Can you swap an old leaking VELUX for a new one?

A damp patch under a roof window does not name its own cause. The glass unit, the frame, the flashing that links the frame to the slates or tiles, and the covering beside the opening are all candidates. Water can also appear on the inside of a roof window. Moisture in warm indoor air settles on whatever is coldest, and in winter that is the glass. In a bathroom or a loft bedroom, beads that gather and trickle can pass for a leak.

The way to tell the two apart is to look at the window and the roof together, from inside and out. Staining around the edges of the opening can point at the flashing or the covering. Fogged glass with a dry frame can point at condensation, and that has its own answer in how the room breathes. A swap makes sense only once the survey has settled which one you have.

Size starts with the type sign, which is a small tag of metal on the head of the sash, in view once the window is propped open. VELUX states that every window it has made since 1968 has one. Read the code off that plate and you have the window's width and height. In VELUX's own table, MK04 is 78 by 98 cm, MK06 is 78 by 118 cm and UK04 is 134 by 98 cm. To decode yours, start with our explainer on VELUX size codes.

With the code in hand, a replacement for the same opening can be picked out, and the roof around the hole is disturbed as little as the survey allows. The flashing is chosen to suit the covering it meets. A like-for-like change from one VELUX to another is set out on our like-for-like VELUX swap page.

A replacement is the one moment the glazing is open for decision. VELUX glazing can include a coating that repels dirt, laminate on the inner pane and rain noise reduction, depending on the window and the variant. For a landing or a bathroom that is hard to reach, an electric version is another route. VELUX says the solar and electric roof windows it sells carry a rain sensor that shuts the sash once rain arrives.

The room steers these choices. A loft bedroom under the slope hears more of a storm than a stairwell does, and a bathroom is where the fogging starts. We talk through the room at the survey and fit what suits it.

Under the Building Regulations a roof window fitted counts as building work, because windows and roof windows are controlled fittings. Welsh guidance treats the jobs differently, as the table shows. A roof window going into an existing home should reach 1.4 W/m²K, or Window Energy Rating band B, under Approved Document L (Wales).

Ventilation counts too. A window with background vents should be replaced by one whose vents are at least as big, working either by hand or automatically. Where a newly built house would need a way out in that position, and the existing window was big enough to climb through, the replacement should give at least the same escape potential.

A listed house brings one more step. In Wales, work that affects a listed building's character as a building of special architectural or historic interest needs listed building consent, which you apply for through the local planning authority. Planning permission has its own rules for roof windows, which our guide to roof windows and planning permission in Wales explains.

Where the work is notifiable, we handle the Building Regulations notification with local authority building control or a registered building control approver. A written 10-year workmanship guarantee comes with the finished swap. Book the free survey on 01792 929062, or use the request form. The VELUX windows page covers the wider range.