How to Measure for a Roof Lantern: A Step-by-Step Guide
Measuring for a roof lantern? Follow our step-by-step UK guide to get the kerb size right, avoid costly errors and order with confidence.
A roof lantern sits on a raised timber or structural upstand, known as the kerb, built on your flat roof. Getting the measurements right is essential, because the lantern is made to measure to fit that kerb precisely. Here is how to measure with confidence.
Understand the kerb first
The kerb is the raised frame built up from the flat roof that the lantern sits on top of. It lifts the glazing above the roof surface so water drains away and the lantern sits proud as a feature. Roof lanterns are almost always measured to the external dimensions of the kerb, so the lantern sits neatly over it like a lid. Before you measure, confirm with your supplier whether they want the external kerb size or the internal aperture, because this varies and getting it wrong is the most common and costly mistake.
What you will need
A steel tape measure, ideally with a second person to help, a pen and paper, and a level to check the kerb is square and flat. Measure in millimetres for accuracy.
Step by step
First, measure the external length of the kerb, from outside edge to outside edge along the longer side. Take the measurement in at least two places, at both ends, because kerbs are not always perfectly square. Record the smallest reading.
Second, measure the external width the same way, across the shorter side, again in two or more places, and record the smallest reading.
Third, check the diagonals. Measure corner to corner both ways. If the two diagonals are equal, the kerb is square. If they differ, the kerb is out of square, and you should tell your supplier so the lantern can be made to suit.
Fourth, check the kerb is level using a spirit level along each edge. A kerb that is not level can cause fitting and drainage problems.
Common mistakes to avoid
Do not measure the internal opening when the supplier wants the external kerb size, or vice versa. Do not assume the kerb is perfectly square, always check the diagonals. Do not measure over the top of the kerb, measure the actual kerb dimensions the lantern will sit on. And always double check every figure before ordering, because a made to measure lantern cannot be resized after production.
If in doubt, ask
Because roof lanterns are bespoke, a quick confirmation with your supplier about exactly which dimensions they need will save you from an expensive error. Send them your kerb sizes and your diagonal check, and confirm before you commit.
Frequently asked questions
Do I measure the kerb or the opening for a roof lantern? Usually the external dimensions of the kerb, so the lantern sits over it. Always confirm with your supplier, as some ask for the internal aperture instead.
What if my kerb is not square? Measure both diagonals. If they differ, the kerb is out of square. Tell your supplier so the lantern can be made to fit accurately.
Should I measure in millimetres? Yes. Millimetres give the accuracy a made to measure lantern needs, and most suppliers work in mm.
Got your measurements? Configure an aluminium roof lantern online and see the price instantly, with free UK mainland delivery.
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Author at SmartaView, writing about aluminium glazing, home improvements and the buying process.
