Can you fit a roof lantern yourself?

The honest answer, and where it turns

Assembling the frame is well within the reach of anyone who has built a decent timber structure. The sections are pre-cut, the corner cleats are drilled, and the sequence in the booklet is clear.

What turns the answer is everything that happens before the first section comes out of the box. The lantern is only as good as the flat surface it lands on, and that surface has to be square, level, dry, structurally adequate and weathered up and over the top before the frame is anywhere near it. A homeowner who has already had a builder form a correct kerb is in a very different position from one starting with a deck and a saw.

Squareness, diagonals, and what a frame will not forgive

A lantern frame is manufactured to a fixed rectangle. It has almost no adjustment in it, and the tolerance it will absorb across a 2.5 metre kerb is a few millimetres, not a few centimetres.

Rear extensions are rarely square. A brick return built off a Victorian wall in Gloucester or Tewkesbury commonly runs out by 15 to 25mm over three metres, and a kerb built off that return inherits the error. The frame then either will not close at one corner or is forced closed, which loads the glass unevenly and puts a twist into the ridge. Diagonals get checked before the kerb top is finished, not after the lantern is delivered, and the frame is ordered to the measured kerb rather than to the drawing.

Glass weight, and the assumption buried in the fitting instructions

Roof lantern glass is a laminated inner pane over a toughened outer, typically around 28mm overall, and it weighs roughly 40 kilograms per square metre. A single 1.4m by 0.9m panel is about 50 kilograms of awkward, slippery, expensive rectangle that has to be lifted above head height and placed onto a bar without dropping the leading edge.

The instructions assume two or three people with suction lifters and a working platform at the right height. On a large lantern the ridge panels go on last and highest, and there is no version of that lift that is safe from a ladder. Where a lorry cannot reach the elevation, which is normal in the Stroud valleys and on the shared lanes around Painswick, the glass has to be carried in by hand or lifted mechanically, and that gets planned before the delivery date is agreed.

The bedding compound, and the joint you get one attempt at

The eaves beam sits on the kerb top on a continuous bed, usually a low modulus silicone or a butyl strip depending on what the manufacturer specifies. That bed is a structural and weathering joint at the same time, and it is compressed once when the frame goes down.

Lift the beam to correct a position and the bed is finished: it has to be cut out and remade cleanly. Leave a gap where two runs of bedding meet at a corner and water tracks along the kerb top under the frame, appearing on the ceiling a metre from the lantern and reading as though the glass is at fault. Silicone smeared over the outside of the joint afterwards is cosmetic, because the waterproofing is the covering dressed over the kerb top, not the bead.

Scaffold, edge protection and a rear extension roof

Most lantern kerbs sit two and a half to three metres above a garden, and the work involves carrying glass across a flat roof with a hole in the middle of it. That is a fall risk from the edge and a fall risk through the opening, at the same time.

A handrailed scaffold platform around the working edge, boards over the opening whenever it is not being worked on, and something better than a leaning ladder for access are the baseline. Hiring a small scaffold for a week is a real cost, and it is one people leave out of a self-fit calculation and then work without. On a Stroud valley property where the extension roof is three storeys above the lane at the back, the calculation is different again.

Building Control, and the certificate a buyer’s solicitor asks for

Forming a new opening in a roof is notifiable work. Structure, thermal performance of the glazing and the reveal, ventilation to the room and safety glazing all bear on it, and a lantern over a knocked through wall usually brings a structural calculation with it.

A homeowner can submit a building notice themselves, arrange the inspections and receive the completion certificate. It is genuinely doable and it costs a fee. What causes trouble is skipping it, because the certificate is the document a buyer’s solicitor asks for years later, and the retrospective route is a regularisation application on a finished roof where the kerb, the insulation and the trimming are all buried. Where Severn does the work, the notification is submitted, the inspections are booked and the certificate comes back in the homeowner’s name.

Manufacturers also make the glass and frame warranty conditional on installation to their published instructions, including the specified bedding, fixing centres and minimum pitch. Keep the paperwork and photographs of the kerb before the frame went on.

Where a self-fit comes unstuck on a Gloucestershire roof

Three local situations account for a large share of the ones that go wrong.

A Severn Vale rear addition in Gloucester or Longford where the existing flat roof already ponds. The kerb gets built to 150mm off the deck instead of off the high point of the finished fall, which on a roof falling the wrong way leaves 90mm of effective upstand at the low corner, and water stands against it every winter.

A Cotswold garden room behind a stone parapet, where the covering has to be dressed up both the kerb and the parapet with a gutter between them, and the outlet through the stone is the only way water leaves. Getting the falls right in that gutter is a lead worker’s job.

A concrete tiled hip on an interwar semi in Longlevens or Hucclecote, where a lantern is wanted on a small flat roof between the house and a later extension. The abutment flashing, the tile course above it and the kerb all meet in one corner, and that corner is where the water goes.

Splitting the job between trades

The common middle route is to keep the parts that are visible and reversible and hand over the parts that are buried and not. A homeowner takes the internal reveal, the insulation, the plasterboard and the decoration, which is a satisfying week’s work and a real saving. The structural opening, the kerb, the covering and the frame go to people who do them every week.

Severn fits new lanterns and replaces units that have reached the end of their life, and the ten year workmanship guarantee is written against the four details nobody can inspect once the job is done: upstand height, falls, flashing laps and where the covering terminates. Send photographs of the roof and the room to info@severnskylightsgloucester.co.uk for a range, and see the costs page for how the fixed price follows a survey.