The kerb decides the job, and it is measured before anything is ordered
A flat roof rooflight is a made to size product. It is manufactured to the dimensions on the order and there is no adjustment in it, which means the order is the moment the job is decided. Everything after that is fitting what has already been settled.
This page is about the measuring and the ordering rather than the construction: what gets measured, when, by whom, and what happens when the number is wrong.
The dimension nobody agrees on
There are at least four dimensions in play on any rooflight opening, and half the problems in this trade come from two people using the same word for different ones.
- The structural opening. The hole in the deck, between the trimmers.
- The kerb outside dimension. The outer face of the upstand, which is what a kerb-mounted unit sits over.
- The kerb inside dimension. The daylight opening you actually see from the room.
- The unit external frame size. The outside of the product itself.
These differ by the kerb wall thickness all the way round, which sounds small until you multiply it by two sides and then discover the unit overhangs or drops inside. Manufacturers quote against different ones of these and do not all use the same convention.
Establishing which dimension the supplier means
Before an order goes anywhere, we confirm which of the four the manufacturer is quoting against, in writing, for that specific product line.
It is a two minute question and it is the single highest value check in the process. The same manufacturer sometimes quotes kerb size on one range and frame size on another, and an installer working from habit rather than from that range’s data sheet is guessing.
Where a unit is being ordered against an existing kerb rather than one we are building, this becomes the whole job, because the kerb cannot be changed to suit a misunderstanding.
What actually gets measured
| Measured | How | What it catches |
|---|---|---|
| Both widths | At each end, not the middle | A kerb that tapers, which is common on older work |
| Both lengths | At each side | The same, in the other direction |
| Both diagonals | Corner to corner | Out of square. Equal diagonals mean square |
| Level across the top | Long level or laser, both directions | A twist that will load the seals unevenly |
| Height above finished surface | At the lowest point of the fall | Insufficient clearance where water arrives |
| Kerb wall thickness | At an accessible corner | The difference between inside and outside dimensions |
Six measurements, twelve numbers, and the whole survey takes a few minutes once you are on the roof. Taking four of them and assuming the rest is how a unit arrives that does not fit.
Why both ends, and not the middle
A kerb is a timber box, and timber boxes built on site are not perfectly rectangular. They bow, they taper slightly, and the bottom is frequently truer than the top because the top has been worked around.
A single measurement across the middle records neither the largest nor the smallest dimension, and the unit has to fit the largest. Measuring at both ends and taking the working figure from that is the difference between a unit that drops on and one that has to be persuaded.
Square matters more than size
A rooflight a few millimetres small can usually be accommodated in the detailing. A rooflight that is square arriving onto a kerb that is not cannot be accommodated at all, because the error is in the shape rather than in the dimension.
An out of square kerb leaves a tapering gap along one side, which cannot be sealed reliably, and it puts the frame into a twist as it is pulled down onto the fixings. Both of those are seal failures waiting to happen.
The remedy is to correct the kerb rather than to order a parallelogram, which is dealt with in full on its own page. The measurement that reveals it is the diagonals, and it takes thirty seconds.
Level, on a roof that is not
The roof falls, because it has to drain. The top of the kerb is level, because the frame cannot take a twist. Those two facts together mean the kerb is deeper on the low side, deliberately.
Checking level across the top of an existing kerb tells you whether whoever built it understood that. A kerb that follows the fall is a kerb that will twist a frame, and it is worth knowing before rather than after.
Who is responsible for the kerb
On most jobs the kerb belongs to the builder or the roofer and the rooflight belongs to the supplier. Neither of them measures the work of the other, and neither of them is in the wrong when the two do not meet, which is precisely the problem.
The gap is not technical. It is a gap in responsibility, and it exists on jobs where everybody has done their own part competently.
We close it by measuring the kerb ourselves before the unit is ordered, whoever built it, and by saying what has to change before the order goes in rather than after the unit arrives.
Ordering from a drawing
A drawing is a statement of intent. A kerb is what got built.
On new work the two are frequently within a few millimetres, and on a well run site they are close enough that ordering from the drawing works. On existing work, on an extension built fifteen years ago by somebody nobody can now ask, the drawing is at best a starting point.
We order from the measured kerb in both cases. On new work that means waiting until the kerb exists, which occasionally puts the rooflight later in the programme than the builder would like, and it is still the right sequence.
What a wrong dimension costs
- Too large. The unit overhangs the kerb, the covering cannot be dressed properly, and there is no fix short of a new unit or a rebuilt kerb.
- Too small. The unit sits inside the kerb line with a gap round it. Occasionally accommodated with a made-up detail, usually not.
- Right size, wrong shape. A tapering gap on one side, an unsealable joint, and a frame under permanent stress.
- Right in every dimension, wrong convention. The unit was ordered to kerb size and quoted as frame size, or the reverse, and it is out by twice the kerb thickness.
All four mean a lead time on a replacement unit with the roof open in the meantime, which is the part that actually hurts.
Lead times, and why they change the sequence
A made to size unit is manufactured after the order. That is weeks rather than days for most products, and longer for anything with a non-standard specification.
Which means the roof is not opened until the unit is in stock and checked. We take delivery, measure what arrived against what was ordered, and only then book the work. Opening a roof against a delivery date is how roofs end up covered in polythene for a fortnight, and on this coast a fortnight includes weather.
Checking what arrives
A unit is measured on delivery, before anything is opened up.
Manufacturing errors are uncommon and they are not unknown, and finding one on the ground with the roof intact is an inconvenience while finding one on the roof with the covering off is an emergency. It takes five minutes.
The same check catches the more common problem, which is the right unit to the wrong specification: clear glass where solar control was ordered, standard coating where marine grade was specified, no opening vent where one was on the order.
Tolerance, and what a made to size product assumes
Made to size does not mean made to fit anything. It means the manufacturer builds to the number you give and expects the opening to match it within a small tolerance.
What the tolerance is for
A few millimetres of clearance all round is designed in so that the unit can be positioned and bedded without being forced. That clearance is taken up by the seal and by the trim, and it is not spare capacity to absorb a measuring error.
What happens when it is used up
A unit ordered at the top of the tolerance onto a kerb measured at the bottom of it has no clearance at all. It goes on, because it can be persuaded, and the seal has nowhere to sit. That is the installation that leaks in year three and looks perfect on the day.
Why we measure to the largest figure
Where a kerb tapers, the unit has to clear the widest point. Ordering to the average, or to the convenient end, produces a unit that fits three quarters of the opening.
The specification is part of the order
The dimensions get the attention and the specification is the other half of the order, with the same property of being unchangeable afterwards.
Glass build-up, coating grade on the frame, whether the unit opens and how it is operated, the colour, whether there is a vent, the type of upstand connection: all of these are set at ordering and none of them can be revisited once the unit is made. On a coastal job the coating grade in particular is a decision that has to be right on the order, because it cannot be applied later.
We write the full specification into the quote rather than the size alone, so that what is being ordered is visible before it is ordered.
Rooflights on an existing kerb
Replacement onto an existing kerb is a large part of our flat roof work, and it is the case where measurement matters most, because the kerb is fixed and the unit has to suit it.
The sequence is to survey the kerb fully, decide whether it is sound and correctable, decide whether the existing unit is a size that is still made, and only then order. Where the existing size is obsolete, which is common on units from the 1990s, the options are a made to size unit to the actual kerb or an adaptation of the kerb to a current size. Both work. The first is usually tidier.
Domes, and the trap in replacing one
A great many flat roofs around Worthing carry a polycarbonate dome from the 1980s or 1990s sitting on a kerb.
Replacing a dome with a modern flush glazed unit on the same kerb is a popular and generally excellent change, and it has a measurement trap in it. Domes were frequently sold by a nominal size that matched neither the kerb nor the frame, and the kerb underneath was built to whatever that particular manufacturer expected.
The old unit tells you very little. The kerb tells you everything, and it gets measured as though nothing had ever sat on it.
What the visit covers on a flat roof
We survey the kerb before quoting, take all twelve numbers, confirm with the manufacturer which dimension their product is quoted against, and order only once the kerb is measured and correct. We take delivery and check the unit before the roof is opened, and we do not open a roof we cannot close the same day.
Everything else about a flat roof rooflight is recoverable. The dimension on the order is not.
Thinking about this job? We survey across Worthing, West Sussex and the South Downs and give you a fixed price before any work starts, with a 10-year workmanship guarantee.