How do you measure for a roof lantern?

Measure the outside of the kerb, check it is square with the diagonals and give width before length. A step-by-step guide with the mistakes to avoid.

Roof lanterns are sold by the external size of the upstand, or kerb, they sit on. That single fact causes most ordering mistakes: people measure the hole in the ceiling, the internal size of the kerb or the size of the old lantern's glass, and order the wrong lantern. Measure the outside of the kerb and you will get it right.

What exactly should I measure?

1. Measure the external width of the kerb, outside face to outside face, at both ends and in the middle. Note the smallest figure.
2. Measure the external length the same way, at both sides and in the middle.
3. Measure both diagonals, corner to corner across the outside of the kerb. If the two figures are within a few millimetres, the kerb is square.
4. Check the top of the kerb is level along each side with a spirit level.
5. Measure the kerb height above the finished roof surface on each side.

Write the size as width × length in millimetres, with the shorter side first: 1000 × 2000 mm, not 2 × 1 m.

What if the kerb is not built yet?

Then you have the best option of all: build the kerb to suit a standard lantern size. Pick the lantern first, then give your builder its external kerb size and the manufacturer's upstand details. Standard sizes cost less and arrive sooner than made-to-measure.

What if my kerb is not square?

If the diagonals differ by more than the tolerance in the manufacturer's instructions, the kerb should be corrected before the lantern arrives. A lantern will not pull an out-of-square kerb straight, and forcing it can stress the glass and seals.

Common measuring mistakes

• Measuring the ceiling opening or the internal kerb size instead of the outside of the kerb.
• Giving the size of the old lantern's glass or frame instead of the kerb.
• Rounding to the nearest 10 cm. Made-to-measure lanterns are usually built in 10 mm steps, so give us the exact kerb size and we round it correctly.
• Forgetting that roofing membrane and trims on the kerb add a few millimetres.

When you request a quote, send your measurements and a couple of photos of the kerb. We check them against the range you have chosen before anything is ordered.

Sources

• Approved Document L, GOV.UK

Written by the Buy Roof Lanterns team. Last reviewed 19 October 2026. General guidance only; your designer, building control body or council confirms what applies to your project.