Do roof lanterns need a survey before ordering?
Yes, and the reason is narrower than it sounds. A roof lantern is built to order from a set of dimensions that describe a structure which, on most jobs, has not been built yet. The kerb it will stand on is timber that somebody is going to make, to a size somebody has to decide, on a deck whose falls nobody has measured. Order the lantern first and every one of those unknowns has to bend to fit a frame that is already welded, glazed and on a lorry. A survey turns the unknowns into figures before anything goes into production. The wider sequence of the job is on the roof lantern installation page. What follows is the ordering stage specifically, and what a visit gets right that a phone call cannot.
What a manufacturer needs before a lantern goes into production
A lantern order is not a single number. The factory needs the external kerb size it will sit down over, the pitch of the sloping planes, the ridge configuration, the glass build-up with its coatings, the frame finish, and whether any pane is an opening vent with an actuator behind it.
Each of those depends on the others. Raise the pitch and the ridge gets longer and the panes get bigger. Add solar control to the outer pane and the glass gets thicker, which changes the weight per pane and sometimes the rafter section. Put a vent in one plane and that plane needs a different frame member. None of it can be settled from a rough width and length written down over the phone.
The kerb sets the size, and it usually does not exist yet
A lantern is ordered against the kerb, not against the hole in the ceiling and not against the room. That single fact causes more ordering errors than anything else, because the two figures can differ by 200mm or more once the kerb thickness, the insulation and the plasterboard reveal are counted.
Where the extension is still being built, the survey fixes the kerb size on paper and the carpenter builds to it. Where the roof is already finished, the kerb is measured as built, including how square it actually is. A kerb 15mm out of square across a three metre opening will show as a tapering shadow line along one side of the finished lantern, and glass cannot be persuaded to take up the difference.
Three ways a tape measure figure goes wrong
Homeowners measure carefully and still hand over figures that will not work. The pattern repeats.
- The wrong face. Outside of kerb, inside of kerb, and the finished plaster line are three different rectangles. A lantern quoted against one and built against another is out by twice the kerb thickness.
- One reading instead of four. A timber kerb is measured at both ends and both diagonals. Old rear additions in Gloucester rarely give the same answer twice.
- Nothing above the ceiling. The joist depth, the direction of span, the trimmers around the opening and whether the deck falls towards the new kerb are invisible from a stepladder in the kitchen.
A made to order lantern has nowhere to go back to
Manufactured roof windows come in fixed size codes and a supplier will usually take an unopened one back. A lantern is different. The aluminium is cut, the corners are machined, the powder coat is applied in the chosen finish and the sealed units are made to the exact pane sizes for that frame. It is a one-off item from the moment the order is confirmed.
That is the commercial reason a survey happens before the order rather than after it. A frame 40mm too narrow is not a small problem to solve on site. It is a new frame, a new lead time, and a roof waiting with a temporary covering over the opening.
What gets recorded in the room below
Inside, the survey works from the ceiling upwards. Joist size, spacing and span direction. What sits over the opening now, and whether an existing trimmer is doing any real work. The position of any beam over a knocked through wall, and the ceiling height, because a deep reveal in a room with a 2.3 metre ceiling reads very differently from the same reveal at 2.7.
Then where the daylight is wanted. A lantern centred on the roof and a lantern centred on the island are frequently not in the same place, and the compromise between the two is a conversation held in the room, with the tape out, rather than settled by a plan view.
What gets recorded on the roof itself
The deck is where a lantern job is won or lost. The survey checks the actual fall with a level rather than trusting the design fall, notes where the outlet sits relative to the proposed kerb, and looks for staining, blistering or a shallow depression that says water has been standing there.
On a Severn Vale rear extension with a felt roof laid twenty years ago at something closer to 1:120 than 1:40, the honest finding is that the deck needs re-falling before a kerb goes on it. That is far better established in April with a tape than in July with a lantern in the yard.
Access, the lorry and the last twenty metres
A three metre lantern arrives as a frame in sections and glass in a stillage, and the glass is the heavy part. A single pane on a decent sized unit can weigh 60 to 90 kilograms.
So the survey walks the route. Whether an artic or a rigid can reach the address at all, where it can turn, whether there is a side gate wide enough for a stillage, and what the ground is like. In Chalford or Rodborough, where a terrace steps up a hillside and the only approach is a footpath and a flight of steps, the answer is often extra hands or a mechanical lift, and that has to be known before a delivery date is agreed rather than on the morning.
Consent and notification, checked on the same visit
A lantern adds height above the roof plane, which is exactly what permitted development rules measure. The survey establishes the proposed ridge height above the existing roof surface, whether the property sits in a conservation area or under an Article 4 direction, and whether the building is listed. Cheltenham and central Gloucester both have streets where the answer is not the default one, and it is set out on the roof lantern planning permission page.
The structural notification is settled at the same time. Where a beam or a designed trimmer is involved, the engineer needs the survey figures before anybody can price the steel.
Where the survey sits between the two figures
The sequence is deliberate. First an indicative range by email from photographs, a rough size and the address. It is free, it comes back quickly, and it exists so nobody books a morning for a project that was never going to work on budget. Then the survey. Then a fixed price, itemised so the unit, the labour, the scaffold and the Building Control fee are each visible, and it holds unless the scope changes or something documented as a risk turns out to be real. That process is set out further on the costs page.
Useful things to send before a visit is arranged: a photograph of the flat roof from an upstairs window, one of the room from inside, the rough size of the roof area, and the address. The survey is where those become the numbers on an order form.
Next on this subject.
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- Question Do roof lanterns need planning permission? Usually not. A roof lantern on a single storey rear extension or on an existing flat roof at the back…
- Question Can a roof lantern be fitted to an existing extension? Yes, and it is the most common lantern job in this county by a distance. A lantern goes into an…