Which size is the maker quoting?
A rooflight has at least four sizes, and makers do not all quote the same one. Before measuring anything, find out which size your product is sold by.
| Size | What it is | Where you measure it |
|---|---|---|
| Builder's opening | The hole inside the kerb, timber to timber | Inside face to inside face of the upstand |
| External kerb size | The outside of the upstand that the frame sits over | Outside face to outside face, at the top |
| Overall frame size | The full footprint of the unit, including any overhang | From the maker's drawing, not the roof |
| Daylight size | The clear glass you see from below | From the maker's drawing |
One large online seller quotes by internal hole size, timber to timber. Others quote the external frame. A product page that just says 1000 × 1000 does not tell you which, so check the technical drawing before you order.
Why can a 1000 mm rooflight leave a smaller hole?
The frame has to sit over the kerb, and the kerb has thickness: timber, insulation and the roof membrane turned up its face. All of that comes off the clear opening. One supplier gives the example of a 1000 × 1000 mm external unit with an internal size of only 850 × 850 mm.
The difference matters inside the room too. If you planned a 1000 mm square hole in the ceiling, ordering a 1000 mm external unit will give you a noticeably smaller one, with a deeper, narrower reveal. Decide which dimension you care about, the ceiling hole or the roof footprint, and order to that one.
How do you measure an existing kerb?
Measure the kerb that is there, never the old dome or the drawings. Work in millimetres and write each figure down as you go.
- Measure the outside of the kerb across the top, in both directions, at each end. Kerbs are rarely perfectly parallel.
- Measure the inside, timber to timber, in both directions.
- Measure both diagonals of the outside. If they differ, the kerb is out of square.
- Measure the height of the kerb above the finished roof at each corner. The difference between corners tells you the fall.
- Measure the width of the kerb top.
- Take photos with the tape in shot, from above and from the side.
If the roof is going to be re-covered or insulated, remember the finished roof will end up higher than it is now, and the kerb height above it will drop.
Where an old dome is still in place, you often cannot see the kerb at all. Measure what you can from inside the room, at the ceiling hole and the depth of the reveal, and photograph the dome's edge from the roof. The true kerb size is confirmed once the dome is lifted, so treat any figure taken over a dome as provisional until then.
How much tolerance is there?
Less than most people expect. Each maker states its own allowance, and they differ:
- One lantern maker says the external upstand must equal the nominal size plus 20 mm, minus nothing. Slightly bigger is acceptable; smaller is not.
- One flat glass maker sizes the overall frame at the external kerb size plus 40 mm, and wants at least a 10 mm gap to the drip flange.
- Another allows about 5 mm of clearance on each side.
- Another says the internal upstand must not be built smaller than the daylight dimensions.
The safe rule is to build the kerb to the unit you have, or order the unit to the kerb you have, using that maker's figures. Glass that is made to order cannot be trimmed once it has been cut.
Remember that the membrane adds thickness to the outside of the kerb. A rubber or felt finish dressed up the face and over the top adds to each side, so a kerb built exactly to the limit in bare timber may end up too big once it is covered. Measure the kerb after it has been weathered, or allow for the covering when you set it out.
How do you measure for a pitched roof window?
Pitched roof windows sit between rafters, so the rafters set what fits. From inside the loft, measure the clear gap between the two rafters where you want the window, and the spacing from the centre of one rafter to the next. Note the roof pitch, which you can read with a digital level or an angle finder held on a rafter, and check it falls inside the window's stated pitch range.
If the window you want is wider than the gap, a rafter has to be cut and trimmed, which is structural work that needs building regulations approval. Note the roof covering and tile profile as well, because the flashing kit is chosen to suit it. Our roof window installation guide covers the fitting steps, and roof window installation explains how we price the work.
What about a rooflight where there is no opening yet?
For a new opening, the joists decide the size as much as the product does. Find which way the joists run, how far apart they are and what is in the ceiling void, such as cables, pipes or extractor ducting. A unit that lines up with the joists needs less trimming than one that straddles them.
Do not order a unit for a new opening until it has been surveyed. The survey confirms joist sizes and direction, the roof build-up and the finished roof height, and then the unit and kerb are sized together. Read adding a rooflight to an existing flat roof for the full process.
Which measuring mistakes cause the most trouble?
- Measuring the old dome instead of the kerb under it. Domes overhang their kerbs.
- Mixing up internal and external sizes between the order and the drawing.
- Measuring from plans rather than the built kerb.
- Forgetting that new insulation or a new membrane raises the finished roof.
- Ignoring the fall, so the unit cannot drain the way its maker intends.
- Rounding to the nearest 50 mm.
If you are unsure, send your figures and photos before ordering through our quote form, and we will check them against the product you have in mind as part of pricing the fitting. For units you have already bought, see fit-only installation.
Questions
Should I order the rooflight before the kerb is built?
Ideally, choose the exact product first and build the kerb to its drawing, because each maker sizes kerbs differently. You do not have to take delivery first, but the kerb should never be built to a guessed size. If the kerb already exists, measure it carefully and order the unit to suit it.
Can you check my measurements before I order?
Yes. Send the product link or drawing, your kerb measurements and photos with a tape in shot. We will tell you whether the unit suits the kerb as part of pricing the fitting. The final choice of what is ordered stays with the buyer, so check the maker's own figures as well.
What if my unit is slightly bigger than the kerb?
That depends on the maker. Some units allow the kerb to be a stated amount smaller than the frame, others need a close fit. Packing out the kerb with timber and re-dressing the membrane is often possible. A unit that is too small for the kerb is a harder problem, and usually means rebuilding the kerb.
