling, giving it a neat finished appearance. Velux sell their own linings in a range of finishes, and they are designed to fit the frame exactly.

If you are finishing in plasterboard, allow time for this to be taped, jointed, and painted before the room is back in use. The window will look a great deal better once the lining is on — it is worth not rushing the last ten per cent.

When to call a handyman

A Velux installation requires safe roof access, confidence working at height, and comfort with roofing felt and tile work. If any of those give you pause, call Richard. It is also worth getting a professional eye on the trimmer joists if you are removing a structural rafter — that is not a cut to make on a hunch. And if the flashing sequence is anything other than standard, a roofer or experienced handyman will save you a lot of trouble.