Roof lanterns in Swansea

Ask where the water goes once it leaves the glass, and the answer is the upstand. This is the short wall that rises above the flat roof and carries the lantern frame. The roof covering is dressed up its sides, so rain coming off the panes meets a weathered edge and runs away across the roof. The junction between that upstand and the roof around it is what keeps the room below dry in a storm.

Manufacturers set out how they want the upstand built. Brett Martin's installation guidance, as one example, asks the builder for an upstand no lower than 150 mm and recommends setting it dead level. The lantern itself will tolerate as much as 5° of pitch, but the firm cautions that it is likely to look lopsided. A level upstand with the lantern sitting square on it lets the pitched glass do the draining, which is why the roof around the upstand needs its own falls to carry water on to a gutter or outlet.

Measuring the opening

Where a lantern is made to measure, the dimensions that count are the upstand's finished ones. A lantern sized to the inside of the opening and one sized to the outer edge of the kerb are different products, and the frame has to suit the upstand it will actually sit on. On an existing extension, an upstand may already be there from an old lantern or a flat rooflight. Whether it is level, sound and the right height decides whether it can stay.

Roof structure and trimming

A new lantern needs a new hole, and the roof has to be built to take it. Cutting an opening usually means altering the roof structure: the joists that run through the opening are cut back and their ends picked up by new timbers fixed across each side, a job known as trimming. The roof must also be able to carry the weight of the lantern, or be strengthened so it can. The Welsh Government gives both as reasons why approval under the Building Regulations is generally needed when a new rooflight goes in.

On a new extension the opening can be framed in as the roof goes on, which is tidier than cutting into a finished roof later. Where the flat roof already exists, the survey looks at which way the joists run, the depth of the roof build-up and where the water drains, because those three things decide where a lantern can sit and how large it can be. Rafters on a pitched roof are a separate matter; a lantern only ever goes on the flat.

A replacement lantern on a sound upstand is a simpler job. The old frame comes off, the upstand is checked, and the new lantern goes onto the same footprint. If the old upstand is out of level, too low or past its best, a new one is built before the glass goes on. Either way, the new frame is ordered to the finished size of the upstand it will sit on.

Because a lantern stands well clear of the roof, planning comes first. In Wales, a rooflight or roof window can be permitted development if it sticks out no more than 15 cm (150 mm) from the slope of the original roof, measured at right angles to the roof surface, and it must not rise above the highest part of the original roof. A lantern typically rises well over 15 cm above the roof surface, so putting one on an existing flat roof will normally take it outside permitted development, and planning permission is then needed. We confirm this with the council at survey. For a lantern built as part of a new extension, we confirm the position with the council for that particular job.

Protected places and listed houses

The Welsh rules go further in some places. Adding a rooflight to a roof slope of a house in a conservation area, a National Park such as Bannau Brycheiniog, an Area of Outstanding Natural Beauty such as Gower, or a World Heritage Site is not permitted development in Wales, so it needs a planning application. On a listed house, any alteration that would affect its character as a building of special architectural or historic interest needs listed building consent under the Historic Environment (Wales) Act 2023. Consent is applied for through the local planning authority, and Cadw explains the process. Doing such work without consent can be a criminal offence.

A flat or a maisonette has no householder permitted development rights at all; the allowances apply to houses alone. Even on a house, a council can withdraw them with an Article 4 direction, and they can also be taken away by a condition attached when the house was first granted planning permission. Where the position is unclear, the council's planners can say, and a Lawful Development Certificate settles it formally. Our guide to roof lanterns and planning permission in Wales takes each case in turn, and the wider planning guide covers other rooflights.

Building Regulations

Fitting a rooflight counts as building work under the Building Regulations, because rooflights are controlled fittings. For a new rooflight, the Welsh Government says approval will generally be needed: forming the opening usually means altering the roof structure, the roof has to bear the unit's weight or be strengthened, the rooflight has to be energy efficient, and close to a boundary its fire performance comes into it. A new lantern on a new opening raises every one of those questions.

A replacement on the existing opening is still building work. In Wales, a replacement roof window or rooflight in an existing home can be self-certified by an installer registered with a Welsh Government-authorised competent person scheme, who notifies the local authority and issues a certificate of compliance. Swapping just the glass and leaving the frame in place does not count as building work under the energy rules.

Where the work is notifiable, we handle the Building Regulations notification with local authority building control or a registered building control approver. Our Building Regulations guide for Wales has the detail on both routes. You are free to choose which kind of building control body oversees the work.