Do roof lanterns need a survey before ordering?
Yes, without exception. A roof lantern is made to a measurement, in a factory, for one opening, and once the aluminium is cut and the sealed units are made there is no adjusting it. A window that is ten millimetres out can sometimes be packed. A lantern that is ten millimetres out sits on the kerb wrong and the flashing detail never quite works.
That is the practical reason. There is a second one, which is that the survey is where the awkward truths about a roof surface, and finding them before an order is placed is the difference between a fixed price and a conversation on the day. Both sit inside how roof lantern installation is run here.
Why a lantern is less forgiving than a rooflight
A flat rooflight is a rectangle sitting on a rectangle, and small discrepancies hide under the frame flange. A lantern is a three dimensional assembly: four eaves beams, a set of rafter bars converging on a ridge, hip bars at the corners, all cut to angles derived from the base dimensions.
Change the base dimension and every angle changes with it. That is why lantern manufacturers will not accept a “roughly” measurement and why the order form asks who took it. The geometry is unforgiving in a way that flat glazing is not.
The measurement that is taken, and from where
There are three sizes in circulation for any lantern and they are all different. The structural opening in the roof. The external size of the kerb, measured across its top face. And the external size of the lantern frame itself, which usually overhangs the kerb by a defined amount.
A lantern is ordered to the kerb size, measured on the top of the finished upstand, corner to corner and along each face. Quoting the wrong one of the three is the most common ordering error in this trade, and it is why a survey record states plainly which dimension has been taken and from what surface.
Squareness, and why a rectangle is an assumption
Kerbs are built on site, often by a different trade, and very few are perfectly square. The survey therefore measures both diagonals as well as all four sides. If the diagonals differ, the opening is a parallelogram and the lantern has to be ordered to accommodate it, either by sizing to the smallest dimension or by correcting the kerb first.
Level matters as much as square. A kerb that is out of level across its length puts the ridge out of horizontal, which is immediately visible from the room below because the eye reads a ridge line against the ceiling. Both checks take a few minutes with a long level and a tape, and neither can be done from a photograph.
What can honestly be settled by email first
Quite a lot, and it is worth doing before anyone visits. A photograph of the roof from an upstairs window, a photograph of the ceiling below, the rough internal dimensions of the room and a note of which way the extension faces will support a sensible size range and a price range.
That first exchange narrows the field: whether a lantern suits the room at all, roughly what size is proportionate, whether the structure is likely to be a factor, and what glass specification the orientation calls for. What it cannot do is produce a number that will not move, which is why the process is two figures rather than one.
New build kerbs, and measuring something that does not exist
On a new extension the kerb has not been built when the lantern is ordered, since lead times run to several weeks and nobody wants the roof open waiting. The sequence reverses: the lantern size is agreed first, a kerb dimension is issued to the builder, and the kerb is constructed to that figure.
This works well provided the tolerance is written down and somebody checks the kerb against the drawing before the lantern arrives. The survey visit on a new build therefore happens twice in effect: once at design to agree the size and the structural implications, and once as a check measure when the kerb is up. The second visit takes twenty minutes and prevents the expensive version of this problem.
Structure, and the questions that only appear on site
The survey establishes which way the joists run, roughly what depth they are, where the opening falls relative to them, and whether the roof needs a beam. Detail on that sits on structural openings and steel for a roof lantern, and the point here is only that the answer arrives before the order rather than after.
Other things surface at the same time and none of them are visible from the garden. A soil vent pipe passing through the roof void exactly where the opening was drawn. A bathroom extract duct discharging into the void. A steel from an earlier alteration sitting in an unexpected position. Each of those moves a lantern by a few hundred millimetres, and moving it on paper is free.
The roof surface, and what it tells you
The covering gets checked for its type, its age and its condition, and levels are taken across the surface to establish the fall. A roof with standing water on it needs the drainage sorted before glass goes into it, which is a separate piece of work with its own price and its own place in the programme.
The deck gets tested underfoot for softness, and the perimeter details get looked at: how the covering terminates, what the existing upstands look like, where the outlet is. On older flat roof extensions across the Chelmsford suburbs and the Harlow and Basildon estates, this is where most of the surprises live.
The internal side, which decides how it looks
Half the survey happens indoors. The ceiling line is established, the depth of the roof build-up is worked out, and the reveal is set out, because the reveal is what you actually see. A splayed reveal spreads light across the ceiling and roughly doubles the apparent size of the opening, and deciding to splay it is a decision made before the plasterer arrives.
The room is also measured properly, so the lantern is proportionate rather than merely large. Ceiling height, room depth and where the light needs to reach all feed into the size. The reasoning is set out on sizing a roof lantern for the room below.
Access, waste, and from survey to fixed price
The last part is practical. How materials get to the roof, whether a scaffold or a tower is needed, where waste goes, whether there is parking, and whether the lantern can be carried through the house or has to go round the side. A large sealed unit is heavy and awkward, and a side return too narrow to pass it through changes the method.
Everything above turns the emailed range into a fixed written figure that names the unit, the glazing specification, the structural work and who is making good. Sunspire settles all of it at survey, handles the Building Control notification as part of the job, and orders nothing until the kerb dimension is confirmed against the roof as it actually is.
Tell us about your roof
We start with the building, the covering and the slope. Then we tell you what will suit it, and what it will cost, as a fixed written number.
- Surveyed before it is priced
- 10-year workmanship guarantee
- Building Control notification handled
- New installations and replacements