A roof lantern is fitted by cutting and structurally framing an opening in the flat roof deck, building an insulated timber upstand at least 150mm above the finished roof surface around that opening, dressing the roof covering up and over the top edge of the upstand, then bedding the lantern’s base frame onto the weathered upstand and assembling the rafters, ridge and glazing on top. The upstand and the membrane keep the water out. The lantern sits on that work; it does not replace it.
That is the whole job in one paragraph, and the order matters more than any individual step. Almost every failed lantern we are called out to look at went wrong because the sequence was compressed: the frame went on before the covering was properly turned up, and sealant was asked to do a job that geometry should have done.
The eight stages, in order
- Survey and set out. The opening position is marked from inside and out, checked against joist positions, and checked against the roof’s fall so the lantern does not end up sitting in the low corner where water collects. Lanterns are made to a surveyed kerb dimension, so this measurement is the one the factory works to.
- Access. A tower with proper edge protection on most single-storey extensions, full scaffold on two-storey or awkward sites. This goes up before anything is opened, usually the previous working day.
- Strip and cut the opening. The covering and insulation come off over the opening area, the deck is cut back, and the joists crossing the opening are removed. On a replacement, the old lantern is lifted off first and the existing upstand exposed for inspection.
- Structural framing. Doubled trimmers on all four sides for openings up to roughly 1.2m, with the cut joists hung off them on joist hangers. Beyond about 1.5m, a ring beam in steel or engineered timber to a structural engineer’s design. We build to the engineer’s detail; we do not design structure.
- The upstand. An insulated timber box is built around the opening, minimum 150mm above the finished roof surface measured at the lowest point of the fall, 200mm on exposed coastal sites. Insulation runs on the outer face and meets the roof insulation so there is no cold bridge round the perimeter. The top edge is levelled across both diagonals, because a twisted upstand will twist the frame that sits on it.
- Weathering in. The roof covering, EPDM, GRP, single-ply or felt, is dressed up the full height of the upstand and over the top edge, with laps mechanically terminated rather than glued and hoped. The roof is watertight at the end of this stage, with or without a lantern on it. That is the hold point we work towards on day one of every job.
- Base frame, rafters and ridge. The eaves beam is bedded onto the weathered upstand and mechanically fixed, with 316L stainless fixings on coastal work. The frame is squared and levelled, then the hip bars and ridge are assembled onto it. A lantern that is out of square by even a few millimetres will not take its gaskets evenly.
- Glazing, capping and internal finish. Sealed units are lifted in, seated on their gaskets, and the external capping bars are clipped or screwed down over them. Internally the reveals are formed and boarded. Finally the installation is hose tested before handover.
Where the waterproofing actually lives. Water has to travel uphill twice to get past a correctly built upstand: up the vertical face, then over the dressed top edge. That is a geometric defence. Sealant is a gasket between two components, not a waterproofing layer, and any detail relying on it will start letting water in within about five years.
What changes on a replacement
A like-for-like replacement skips stages three and four almost entirely, which is why it runs faster. The old lantern comes off, the upstand is exposed, and then there is a decision point: is the existing upstand tall enough, level enough, insulated and dry? On lanterns from the mid-2000s the honest answer is often no, and the upstand gets rebuilt as part of the works. That is covered in detail on our page about when the upstand is the problem rather than the lantern.
Modern lantern profiles also rarely match older kerb dimensions exactly, so a replacement usually involves some adjustment of the upstand footprint regardless. We survey the existing opening before ordering rather than assuming the old size transfers.
What it means in Suffolk and north Essex
Two local factors change how this sequence is executed. Within about a mile of open sea at Felixstowe, Harwich, Frinton or Aldeburgh, the upstand goes to 200mm rather than 150mm, because wind-driven rain arrives horizontally and climbs a vertical face in a way it does not inland. Every fixing on the job is 316L stainless, and dissimilar metals are isolated with proprietary washers rather than left in contact. The estuarine towns, Woodbridge, Wivenhoe, Manningtree, Shotley, carry moderate salt loading rather than open-sea exposure, so upgraded fixings make sense there without the full marine package.
Inland, the constraint is structural instead. Timber-framed and historic roofs across mid Suffolk have irregular joist sizes and spans that were never documented, so stage four takes longer and an engineer’s calculation is not optional. Modern trussed roofs on the newer estates around Kesgrave, Martlesham and Sproughton are predictable by comparison, but trusses cannot be cut without a designed replacement detail.
Related questions
- How long does roof lantern installation take? covers the programme for each of these routes.
- What is a roof lantern upstand? explains stage five in more depth.
- What fall does the roof around a lantern need? deals with the drainage the upstand depends on.
For the full technical treatment, see our roof lantern installation process guide and the upstand construction page. Both sit under roof lantern installation.
Get a fixed price
Every installation carries a 10-year workmanship guarantee, and the installers have 25 years fitting rooflights behind them. Tell us the room size, whether it is a new extension or a replacement, and roughly where the property is. Request a fixed quote and we will survey the roof before we price it.
Thinking about this job? We survey across Ipswich, Suffolk and north Essex and give you a fixed price before any work starts, with a 10-year workmanship guarantee.