Fitting a roof lantern to an existing flat roof

Is your roof a good candidate?

Almost any flat roof that is sound can take a lantern of some size. The questions are which size, and where. The answers come from four things we look at on the survey.

The direction of the joists. Flat roof joists usually span the shortest way across the room, from wall to wall. A lantern set lengthways along the joists cuts through fewer of them than one set across. That can steer the shape of the lantern as much as the room below does.

The depth and condition of the timbers. Older extensions were sometimes built with joists that only just carried the roof as designed. Cutting one or two and passing their load sideways asks more of the joists either side, so we check their size and look for rot at the bearing ends, where water tends to sit if a roof has ever leaked.

The deck. Boards, ply or chipboard can soften if water has got under the covering. A soft deck round the opening is replaced before an upstand is fixed to it.

The covering. Whether it is felt, EPDM rubber or GRP, the new upstand has to be sealed into it. A covering near the end of its life will not take a sound joint, and we would rather tell you that at the survey than find it with the roof open.

Cutting the opening and carrying the load

Each joist that crosses the opening has to be cut. Once it is cut, it no longer carries the roof on its own, so a new timber is fixed across its cut ends. That timber is a trimmer, and it passes the load sideways to the joists either side of the opening, which are usually doubled up and called trimming joists. The result is a framed hole, as strong as the roof was before, with a clear space for the lantern.

How big the trimmers need to be, and how they are fixed, depends on the span, the size of the opening and the weight of the lantern. A large lantern with triple glazing is heavy, and the roof also has to take snow and the occasional person on it for maintenance. On anything beyond a small opening, the sizes come from structural calculations, which also go to Building Control. That is the reason the work is notifiable: it is a change to the structure of the house under Part A of the Building Regulations, not just a new window.

Inside the room, the ceiling is cut back to the line of the new framing before the timbers are touched. The room is sheeted and protected, because cutting a ceiling makes dust and plaster that would otherwise travel through the house. On a kitchen extension this is often the messiest hour of the job, and we plan around worktops, appliances and anything that cannot be moved.

Building the upstand

With the opening framed, the upstand goes on. It is a box of treated timber, fixed down to the trimmers and trimming joists, with its top edge set dead level even though the roof itself falls towards a gutter. That levelling matters. A lantern is a rigid frame, and set on a sloping seat it would twist, strain its glass and open its joints.

Its height comes from the lantern's own installation guide and the depth of the roof build-up, so that the base of the lantern stays clear of water running across the roof. On a roof that sits close beneath a first-floor window or a parapet, there may be a limit on how tall the whole thing can be, which we check before the lantern is ordered.

The upstand is insulated on its inner faces. A warm flat roof has its insulation above the deck; cutting a hole leaves a ring of bare timber where the insulation stops, and that cold edge is where condensation forms on a winter morning. Lining the upstand closes the gap and keeps the reveals inside the room warm.

Joining new work to an old roof covering

This is the step that needs the most judgement on an existing roof. The waterproof layer must climb the kerb's outer face without a break and fold across the top, so the lantern's base frame ends up sitting on it. On a new roof that is laid in one go. On an existing one, new material has to be bonded to old, and each covering does that differently.

Whatever the covering, the old material round the opening is cleaned and prepared before anything is bonded to it, and the joint is lapped so water running down the roof passes over it, never into it. Once the upstand is covered, the trim that caps the top edge goes on, and the base of the lantern seals onto that.

When we survey, we look closely at how much life the covering has left around the area we will open. If it is sound, the joint is local and neat. If it is blistered, split or soft, a new joint will be only as good as the material it is bonded to, and we say so plainly in the quote.

Setting the lantern and finishing inside

A lantern usually arrives as a kit of base frame, bars, ridge, caps and sealed glass units. Smaller ones can be lifted on part-assembled; larger ones are built up on the roof. First the perimeter frame is screwed down onto the kerb. Bars and ridge follow the maker's drawing, then every sealed unit is bedded on its gaskets and clamped by a top cap. End caps close the ridge and the eaves.

Indoors, the short walls that rise from the ceiling to the lantern's base are boarded, taped and left ready to paint. Some owners take the chance to add a ring of downlights round the reveal or a pendant rail inside the lantern, and if the lantern has electric vents the cable runs are planned before the boarding goes up, with a qualified electrician making the connections.

Before we leave, we put water on the roof round the upstand and watch the inside for any sign of it. Then the glass is cleaned, the room is cleared and the Building Control inspection is arranged where it has not already happened.

Getting a lantern onto a Sheffield back roof

Access shapes the job more than people expect. Plenty of extensions round Sheffield sit at the back of terraces and semis on sloping plots, with the garden either well above or well below the street. A lantern's glass units are large and heavy, and they cannot be bent round a tight passage or carried flat through a narrow hall. So the survey includes the route: through a side gate, round a shared ginnel, over a garden wall or, as a last resort, through the house itself with the floors protected.

The roof also has to be reached safely. On a single-storey extension that usually means a tower or a short run of scaffold at the back. Where the only sensible place for scaffold is the pavement at the front, the council licence is part of the planning of the start date. On a steep street we think about where materials will be unloaded and how far they travel uphill or down before they reach the roof.

None of this changes the lantern, but it does change the time and labour, which is why it sits in the quote, in writing, rather than turning up as a surprise on the morning.

Planning, paperwork and the life plan

The Building Regulations side is clear. The new opening is structural work, and the lantern is a rooflight for the energy rules, so the whole unit has to come in at 2.2 W/m²K or lower, the figure that Table 4.2 of Approved Document L (2021) sets for rooflights on existing homes. Notifying Building Control is our job, and the inspection may come from the council or from a private approver registered to do the work.

Planning is less certain. Because its ridge usually rises further than the 0.15 metre projection that permitted development allows a rooflight, a lantern can fall outside those rights; how tall it is and what kind of house it sits on settle the matter. Listed buildings, flats and some homes in conservation areas have their own rules. At the survey we look up the property, measure the proposed height and tell you which route applies before you sign. Our rooflight planning guide sets out each condition.

The quote that follows the free survey is fixed and in writing, and it comes with a life plan. For a lantern on an existing roof, that plan names the lantern and its size, lists the parts that will wear, such as the gaskets, the sealed glass units and any vent motors, and says how each is changed later. It also records the covering we sealed into, and how the upstand joint was made, so anyone who works on the roof later knows what is there.

When an existing roof is not the right place

Now and then the survey shows a reason to pause. A roof with sagging joists or rot at the walls needs that dealt with before any opening is cut. A very shallow roof void may leave little room for the trimmers a large lantern needs. A roof that sits right under a bedroom window can limit the height. In those cases a smaller lantern, a flat rooflight on a low kerb, or two rooflights in place of one can give the room its light with less disturbance. Our hub page on roof lanterns and the short answer on lantern or flat rooflight set out how we weigh those options.