Flat rooflight standard sizes and made-to-measure

Three different numbers, all called the size

This is where most ordering errors begin. A flat rooflight has at least three dimensions and people quote them interchangeably.

The structural opening is the hole in the deck, measured between the trimming timbers. The kerb size is the outside of the timber box built on the deck, or the outside of the manufacturer’s proprietary upstand. The clear glass or daylight opening is what you actually see from the room, and it is smaller than both by the width of the frame all round.

Most flat rooflight manufacturers quote the size of the kerb the unit is designed to sit on, so a “1000 x 1000” unit sits on a kerb with a 1000mm by 1000mm external face and gives perhaps 850mm by 850mm of visible glass. Establish which convention a quotation uses before comparing two of them.

The stock sizes you will actually be offered

Ranges differ but they cluster around the same figures, because they are driven by glass cutting and by what fits on a lorry.

Note the pattern: the short dimension rarely exceeds 1500mm in a stock range, because the glass width is the constraint. Long and narrow is cheap. Large and square is not.

What the step between sizes is really costing

Going from 1000 by 1000 to 1200 by 1200 adds about 44 per cent to the glass area and rather less than that to the price, because the frame, the delivery and the labour are much the same.

Going from 1500 by 1000 to 2000 by 1500 is a different matter. The glass is now heavy enough to need more people or a lift, the frame section is deeper, the trimming timbers get bigger, and the delivery is a specialist one.

The practical consequence: if you are within one step of a bigger stock size, take the bigger one. The extra light is nearly free. If you are two steps above what you need, you are paying for weight and handling as much as for glass.

When a standard size is the right answer

On new work, almost always. If the extension is not yet built, the opening can be set out to suit a stock unit, and there is no reason not to. The joists get trimmed where the unit wants them.

On a replacement into an existing kerb, sometimes. If the existing kerb happens to sit close to a stock size and is sound and tall enough, a stock unit plus a small adaptation of the kerb is quicker and cheaper than a bespoke one.

Stock also means availability. A common size is on a shelf. A bespoke unit is a production slot. Where a client needs the job done before a plasterer arrives, that difference is the whole decision.

When made-to-measure earns its price

Four situations justify it.

An existing opening you cannot move, typically in a replacement where the trimmers are structural and cutting them means an engineer. Measuring the kerb and ordering to fit is far cheaper than rebuilding the opening.

A specific proportion that matters visually, usually a long slot over a kitchen run or a unit sized to line up with a window head below.

A shape that is not rectangular: a raking end where the extension is not square to the house, or a unit following a boundary line. Oxford back gardens are full of plans that are not right angles.

And any size outside the stock range in either direction, which in practice means anything over about two metres in the short dimension.

What bespoke actually adds to the bill

The honest position is that a made-to-measure rectangular unit within the normal size band usually costs somewhere in the region of a quarter to a half more than the nearest stock size, and the lead time roughly doubles.

Non-rectangular shapes cost more again, because the glass has to be cut and toughened to a template and the frame mitred to match. A raking corner is not a small surcharge.

We give a range by email at enquiry stage that reflects which of these you are likely to be in, and a fixed price after survey once the shape and the exact dimensions are established. We will not price a bespoke unit off a photograph.

Lead times, and how they affect the programme

A stock unit in a common size is often available within days. A stock unit in a less common size is typically one to three weeks.

Made-to-measure rectangular units usually run four to eight weeks. Non-rectangular or very large units can run longer, and glass supply has been unpredictable enough in recent years that we take the manufacturer’s date and add a margin before committing to a start.

The rule we work to is that nothing is booked in until the unit is confirmed in the country. Cutting an opening on the promise of a delivery is how roofs end up covered in sheeting for a fortnight, and the roof is never left open overnight on our jobs.

Measuring an existing opening for a replacement

Measure the kerb, not the glass and not the hole in the ceiling.

Take the external dimensions of the kerb at the top, in both directions, at both ends and in the middle, and record all six figures. Old kerbs are rarely square and the difference between the ends is exactly what you need to know.

Then measure the height of the kerb from the finished roof surface, because that decides whether it can be reused at all. And check whether the existing unit sits on top of the kerb or inside a rebate, because that changes which dimension the new unit has to match.

None of this is difficult and all of it is worth doing before an enquiry, because it turns a vague conversation into a specific one.

Tolerance, and why we survey before ordering

A made-to-measure unit is made to the millimetre you gave. If that figure is wrong, the unit is wrong, and it is not returnable.

Manufacturing tolerance on a bespoke frame is typically a couple of millimetres. Site tolerance on a timber kerb built by somebody else fifteen years ago is more like fifteen. The two have to be reconciled on the drawing rather than on the roof.

So we measure the kerb ourselves before ordering, always, even where the client has already sent dimensions. It is not a lack of trust. It is that we carry the cost of a wrong unit and the client does not, and that arrangement only works if we take the measurement.

Non-rectangular and raking units

Trapezoids, parallelograms and units with one raking end are all made. Triangles are made and are rarely a good idea, because the acute corner is difficult to seal and difficult to trim.

A raking unit needs a template rather than a set of dimensions: the manufacturer wants the actual angles, and the safe way to give them is a full-size template or a surveyed set of coordinates.

Costs rise steeply and lead times with them. Where a plan is only slightly out of square, it is usually better to build the kerb square within the out-of-square roof and absorb the difference in the ceiling detail, which is cheap, than to buy a bespoke parallelogram, which is not.

The limits of a single pane

Glass for these units is toughened and laminated, and the practical maximum for a single insulated pane in a domestic rooflight is set by what can be made, transported and lifted rather than by any rule.

Weight is the honest limit. A double glazed unit runs roughly 25kg to 30kg per square metre, and a laminated or walk-on build-up considerably more. A three square metre unit is therefore around 80kg to 90kg of glass in one piece, which is at the edge of what a crew can safely carry up a scaffold.

Beyond that you are into mechanical handling, a lorry-mounted crane, road closures on a narrow Oxford street, and a job that is planned weeks ahead. It is all doable and it stops being a routine installation.

Two units on a shared kerb instead of one large one

Where a client wants a wide glazed area and the single pane is getting heavy, a pair of units set close together on a common kerb structure is often the better answer.

Each pane is manageable, the cost stays in the stock range, and the structure is simpler. What you get is a mullion between them, and whether that is acceptable is a matter of taste rather than of performance.

The alternative, joining two units into one continuous run, is discussed on whether two rooflights can be joined together. It is possible and the junction detail is the whole job.

Access, and the size your house will physically accept

The unit has to get from the road to the roof. On a detached house with a drive this is rarely a question. On a terraced house in East Oxford with a side passage 800mm wide and no rear access, it is the first question.

We measure the route at survey: gate widths, passage widths, the turn at the end of the passage, and whether there is anywhere for a lorry to stop. A 2000mm unit that cannot be turned through a 900mm side gate is not a 2000mm unit for that house.

Where the route is impossible, the options are through the house on boards and dust sheets, over the roof with a lift from the street, or a smaller unit. Establishing that before ordering saves a genuinely bad day.

Ordering mistakes we see most often

Ordering to the ceiling opening rather than the kerb, which produces a unit far too small once the build-up depth is accounted for.

Ordering to the old unit’s outside frame dimension rather than to the kerb it sat on, when the two are different systems.

Assuming a stock size is exactly what it says: some manufacturers quote kerb size, some quote external frame, and one range’s 1000 x 1000 is another’s 1040 x 1040.

And ordering before the kerb height has been checked, which is how a new unit ends up on a 75mm kerb and fails for the same reason the old one did.

What you should get in writing before anything is ordered

The manufacturer and range. The size, stated as kerb dimensions with the clear glass figure alongside. Whether it is stock or made to order, and the lead time. The glass specification, including whether it is laminated and whether it has a solar control coating. The kerb height in millimetres from the finished roof surface. And whether the unit is fixed or opening.

Six lines. A quotation containing all six can be compared with another quotation containing all six. Anything less is a number without a specification behind it, and the difference between two such numbers tells you nothing.