Mountain Ash rooflights and skylights

Wipe the inside of a bedroom roof window on a cold morning and the cloth comes away wet. By the time you wake, the night's breathing has landed on the nearest cold surface, which in a loft room is the glass.

What to do about it is a question for the whole room. A window that opens lets the moist air out. A sash that can be left slightly ajar through the night can help. Ventilation guidance counts here too. Approved Document F (Wales) says a home's ventilation should end up no worse than before once a window is swapped, so tell us whether the frame you have now carries a vent along its upper edge.

The page on energy efficient rooflights covers the glazing choices, and Approved Document L (Wales) gives 1.4 W/m²K, or Window Energy Rating Band B, as the figure a roof window should meet. For a rooflight on a kerb, the figure is 2.2 W/m²K.

The blind comes into it as well. A blackout blind that sits close to the glass keeps morning sun out of the room, and it also makes a dark ceiling at night in place of a black pane showing the lamp. Whether to choose a blind for darkness or for warmth is a question for the survey, with the room in front of us.

If you are replacing an existing window, the swap is a smaller job than a new opening, and our answer about swapping an old skylight shows what is involved. A fresh opening needs more, because the timbers of the roof usually need altering and the work counts as building work. Where it is notifiable, the notification to local authority building control or a registered building control approver is ours to handle.

Last, think about when the water appears. Condensation that clears by ten in the morning is a different matter from glass that stays wet all day. Telling us which it is at the survey helps us understand how much moisture the room produces and how the air moves through it.

Bedrooms ask more of a window than other rooms do, because they are used when the rest of the house is quiet. A window that is easy to open without climbing on the bed, a blind that darkens the room fully, and glass that does not drip on the pillow are three simple tests. Each can be checked at the survey, standing where the bed will be.

A walk-on rooflight is glass you can stand on. It is set in a flat roof so that daylight reaches the room below, and the same pane forms a small patch of floor above. The walk-on rooflights hub sets out the product, and the explainer on load ratings for walk-on glass deals with the numbers.

What happens to it after dark is simple. The pane goes black, and whatever room is lit shows through it. If the room below is lit, the glass reads as a light square in the floor above; if the room below is dark, it reads as a black one. Neither is a fault, and both are worth picturing when the position is chosen.

The position matters most. A pane can go over a room that otherwise gets little daylight, and the route people take across the roof above shapes where it belongs. Furniture below matters as well, because a pane directly over a bed or a desk feels different from one over a passage.

Building control confirms whether a given rule applies to a particular walk-on installation, and that conversation comes before the order. We talk it through at the survey.

The build-up around the pane is part of the survey, and so is the structure beneath it.

Two questions settle most of the choice: who will be walking on it, and how often. A pane over a store room that nobody visits is a different proposition from one beside a kitchen door.

If the pane is over a room people use at night, ask how it looks with the light on. A lit room beneath shows through the glass, and in a hall that can mean a bright square seen from the upper floor. Some households like that, and some prefer to keep the lights low or choose a spot away from the bedroom door. Neither choice is wrong, and the position can be changed on paper far more easily than on the roof.