What U-value should a rooflight have in Wales?

Wales gives a rooflight and a roof window different limits, and the two are easy to swap by mistake. The difference comes from the way each is built into the roof.

Approved Document L describes a rooflight as a glazed unit set out of plane with the roof surface, on a kerb or upstand. The same entry notes that people also call it a skylight, so the two words mean one product here. A roof window is one installed in the same orientation as the surrounding roof, and in plane with it. A flat-roof dome on a kerb is the first kind, and a window lying in a pitched slope is the second.

These numbers are guidance. A new or replacement roof window should meet 1.4, and a rooflight should meet 2.2. They are the same figures that apply in England, though the Welsh document numbers its tables differently, so a table reference copied from an English page will point at the wrong thing.

For a rooflight, the calculation looks at the unit as a whole. The U-value counts the full surface of the unit, which is often larger than the opening it covers in the roof. A kerb that comes sold with the rooflight in one piece sits inside the same 2.2 figure. When someone quotes you a number, find out whether it describes the glass alone or the complete unit, because the two are not the same thing.

The document does not name roof lanterns when it defines a rooflight. So we do not apply the 2.2 figure to a lantern by name. A lantern on an existing flat roof is a case for the survey, and so is one in a new extension, where we confirm the approach with the council.

Older and historic houses get a little leeway. Where a replacement roof window on a home of architectural or historic interest cannot reach 1.4, the document allows a fallback of 1.2 W/m²K measured at the centre of the pane, or low-emissivity secondary glazing. The document offers nothing equivalent for rooflights, which stay at 2.2 whatever the age of the house.

Start by asking which of the two a unit counts as. Then ask for its declared figure, and ask whether that figure covers the glazing, the frame and, for a rooflight, its kerb. A roof window can be shown to comply by either route: a U-value of 1.4 or better, or a WER in band B.

The glazing makes the difference. Double or triple glazing and the coatings on the glass all feed into the rating. The question of whether better glazing costs more has its own answer in does an energy-efficient rooflight cost more. Our guide to rooflight U-values explained shows how the number is built up, and the Welsh Government's Approved Document L page carries the document itself.

Under the Building Regulations a rooflight or roof window is a controlled fitting, which makes installing one building work. A new rooflight will generally need Building Regulations approval, and where the work is notifiable, we handle the Building Regulations notification with local authority building control or a registered building control approver.

Planning is separate. In a conservation area, a National Park, the Gower AONB or a World Heritage Site, a new rooflight in a roof slope needs planning permission. Our skylight planning permission guide covers the rest. To have your own roof looked at, call 01792 929062 or book a free survey.