Can a flat rooflight be replaced without redoing the roof?

A flat rooflight sits on a kerb, and the kerb is wrapped in the roof covering. Replacing the rooflight means lifting the old unit and fitting a new one. The other two parts stay if they are in good order.

That is why the answer can be a smaller job than people fear. Where the opening is the right size, the structure and the lining below are left alone, and the work is concentrated at the top. A kitchen extension with a rooflight over the worktop is a good example: the room below stays as it is while the unit above changes.

Where the kerb and the covering are sound, the work is the unit alone: the old rooflight comes off and a new one, matched to the opening, goes on. The survey is where the kerb and the covering are checked, along with the size of the opening they surround.

The old kerb is looked at for its size, its fall and its condition. The new unit has to match the opening, and products can be made to measure where the old size is no longer made.

The fall matters because water has to run off. Rainclear's advice for its flat glass rooflights is a kerb slope of no less than 5 degrees, to reduce pooling. VELUX says its flat roof dome suits roofs pitched between 0 and 15 degrees. Those are the makers' figures for their own products, and the survey compares them with what your roof offers.

If the kerb has moved, or the covering has worn at its foot, that part is reworked along with the swap.

Size is the last check. A replacement the same size as the old unit goes back into the same opening. A different size changes the opening, and that moves the job from a swap to a small alteration of the roof structure.

The covering is sealed up the kerb, so lifting the unit shows how well that seal has lasted. A sound covering can stay. A tired one around the opening is dealt with while the access is there.

Inside, the lining of the opening and the ceiling edge are checked. A like-for-like replacement leaves them in place, and a bigger unit means they are altered and made good.

If the old unit has misted, our page on sealed unit or whole rooflight sets out the choice between a new glazed unit and a new rooflight.

Getting the old unit down and the new one up matters on a larger rooflight. Access is looked at during the survey, from the ground and from the roof, including how a heavy unit reaches the roof.

Approved Document L (Wales) is guidance. It says a new or replacement rooflight in an existing home should have a U-value no worse than 2.2 W/m²K. That figure is measured over the unit's whole developed surface, and that surface is often larger than the hole it sits over. Where the kerb comes as one assembly with the rooflight, the 2.2 figure covers the kerb as well. In Wales the energy rules do not treat a glass-only change, with the existing frame kept, as building work, which matters if the old unit has simply misted.

Where the work is notifiable, we handle the Building Regulations notification with local authority building control or a registered building control approver. Our flat rooflight Building Regulations guide goes into it, and the Part L guide covers the energy side.

Whether a replacement is permitted development turns on how far it stands proud of the original roof slope and on where the house is, and a listed house or a house on protected land has its own rules. Our answer on planning for flat rooflights goes through them.

One thing changes at night. After dark the glass turns black and gives back a reflection of the lit kitchen below, table and all, at supper. To book the visit, use the survey form. The 10-year workmanship guarantee is given in writing.

Photos sent to WhatsApp at 07880 284133 help us prepare for the visit, and they show the room as well as the roof. The price is set after the survey.