How to measure for a flat rooflight

A rooflight is made before it reaches your roof and toughened glass cannot be cut afterwards. Ten careful minutes with a tape now is what stops the wrong size arriving. Measure the finished kerb if you can, not the plan.

Rooflight kerb from above and from the side, showing inside width, outside width, kerb height and fall

1. Measure the kerb, inside and outside

A flat rooflight sits on a raised kerb, also called an upstand. Use a steel tape and write everything in millimetres. Take these four sizes:

Outside width and outside length of the kerb
Inside width and inside length of the opening

Measure each one at both ends and in the middle. If the numbers differ, write down the smallest. Send us both the inside and outside sizes, and we will tell you which rooflight fits.

2. Check it is square

Measure corner to corner across both diagonals. If the two figures match, the kerb is square. If they differ by more than a few millimetres, tell us the difference, or ask your builder to correct the kerb before we make anything.

3. Check the fall and the height

Rain has to run off the glass, so flat rooflights are normally fitted on a kerb built with a fall. Many installers work to around 5 degrees, but ask your roofer what suits your roof. A level kerb will leave water sitting on the glass.

Your roofer will also want the kerb high enough to finish the roof covering properly. Upstands of around 150 mm above the finished roof are common. Measure the height from the finished roof, not the deck.

4. Plan the lift

Glass weighs about 2.5 kg per square metre for each millimetre of thickness. As an example, a unit made from three 6 mm panes weighs about 45 kg per square metre, so a 1000 x 2000 mm unit of that build-up is about 90 kg before any frame or fixing. Agree how it will get onto the roof before delivery day.

What to send us

Inside and outside kerb sizes, in millimetres
Kerb height and the direction of the fall
A photo of the kerb and the roof around it
Your delivery postcode, and anything that could make unloading difficult

A new roof opening may also need Building Regulations approval. Your builder or local building control can confirm this for your project.

Mistakes we see most often

Measuring the ceiling opening. The plastered hole inside the room is not the kerb. Always measure the kerb on the roof.
Measuring before the roof is finished. Insulation, membranes and trims change the kerb sizes. Measure the finished kerb, or send us the drawing and tell us it is not built yet.
Rounding. Write the exact millimetres. A rooflight cannot be stretched, and a few millimetres matter.
Mixing units. Do not mix centimetres and millimetres in one list. 1000 mm is 100 cm.
Measuring one side only. Kerbs are rarely perfect. Measure at both ends and in the middle and send the smallest figure.

Replacing an existing rooflight

If you are replacing an old rooflight, measure the kerb it sits on, not the old glass or frame. Old units often overhang the kerb by different amounts depending on the maker. If you cannot remove the old unit to measure, send us photos from the roof and from inside, the overall size of the old unit and the inside size of the opening, and we will tell you what else we need.

The kerb itself

Kerbs are usually built in timber and finished with the roof covering, and some are made from insulated or pre-formed upstands. Whatever they are made of, the top of the kerb should be level along each side, square, and strong enough to carry the rooflight. The insulation around the kerb matters too: a cold, uninsulated kerb is a common place for condensation inside the room. Ask your builder or roofer how the kerb will be insulated and finished.

A worked example

Say your outside kerb measures 1220, 1218 and 1221 mm across at the top, middle and bottom. You write down 1218 mm, the smallest. The diagonals are 1725 mm and 1729 mm, a 4 mm difference, so the kerb is close to square. You send us the outside sizes, the inside sizes measured the same way, the kerb height, the fall direction and a photo. We then confirm the rooflight size that fits, or tell you what needs adjusting, before anything is made.

Questions we are asked

Should I measure in millimetres or centimetres?

Millimetres. Glass is made to the millimetre, and one unit throughout avoids mistakes.

Can you make a rooflight for a kerb that is not square?

A small difference is usually fine. If the diagonals differ by more than a few millimetres, tell us the difference and we will advise whether the kerb should be corrected first.