Skylights and rooflights across Llantrisant

At a quarter to eight on a January morning the sky has barely begun to lighten, and a bathroom under the roof still depends on its light fitting. When a roof window is there, it is the first thing to catch the day and the last thing to hold the night.

Steam is the working condition of the room. A shower fills the air with moisture, and cold glass is often where it condenses first. A window that opens a little, with an extractor that keeps running afterwards, can deal with much of it.

Privacy is the second question. A roof window looks at the sky, so by day it is rarely overlooked. Where roofs stand at different heights, one can be in view of an upstairs window opposite. With the light on and the sky dark outside, the same pane turns into a mirror, so privacy is about what you can see of yourself as much as what neighbours can see of you. Stand at the planned position during the survey and look around. A blind usually settles it, and a blind for a damp room is picked from the maker's own information.

Reach matters in a bathroom. A window above the tub or at the back of the basin is awkward to open by hand. Motorised and solar roof windows can be operated by a switch fixed to the wall or by a handset. VELUX builds a rain sensor into the solar and electric models of its roof windows, and says it shuts the sash when rain falls.

Approved Document F (Wales) covers the replacement case. If the old window had no vents and natural ventilation is all the house has, fitting vents of at least 4,000 mm² equivalent area in a bathroom is one way to show that ventilation is no worse than before. Where that size is not technically feasible, the vents should be as near to it as can be managed.

A new roof window means a new opening, and where the work is notifiable we make the Building Regulations notification through local authority building control or a registered building control approver.

Is there a table under the dark part of your extension? A dining room added to the back of a house can end up with its windows on one side and its table on the other, and the daylight falls short of where people actually sit.

A roof lantern fixes that, because it brings light down from above onto the table. Its glass is pitched in the shape of a miniature roof, and it stands up from the flat roof on an upstand that is built to the maker's details. A flat rooflight does the same job with a lower profile. The guide to lantern or flat rooflight weighs them against each other, and the page on planning and building regulations for lanterns deals with the consents.

Late in the evening the glass above the table shows what is below it. With the pendant lit and the table laid, the pane mirrors both. A well-finished reveal looks like a quiet, dark frame; a rough one looks like a gap. The reveal and the lining are therefore best planned ahead of the plasterer.

A light in the opening is a choice to make early. One in the upstand or the reveal has to be thought about before the lining goes up, since its cable runs inside the lining, and an electrician does that wiring.

Planning needs a clear head. Putting a lantern onto a flat roof that is already there would normally sit outside permitted development, which means it needs planning permission; our Welsh planning guide shows where the line is. For a lantern that belongs to a new extension, we ask the council what applies. Brett Martin, which makes roof lanterns, recommends building the upstand level. A little pitch is tolerated, up to 5 degrees, but the company says the lantern will probably look lopsided. A lantern also needs more height than a flat rooflight, so the roof is measured before the choice is made. The same visit shows how the glass would look from the garden, which matters when the dining room is the one that neighbours and visitors see from outside.