Do roof lanterns need a survey before ordering?

What the survey establishes

The kerb, or where it will be. Its size, whether it is square, whether it is level along the top, and whether the stated dimension is internal or external.

The roof build up. Warm or cold roof, the covering, and where the insulation and vapour control layer sit, which decides how the perimeter is detailed.

The fall. Where water goes and whether it will reach the kerb moving or standing.

The structure. Whether the opening needs a steel, which has to be designed before anything is ordered because it sets the kerb position.

The margin. Whether there is enough flat roof left around the kerb for the covering to terminate properly.

Access. Whether scaffold is needed, and whether a large pane can physically be brought to the position.

What goes wrong without one

The common failure is ordering from a builder’s drawing. Drawings show intent; roofs show what was built. A steel installed 30mm from its drawn position is entirely normal and will make a made to size lantern not fit.

The second failure is ordering before the structure is designed, then finding the kerb has to move.

Sequence

Structure designed, structure installed, kerb built, kerb measured, then order. Every step in that order exists because the one before it can move.

The survey is free and so is the indicative range that precedes it. Nothing is chargeable until you accept a quotation.

Yes, and we will not price one without

A lantern is manufactured to your dimensions, it is heavy, and it sits on a structure that has to carry it. Four things have to be established before an order makes sense, and none of them is reliably visible in a photograph.

1. The structure

What the joists are, which way they run, and what they are already carrying. An opening set within the span of existing joists rather than cutting across them frequently avoids steel entirely; a wide opening across them needs a designed trimmer and an engineer.

This is the largest single line on most retrofit lantern quotations and it has nothing to do with the glazing.

2. The fall

Which way the existing roof drains and whether water sits anywhere near the proposed opening. A roof with no fall, or a fall running toward the opening, is the single most common reason a lantern installation causes trouble later.

Correcting the fall locally around the kerb is part of the job rather than an optional extra, and it has to be priced rather than discovered.

3. The kerb

Whether one exists, how high it is, whether it is square, and whether it is insulated. A lantern kerb is doing more work than a flat rooflight kerb because it carries the frame as well as weathering the opening.

4. Getting it there

A large lantern pane can weigh several hundred kilograms and cannot be manhandled. Whether a crane or glass robot can reach the roof, and by what route, decides whether a given size is realistic at all.

A terraced house with no side access and no frontage close enough for a lift is a genuine constraint, and establishing it after the lantern is made is an expensive way to find out.

What the survey produces

An itemised fixed price naming the lantern with its dimensions and glazing specification, the structural work and whether an engineer’s fee is included, the kerb detail including insulation, the access arrangement, making good, disposal and Building Control.

It also produces the exact opening dimensions, which is what the lantern is manufactured to. Getting those from a tape measure held by somebody who has not seen the structural design is how lanterns arrive the wrong size.

Before the survey

We will give an indicative range by email from a photograph showing how the flat roof meets the house and a rough measurement of the room below. That is free, it needs no appointment, and it tells you whether the project is viable.

What the survey establishes that a drawing cannot

Every lantern is made to measure, so the manufacturer works from confirmed dimensions rather than a nominal size. Those dimensions come off the built kerb, not off an architect drawing, because the kerb as constructed and the kerb as drawn are rarely identical to the millimetre.

The survey also settles the kerb height and whether it is square and level. A lantern set on a kerb that is out of square will not sit down evenly, and the resulting gap has to be taken up in sealant, which is where a lantern starts leaking.

And it establishes access. Large panes are heavy, and whether they can be carried through the house or need lifting over it changes the job substantially.

The sequence that works

Structural design, kerb built, survey to the finished kerb, order, then delivery and installation to the confirmed date.

Ordering from a drawing before the kerb exists is the common shortcut, and it produces the classic outcome: a unit that is a few millimetres out, discovered on the day, with a replacement lead time measured in weeks.

Where a programme genuinely cannot accommodate that sequence, the alternative is to build the kerb to a dimension the manufacturer has specified in advance and accept responsibility for hitting it exactly.