New rooflights and skylights in Ynys-ddu

Is there a grey haze between the panes of your flat rooflight that no cloth will reach? That is a misted unit. Moisture sits inside the sealed glass, where it cannot be wiped off, and on a bright morning it shows as a milky patch across the ceiling.

There are two ways to put it right. The first is a new sealed unit in the frame already on the roof. The second is a whole new rooflight, frame and glass together. Which one fits turns on how the frame is holding up, and the kerb underneath it, and that is judged from the roof itself, with the glass, the seals and the edges all in view. The comparison of sealed unit or whole rooflight sets out how the choice is made.

Size and shape are measured at the survey, and the replacement is ordered to match. A flat roof's covering is noted too, so that a new kerb or upstand can be dressed into it. If you can send details of what is up there already, any marks on the frame or a label inside the glass help. Rainclear advises a kerb sloped at no less than five degrees for its flat glass rooflights, to minimise water pooling, and the kerb is measured with that kind of advice in mind.

Moisture inside a unit says nothing about how the rest of the roof is doing, which is why the survey looks past the glass. The flashings, the kerb and the ceiling lining beneath are looked at alongside the glass.

Replacing only the glass while keeping the existing frame is not building work for the energy rules. A whole new rooflight is different: Approved Document L (Wales) is guidance, and it says that a replacement rooflight in an existing home should have a U-value no worse than 2.2 W/m²K.

Cleaning is another consideration for a roof you cannot reach. Roof glazing is among the listed uses of Pilkington Activ, a self-cleaning glass whose coating works with daylight and rain to shift dirt.

Where a whole rooflight is replaced, the notification is something we handle with local authority building control or a registered building control approver, where the work is notifiable.

By the first cold week of autumn, an old roof window can run with water on the glass when the room is warm. Water on the inner pane at night is condensation. The remedy can be a replacement in the same opening, which avoids cutting a new hole in the roof. Where a house has more than one roof window, each can be looked at on the same visit, and the ones still working well can stay.

To match the size, open the window and read the small metal label across the top of the sash. VELUX sets each size code against a width and height, for example MK04 at 78 by 98 cm, and every VELUX window made after 1968 carries a sign. Send us the code or a clear photograph of it, and the replacement can be lined up before the survey.

Swapping the whole window for one the same is building work, whereas a repair is not. Changing just the glass, with the frame left in place, is something the energy rules do not count as building work. Our page about VELUX size codes shows how the codes read, and the walkthrough for replacing an old skylight explains the route.

Brightness can be the reason for asking. A replacement that fits the old opening keeps the view and the position, which suits a room that works well already and is let down only by tired glass. Those who want a bigger window are looking at a different job, since a larger opening usually means altering the rafters.

Ventilation counts as part of the change. Approved Document F (Wales) asks that a replaced window leave the home ventilated at least as well as before, and any background vents the old window had should be matched by vents of at least the same size, controllable by the occupant or automatically.

The sash itself is a choice. The glazing options VELUX lists include a dirt-repellent coating, laminated safety glass on the inner pane, and a rain noise reduction build, and which are available varies by window and variant.