Yes, always, and the reason is specific to lanterns rather than general good practice. A lantern is manufactured to the size of a structural opening that in most cases does not exist yet, is not returnable once made, and cannot be trimmed on site. Get the figure wrong by 30mm and you have a large aluminium frame in your garden and a twelve week wait. Every other rooflight type gives you more room to recover. A lantern gives you none, which is why we measure before anything is ordered rather than after.
A lantern is made to an opening that is not there yet
The ordinary sequence on an extension is that the deck goes on, the opening is formed, the kerb is built and only then does the lantern arrive. So at the moment the order has to be placed, the thing being measured is a hole that exists on a drawing.
That is not a reason to skip measuring. It is the reason to measure everything around it: the joist depth and direction, the wall plate positions, the internal wall thicknesses the reveal will be formed against, and the actual dimensions of the room as built rather than as drawn. Extensions are routinely 20 to 40mm off their own drawings, and on a lantern that difference has to be known before the frame is made rather than discovered when it is lowered on.
The two measurements people mix up
Nearly every ordering error traces back to a confusion between the structural opening and the external frame size. The structural opening is the hole in the deck, measured between the trimmers. The kerb is built up inside or around that opening. The lantern then sits on the kerb, and its frame is sized to the kerb’s outer face, not to the hole in the deck.
Between those three dimensions there can be 100mm or more, and different manufacturers state their sizes differently: some quote the external frame, some quote the kerb size the unit fits over, some quote the daylight opening you will actually see. Ordering the right lantern to the wrong convention is how a unit arrives that will not sit down.
Squareness, which a lantern will not forgive
A lantern is a rigid four-sided frame with a ridge running through it. It has no adjustment. Set it on a kerb that is out of square by 15mm across the diagonals and one corner lifts, the gaskets do not compress evenly, and the frame is under a twist it was never designed to carry.
Measuring the diagonals is therefore part of the survey, and on an existing structure it is often the finding that changes the job. Level matters equally. Two adjacent kerb tops differing by 10mm produce a visible kink along the eaves line that nobody can unsee once it is pointed out. Both are cheap to correct while the kerb is being built and impossible afterwards.
What is under the covering, which nobody knows from a drawing
The survey is not only a measuring exercise. On any existing roof there are questions that only opening up or careful inspection will answer: what the deck is made of, whether the insulation is dry, which way the joists run, whether the build-up is a warm roof or a cold one, where the falls go, and whether there are services in the ceiling void where the opening wants to be.
Extract ducting, a soil vent pipe, and cabling for downlights are the usual three, and any of them can move a lantern half a metre. Finding that at survey costs a conversation. Finding it after the frame is made costs the frame.
Ordering from a drawing, and when that is acceptable
There is one case where it works: a new-build extension not yet started, where the builder is setting out the structure and will build the kerb to a dimension you give them rather than the other way round. Here the lantern size can lead and the opening follows.
Even then it is conditional. Somebody has to own the dimension, in writing, and the kerb has to be checked for size, square and level before the unit is dispatched rather than when it arrives. We will work that way alongside a builder who wants it, and we will still want to see the kerb before the order is released.
Replacing an existing lantern is not a shortcut
People reasonably assume that swapping a lantern is a like-for-like exercise: measure the old one, order the same size, done. It is not, for three reasons.
Frame profiles differ between manufacturers, so a new unit sized to the old frame’s outside dimension may not cover the same kerb. Old kerbs are frequently a different height from what current practice wants, and if the frame is coming off, that is the moment to correct it. And the condition underneath is unknown until the old unit is lifted, which is when saturated timber and a degraded covering tend to appear. A survey on a replacement is arguably more important than on a new opening, not less.
What our survey produces
| Recorded | Why it decides something |
|---|---|
| Structural opening, both diagonals, level across the kerb line | Sets the frame size and reveals any twist |
| Deck construction and insulation type | Determines the kerb build-up and the insulation line |
| Fall direction and outlet positions | Fixes where the lantern can sit |
| Covering type and condition | Decides how the covering terminates at the kerb |
| Internal finishes and reveal depth | Sets the plastering and making good |
| Access route and lifting arrangement | Decides the working platform and the handling |
| Orientation and shading | Informs the glass specification |
All of that goes into a written specification, and the fixed price is written against the specification rather than against a size. That is the difference between a number that holds and a number that moves once somebody is on the roof.
The time it adds, honestly
A lantern survey takes an hour or two on site. The written specification follows within a few days. Against a manufacturing lead time that is usually several weeks and can be longer for a made-to-order frame, the survey adds effectively nothing to the programme.
What it removes is the failure mode that costs months. We give a range by email so you know whether the project is worth pursuing at all, then a fixed price after the survey, and we do not ask for an order before that. Fifteen years fitting rooflights across Oxford and 25 miles around it, Building Control notification handled, and a ten-year guarantee on our workmanship. Call 01865 704245 or email info@heritageskylightsoxford.co.uk.
Helpful reading on this
Costs, comparisons and the questions we are asked most.
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