BEFORE YOU START
What you need
Five minutes and a tape. The measurements below are the ones we ask for on every enquiry, so taking them now saves a round trip.
A tape measure, a phone, and — if the opening is already cut — someone to hold the other end. Write the figures down on the roof rather than trusting them to memory on the way back to the van.
Two rules cover most of what goes wrong:
Measure the opening, not the old rooflight. The unit that came out may have been the wrong size, may have been trimmed to fit, or may have been a different manufacturer's convention entirely. The hole is the fact.
Order by nominal size. Rooflights in this range are listed by nominal size, which is the size of the opening they suit — not the outside dimension of the dome. A 100/100 is not a dome that measures 100 by 100 cm.
STEP BY STEP
Measuring the opening
The four figures we need
- 1 Measure the clear opening across the short side, inside edge to inside edge of the structural deck.
- 2 Measure the long side the same way.
- 3 Measure both again at the opposite end. Site-cut openings are rarely square, and the smaller figure is the one that governs.
- 4 Measure the build-up depth: from the top of the structural deck to the finished roof surface, including insulation and waterproofing.
READING THE CATALOGUE
Nominal sizes
Sizes are written as two figures separated by a slash — 100/100, 100/150, and so on, in centimetres.
The first figure is the short side, the second the long side. A square opening gives you a square unit; anything else is listed with the smaller dimension first.
Your measured opening will rarely land exactly on a listed size. Take the nearest listed size that your opening will accept, and if you are between two, send us both figures rather than guessing — the answer depends on the deck and the detail, not just the arithmetic.
WHAT TO SEND
What to put in the enquiry
Everything below fits in one message. With it, most enquiries come back priced the same working day.
| Measurement | Where from | Why it matters |
|---|---|---|
| Clear opening, short side | Inside edge to inside edge | Sets the nominal size |
| Clear opening, long side | Inside edge to inside edge | Sets the nominal size |
| Build-up depth | Deck to finished surface | Sets the upstand height |
| Upstand height required | Your roofing specification | Sets the upstand height |
| Deck type | Concrete, steel, timber | Affects fixing |
| Roof access | Ladder, hoist, crane | Affects what we can send in one piece |
A photograph of the opening with the tape measure in shot is worth more than any of the above on its own. If in doubt, send the photo.
DIFFERENT PRODUCT, DIFFERENT MEASUREMENTS
Measuring for a Solatube
A tubular daylighting system is not sized by opening. It is sized by the route the tube has to take.
What to check in the roof void
- 1 Measure the vertical distance from the underside of the roof deck down to the finished ceiling.
- 2 Look along the intended route for obstructions — purlins, services, ducting, water tanks. A tube that has to dogleg loses light.
- 3 Note the roof pitch, or confirm it is flat.
- 4 Note the roof covering: tile, slate, metal, or a flat membrane. The flashing differs.
- 5 Measure the ceiling area you are trying to light, and note the room's use.
AVOID
Four ways this goes wrong
Measuring the old rooflight instead of the opening
The unit coming out is evidence, not a specification. It may have been the wrong size to begin with, or fitted to a different manufacturer's convention. Measure the hole.
Measuring across the top of the upstand and calling it the opening
These are two different figures and both are useful — but they are not interchangeable. If an upstand is present, tell us which one you measured.
Measuring at one end only
Openings cut on site go out of square more often than not. Measure both ends. If they differ, the smaller figure governs and the difference is worth telling us about.
Rounding the opening down to make a listed size fit
It fits on paper and not on the roof. If your opening sits between two nominal sizes, send both measurements — enlarging the opening slightly is often the right answer, but that is a decision to make deliberately rather than by rounding.
Send the figures.
Two measurements and a photograph are usually enough for us to come back with a priced specification.
Where your roofing specification sets out its own measuring or tolerance requirements, follow the specification.