How a roof lantern is fitted to a flat roof upstand

A flat roof is never truly flat. It is laid to a slight fall, usually somewhere between 1:80 and 1:40, so that rainwater runs off towards an outlet rather than ponding in the middle. That means water is moving across the surface, and at the edges of any opening it will try to find a way in. The upstand is the answer. It is a raised timber kerb, framed around the hole in the roof, that lifts the base of the lantern clear of the water running over the deck.

The governing number is 150mm. The top of the upstand should stand at least 150mm above the finished roof covering, so the glazing sits above the level any water or lying snow is going to reach. On a lantern this height is not optional detailing, it is the line between a dry room and a damp one. Some proprietary aluminium lanterns are supplied with a low-profile base that assumes you have already built a structural kerb to that height beneath it. Others are designed to be bedded straight onto a builder’s upstand. Either way the 150mm has to exist somewhere in the build-up, whether it is all timber or part timber and part frame.

The upstand also does a second job. It carries the weight of the lantern and everything the lantern catches: the glass, the wind load pushing down and lifting up, the person who will one day stand on the roof to clean it. That load has to travel through the kerb and into the roof structure, which is why the opening is not simply cut, it is framed.

Before any kerb is built, the hole has to be made properly. A flat roof is a deck of joists, and cutting through them removes some of what was holding the roof up. The loads those cut joists were carrying have to be redirected around the opening.

The joists that stop short at the edge of the hole are trimmed, meaning they are cut back and their ends supported on a trimmer: a doubled-up joist running across the opening, fixed to the two full joists on either side. On a larger lantern the trimmers themselves are doubled or upgraded, because they are now carrying the ends of several cut joists plus a share of the lantern load. This is structural work. On anything beyond a modest opening, or where the roof spans are already working hard, the sizing wants checking against the span and the load, and a structural engineer’s note is cheap insurance on a big flat-roof lantern over a knocked-through kitchen.

South Essex has an enormous stock of 1930s semis with single-storey rear additions, and the flat or near-flat roof over that back room is exactly where most lanterns in this area end up. Those roofs were often built with modest joist depths for their span, so opening one up for a lantern frequently means the trimmers do real work rather than token work. It is worth knowing that before the first cut, not after.

With the structural opening framed, the upstand is built up around it. In timber this is typically a studwork box: framing timber to form the walls of the kerb, faced on the outside with exterior-grade plywood, and rising to that 150mm minimum above the finished roof surface. The corners are the weak point, so they are properly jointed and fixed rather than just butted, because a lantern base pushing and pulling in the wind will find a lazy corner.

Two details separate a good kerb from a merely adequate one. The first is insulation. The kerb is a wall sticking up out of your warm roof into the cold outside air, so if it is left as bare timber and ply it becomes a cold bridge: a ring of chilly surface around the opening where warm indoor air condenses. On a heated room below, the kerb is insulated on the inside face or built with an insulated core so that the warm line runs continuously up the kerb and the inner reveal stays warm enough to keep condensation off. The second detail is squareness. The lantern is a rigid, factory-made frame. If the kerb it lands on is out of square or twisted, the frame will not sit flush, the glazing bars will not line up, and you will be chasing gaps. The top of the kerb is made flat, level in both directions and square to itself, and that is checked with a level and a diagonal measurement before anything goes on top of it.

This is the step people assume the lantern handles, and it does not. The waterproofing of the upstand is part of the flat roof covering, not part of the lantern, and it goes on first.

Whatever the flat roof is finished in, the covering is dressed up the outside face of the kerb and over the top of it. On a single-ply or EPDM rubber roof the membrane is taken up the upstand walls and lapped over the top edge. On a GRP fibreglass roof the laminate is carried up and over. On a felt roof the layers are dressed up and a capping detail formed. The principle is the same in every material: the water running across the deck meets the kerb, is turned upwards by it, and is stopped by a covering that continues without a joint from the flat surface, up the wall of the kerb, and over its top. The lantern then sits on top of that already-weathertight upstand. If the lantern’s own seals ever fail, the upstand underneath is still keeping the water out. That is belt and braces, and it is how a flat-roof lantern is supposed to be built.

Only once the upstand is covered and watertight does the lantern come into the picture. If someone is bedding a lantern straight onto bare plywood, the job is being done in the wrong order.

Now the lantern goes on. A modern aluminium roof lantern arrives as a base frame, a set of eaves beams, hip bars that rise to a ridge, and the ridge itself, along with glazing caps and gaskets. Timber lanterns follow the same logic in a different material. The base frame is the part that meets your upstand, and the sequence runs roughly like this.

The two steps that get rushed and should not are the levelling and the mechanical fixing. A lantern that is fixed down properly but sitting a few millimetres out of level will still shed water, but the glazing caps will never look right. A lantern that is dead level but under-fixed can be worked loose by years of wind cycling, and on an exposed estuary roof that wind is real. Both matter. Neither takes long if the kerb underneath was built square in the first place, which is the whole argument for spending the time lower down.

Once the structural opening exists, fitting the lantern itself onto a prepared upstand is usually a one to two day operation for a straightforward domestic unit. The longer pole in the tent is nearly always the work below the lantern: forming and trimming the opening, building and weatherproofing the kerb, and then the internal reveal and plastering afterwards. On a job where the flat roof is being renewed at the same time, the lantern slots into a wider programme and the kerb is built as part of the new roof.

The figures below are industry-typical ranges for guidance, not a quotation. They move with size, with the glass specification, with how much structural work the opening needs and with access.

A common size for a rear-extension lantern is around 1.5 by 1.0 metres up to about 2.0 by 1.5 metres. Beyond that you are into larger glass, heavier lifts, and often a structural engineer’s involvement, all of which push the numbers up. The full picture of what drives a lantern installation is covered on our roof lantern installation service page.

Everything above is about keeping water out and holding the lantern down. There is a separate question that a flat-roof lantern makes more urgent than almost any other rooflight, and it is what happens on a hot afternoon.

A lantern on a flat roof points straight up. It faces the whole sky. In the middle of a summer day, when the sun is high, a horizontal or shallow lantern collects far more solar energy per square metre than a wall window on the same house, because the sun is striking it much closer to square-on. All of that arriving energy enters at ceiling height, at the top of the room, where warm air already gathers and sits. A lantern over a south-facing rear extension, which describes a great many of the ones fitted across this stretch of Essex, is in effect a solar collector mounted over your kitchen table.

This is the part of a lantern job that most quotes leave off the page, and it is the part you feel most. Southend faces south across the Thames Estuary, with an open horizon and reflected light coming up off the water, so a south-facing lantern here is working harder than the same lantern would inland or on a north coast. The number that governs it is the G-value of the glass, the share of the sun’s energy the glass lets through. Clear double glazing sits around 0.6, meaning most of the heat comes in. A solar-control unit brings that down towards 0.3 or below, cutting the summer heat gain roughly in half while keeping the room bright. It also takes the edge off glare, which on a lantern falls straight down onto worktops and screens rather than across a room.

So when the glass is specified for the frame at step five above, the orientation of the roof decides which glass goes in. A north-facing lantern can take clear glass and all the daylight it can gather. A south or west-facing one wants solar-control glazing, chosen for the aspect rather than picked off a default spec sheet. A lantern is a long-term fixture, and the glass is the one decision you cannot cheaply change afterwards, so it is worth settling at the survey.

A new or enlarged rooflight, and a lantern certainly counts, is notifiable under Building Regulations. Two parts apply directly. Part L covers thermal performance and sets a limit on the U-value of the glazing, which any decent lantern unit beats comfortably. Part K covers safety glazing and guarding, and it is the reason the inner pane of an overhead lantern is laminated: if it ever broke, laminated glass holds together rather than falling into the room below. Toughened outer panes are standard on top of that.

The notification is not paperwork the homeowner has to chase. On the installations we carry out we make the Building Control notification to Southend-on-Sea City Council, or to the relevant local authority for your address, as part of the job, so the finished lantern is signed off against the regulations rather than left in limbo. On most houses a rooflight is permitted development, but flats, maisonettes, listed buildings, conservation areas and the Article 4 zones around Leigh are the exceptions where a planning application may be needed, so the aspect worth checking with the local authority before ordering glass is the planning position, not the building work itself.

The order of the whole job is the thing to hold onto. Frame the opening, build the kerb square and to height, weatherproof the upstand until it is dry on its own, then fit and fix the lantern, then glaze it for the way the roof faces, then sign it off. Skip down that list and you buy a problem. Follow it and the lantern sits on a kerb that was already watertight before it arrived, which is exactly where you want it. If you are weighing up a lantern for a flat roof and want the sizing, the glass and the numbers worked out for your own roof, ask us for a specification before you commit to anything.