What happens on a roof lantern installation day

An installation day that goes smoothly is one where all the thinking was done in advance. A roof lantern is not a stock item pulled off a shelf. It is made to the size of your opening, in the finish you chose, with a specific glass in it, and the lead time on a made-to-measure unit is usually somewhere between three and six weeks. Nothing about the day works if the survey underneath it was rushed.

The survey settles four things. First, the exact size of the structural opening, measured to the brick or the timber, because the lantern is built to sit on a kerb of a known footprint and a few millimetres out at this stage becomes a real problem on the roof. Second, the kerb or upstand: whether one already exists, what height it is, and whether it is square and level. Third, the finish and the sightlines, which is where a bespoke lantern design gets pinned down, ridge bar colour inside and out, the number of glazing bars, the pitch of the glass. Fourth, and this is the part most quotes skip, the glass specification itself.

That last point matters more in this part of Essex than almost anywhere. Southend faces south across the Thames Estuary, and a lantern over a rear extension with a south-facing garden is a solar collector with a dining table under it. The survey is where the roof plane’s aspect gets written down and the G-value of the glass gets chosen to match it. A south-facing lantern wants solar-control glazing with a G-value down around 0.28, so that the room stays usable on a July afternoon rather than becoming the hottest space in the house. A north-facing one wants clear glass and all the light it can get. None of that can be changed on installation day. The glass that arrives on the van is the glass that was specified at survey, so the survey is the moment to get it right.

The team usually arrives between eight and nine in the morning. The first thirty to forty-five minutes are not glamorous and they are the part homeowners least expect: nothing goes onto the roof yet. The crew unloads, walks the route from the van to the work area, and lays protection. Dust sheets go down over floors, furniture that cannot be moved gets covered, and on an internal opening a temporary screen often goes up to keep plaster dust and debris out of the rest of the house.

Then comes the check that saves the day from going wrong: the delivered unit is inspected against the order before anything is lifted. Frame size, glass specification stamped on the spacer bar or the glass data sheet, finish, opening mechanism if it is an electric opening lantern, and the ridge bar colour are all confirmed on the ground where a problem is easy to deal with, not forty minutes later at roof level where it is not. Glass is heavy and a large lantern pane can weigh a great deal, so access and lifting are planned at this point too: where the ladder or tower goes, whether the glass can be hand-balled up or needs more people, and where offcuts and the old unit will be stacked.

What happens next depends on whether this is a replacement into an existing opening or a lantern going into a newly formed one.

On a replacement, the old unit comes out. If a tired old dome or a failed flat rooflight is being taken off and a lantern put in its place, that is replacement work, and the old glazing and its frame are lifted clear, the kerb is exposed, and everything is checked before the new unit is offered up. A sealed unit that has misted, a frame that has moved, an old skylight that no longer holds the weather: swapping the whole thing for a new one is a replacement, and it is done in a controlled sequence with the roof kept covered against the weather at every stage. If what you actually have is a localised leak around otherwise sound glazing, that is a job for a roofer rather than a reason to replace the unit, and it is worth being told so honestly.

On a new opening, the aperture is formed in the roof structure. On a flat roof this means cutting through the deck and trimming the joists around the opening with new timbers to carry the load that the cut joists used to carry. On a pitched roof it means trimming the rafters in the same way. This is structural work and it is why the survey and, on larger openings, a structural engineer’s input matter: the roof has to keep doing its job with a hole in it. Once the opening is trimmed, the deck around it is made good and the surface is prepared to take the kerb.

Every roof lantern sits on a kerb, also called an upstand: a raised frame around the opening that lifts the glazing above the plane of the roof. This is the single most important detail on the whole job, and most lantern problems trace back to it. Get the upstand right and the lantern stays dry for decades. Get it wrong and no amount of good glass will save it.

The upstand does several things at once. It raises the base of the glazing clear of standing water and driving rain, which matters on a flat roof where water can pond. It gives the roof covering something to turn up against and be dressed to, so the waterproofing runs continuously from the roof, up the side of the kerb, to just below the lantern frame. And it provides a level, square, correctly sized base for the lantern to bolt down to. There is a general guide worth knowing: rooflights on a house are usually permitted development where they project no more than 150mm above the roof plane, which is part of why upstand heights are what they are, though a lantern’s own frame and glazing sit above that and the permitted-development position always wants checking with the local authority for your address.

If a good kerb already exists and is square, level and the right size, the day moves quickly from here. If it does not, one is built or corrected before the lantern goes anywhere near it. Insulating the upstand is part of this, because a cold kerb is where condensation forms, and the internal reveal is designed at this stage so the finished ceiling line splays outward and throws the light down into the room rather than trapping it in a deep, dark box.

With a sound upstand in place, the lantern itself goes on. The base frame is bedded onto the top of the kerb on a continuous bead of low-modulus silicone or the manufacturer’s specified sealant, set square and checked for level in both directions, and mechanically fastened down. Square matters here more than it sounds: a lantern that is even slightly out of true will fight the glass when the panes go in, and the glazing bars will not line up cleanly at the ridge.

On a self-supporting aluminium lantern the ridge bar and the rafters that run from the corners up to the ridge are assembled and set. The geometry of these bars is what gives a lantern its shape and its slim sightlines, and it is why a well-made aluminium lantern can carry large panes on surprisingly thin frames. The pitch of the glass, usually somewhere in the region of 25 to 30 degrees on a typical hipped lantern, is set by this frame, and that pitch is chosen partly so rain runs off cleanly and dirt does not sit on the glass.

This is the stage where a lantern stops being a flat-pack of parts and starts to look like the thing you ordered. It is also the point of no return on size, which is exactly why so much care went into measuring the opening weeks earlier.

Now the glass goes in. Each pane is a sealed double or triple-glazed unit, and on an overhead installation the inner pane is laminated glass, which is a requirement under Part K for safety glazing: if it ever breaks, the interlayer holds it in place rather than letting it fall into the room. The panes are lifted up, set onto the glazing bars on their gaskets, and clamped or capped down under the ridge and rafter caps that also form the external weather line.

The glass that goes in is the glass that was specified, and on a south or west-facing lantern that means the solar-control coating doing its work. It is worth restating what that buys you, because it is the whole reason to care about the specification. A large clear-glazed lantern over a south-facing room can pass more than half of the sun’s energy straight through at ceiling height, all afternoon, into the top of the room where hot air already wants to sit. A solar-control unit turns most of that off while keeping the room bright, and glare on a screen or a worktop goes with it. That decision is invisible on installation day because the two panes look almost identical, but the room feels the difference every summer afterwards.

With the glass in, the perimeter is weathered. This is where the roof covering, the upstand and the lantern frame are married into one continuous waterproof line. On a flat roof the membrane or GRP is dressed up the kerb and under the frame skirt. On a pitched roof, flashings and soakers weave the lantern into the tiles or slates. The external sealant lines are run, tooled and left clean. Done properly, the finished detail sheds water without relying on the sealant alone to do the sealing, because sealant is the last line of defence, not the first.

Inside, the reveal is completed. The plasterboard line around the opening is set out so the reveal splays outward from the glass towards the room, which is what makes a lantern feel like it is pouring light in rather than looking at you through a shaft. On most installations the plastering skim itself is left to a plasterer to finish, so that it can be done and dried and decorated as part of the room, though the plasterboard and the reveal geometry are set correctly first so the finished line is right.

Then the site is cleared. Protection comes up, the old unit and offcuts are removed and taken away, the glass is cleaned inside and out, and the opening mechanism, if there is one, is demonstrated and handed over with its controls. On an electric lantern this is when the rain sensor and the remote or wall switch are shown working, so you leave the day knowing how to open and close it and confident the auto-close on rain does what it should.

A new or enlarged rooflight is notifiable under Building Regulations. Part L governs the thermal performance, meaning the U-value of the glazing has to meet a maximum, and Part K governs the safety glazing overhead. We make the Building Control notification to Southend-on-Sea City Council, or to the relevant local authority for your address, on your behalf, so that the paperwork trail exists for when you come to sell the house. Every installation is carried out to current Building Regulations and backed by a ten-year workmanship guarantee, which is what that notification and that sign-off underwrite.

Two practical things about the days afterwards. First, if a plasterer is finishing the reveal, give the plaster time to dry fully before decorating, which in a cool room can take a week or more. Second, a brand-new sealed unit and a freshly plastered reveal can show a little condensation on the inside face in the first cold mornings while everything settles and dries out, and that usually clears as the room’s moisture balance settles.

Here is a rough shape of how the day divides up on a typical single lantern into an existing or straightforward opening. Larger units, difficult access, or a new structural opening will stretch these figures or push the job into a second day.

A few things you can do to help the day run. Clear the room below and the access to it the night before, so no time is lost moving furniture. Keep pets and small children well away from the work area and the ladder line. Make sure the crew can park close to the house and reach the rear, because a lantern and its glass are heavy and every metre of carry adds time. And if you have any doubt about the glass that was ordered, raise it before the day rather than on it, because installation day is for fitting what was specified, not for changing the specification.

That is the whole sequence: survey the roof and choose the glass for its aspect, protect the house, open up, get the upstand right, set the frame, glaze it, weather it, finish the reveal and notify Building Control. If you want to talk through what a lantern would take on your own roof, from the structural opening to the G-value of the glass for your aspect, our roof lantern installation service is the place to start, or you can ask us for a quote with the specification written down for each plane of your roof.