Flat rooflight standard sizes and made to measure

Where the standard grid came from

The sizes are not arbitrary and they are not marketing. They come from three pressures pulling at once.

The first is glass. Sealed units are cut from stock sheet, and a size that nests well into a sheet with little waste is cheaper to make than one that leaves an offcut nobody can use. The second is the roof. Domestic joists run at 400mm or 600mm centres almost everywhere in England, so an opening that lands between a pair of joists, or trims neatly across two bays, is easier to form. The third is history. The domed rooflights fitted across Essex in the sixties and seventies were imperial, in two foot and three foot increments, and the metric sizes that replaced them were rounded from those. That is why 600 x 600, 900 x 900 and 1200 x 1200 keep appearing: they are 2ft, 3ft and 4ft wearing new clothes.

The sizes you will see quoted most often

Across the UK flat rooflight market the recurring kerb sizes are 600 x 600, 600 x 900, 800 x 800, 900 x 900, 1000 x 1000, 1000 x 1500, 1000 x 2000, 1200 x 1200, 1200 x 1800 and 1500 x 1500. Most manufacturers hold a version of that list, and most will have a handful of them physically in stock rather than making to order.

Beyond those, the grid usually continues in 100mm or 200mm steps up to around 2000 x 3000, but as made-to-order rather than stock. The distinction between a listed size and a stocked size is worth asking about directly, because a size can be on the chart and still be a six week wait.

Why 1000 x 1000 and 1000 x 1500 are everywhere

These two do more work than the rest of the grid combined, and there is a structural reason.

A 1000mm opening across joists at 400mm centres cuts two joists and lands on trimmers, which is routine carpentry with a doubled joist either side and no calculation needed. Push to 1200mm and you are often cutting three, which starts to want checking. The 1500mm length runs with the joists rather than across them on a typical rear extension, so the long dimension costs nothing structurally at all.

The result is a unit that suits a four metre by three metre kitchen extension, sits comfortably in the deck, and does not need an engineer. That combination is why it appears on so many Essex roofs from Springfield to Witham.

Reading a manufacturer size chart correctly

Charts are laid out width first, then length, but not universally, and on a rectangular unit the difference decides which way the drainage falls and where the opener sits.

Two things are worth confirming in writing before an order goes in. Which dimension the manufacturer calls width, and whether the quoted figure is the kerb size or the external frame size. On a hinged unit, also confirm the hinge side, because on a rectangle it is normally on a long edge and a unit delivered hinged on the wrong side is a unit that goes back.

The point at which a unit becomes a special

There is a genuine step in the process, and it usually sits at three thresholds rather than one.

The first is the size grid itself: any dimension not on the chart is a special, even if it is 5mm off. The second is around 1500mm in the shorter dimension, where the glass often moves to a thicker make-up and the frame section changes. The third is around 2500mm to 3000mm in the longer dimension, where the sealed unit becomes a jumbo pane, needs specialist handling to get onto the roof, and may want a lifting frame or a crane rather than four people and a stair.

Crossing the first threshold changes the lead time. Crossing the third changes the whole day.

What made to measure changes at the factory

A bespoke unit is not a different product. It is the same frame extrusion cut to a different length and the same glass make-up cut to a different pane size, which is why the cost step is usually modest rather than dramatic.

What does change is the sequence. A stock size is picked from a rack. A special goes into a glass order first, then into a frame batch, then into assembly, and each of those has a queue. Six to eight weeks is normal against one to two for stock. On a build programme that difference decides whether the roofer waits for you or you wait for the roofer.

Tolerances, and the millimetres that decide the fit

Flat rooflights are ordered to a kerb size with a build tolerance already designed in, typically a few millimetres of clearance on each side so the unit lands without being forced.

The practical consequence is that the kerb has to be built square and to size, not approximately. A kerb 8mm out of square on a 1200mm unit shows as an uneven shadow line along one edge and, more seriously, as unequal compression on the perimeter seal. Diagonals get measured before the unit is ordered, not after it arrives. Where a kerb is being built new, it is built to the manufacturer’s stated size and checked corner to corner.

Lead times, stock and the programme

The size decision and the programme decision are the same decision, which is not obvious until a roof is open.

On a new extension the roof structure goes up, the deck goes down, the kerb is formed and then the covering is dressed around it. The unit needs to be on site by the time the covering is being laid, because the roofer terminates the membrane against a kerb that has to be the right size. Ordering a special after the roof is framed is how a build ends up with a temporary cover over an opening for a month.

Glass thickness, and the size where it steps up

A flat rooflight is glazed overhead, so the inner pane is laminated as a matter of course and the outer pane is toughened to cope with thermal stress. As the pane grows, both have to grow with it.

A modest unit might run a 6mm toughened outer and a 6.8mm laminated inner. Take the same specification to 1500 x 2500 and the deflection under wind and snow load becomes the governing case, so the outer moves to 8mm or 10mm and the laminate to 8.8mm or 10.8mm. Weight roughly follows. A square metre of double glazed unit at the lighter make-up is around 25kg. At the heavier one it is closer to 40kg, and a three square metre unit is then a serious lift.

Sizing to the joists on an Essex flat roof

The roof decides the size more often than the room does, and on the flat roofs across Harlow, Basildon and the postwar estates around Chelmsford the joists are usually 47mm by 150mm or 47mm by 175mm at 400mm centres, spanning the short way.

An opening that stays within a single bay needs no trimming at all, which caps the width at around 350mm and is only useful for a small unit. Cutting one or two joists and trimming across them is standard practice and covers most sizes people actually want. Cutting three or more, or forming an opening close to a bearing wall where the joist is doing its hardest work, is the point at which a structural engineer earns their fee. The wider question of a trimmed opening is covered on the main flat roof rooflight page.

Fitting a size to a garage or side return roof

Narrow roofs constrain the width first. A single garage is typically 2.4m to 2.8m internally, and once the kerb and the covering upstand at each edge are allowed for, the practical maximum width for a single unit is well under two metres.

A side return is narrower again, often 1.2m to 1.8m between the flank wall and the boundary. There, a run of two or three units at 800 x 800 or 900 x 900 down the length usually beats one long rooflight, because it keeps every opening inside a single joist bay and spreads light along the room rather than pooling it at one end. Where a garage is being brought into use as a room, the same width logic applies and the roof is usually shallower still.

Replacing into an opening that already exists

When an old unit comes out, the kerb size is already fixed, and the honest question is whether that size is on anybody’s chart.

Old domed units fitted in the seventies and eighties are frequently imperial, so a kerb might measure 762mm or 1219mm rather than 750mm or 1200mm. Three routes exist. Order a made to measure unit to the exact kerb, which is usually cleanest. Re-form the kerb to the nearest standard size, which is quick where the timber is sound. Or fit an adaptor kerb over the existing one, which raises the unit and is worth checking against the roof’s drainage before committing. Replacing a unit into a sound kerb is often a single day.

Repeating one size rather than ordering three

Where a roof takes more than one rooflight, ordering the same size more than once is worth real money and real time.

One glass make-up, one frame batch, one delivery, one set of kerbs built to identical dimensions with the same templates. It also looks better. Three identical squares spaced evenly down an extension read as a deliberate rhythm, while three different rectangles read as a series of decisions taken separately. The exception is where one opening genuinely serves a different purpose, an opener over a cooking area for instance, in which case the difference should be obvious rather than nearly the same.

Locking the size down before the roof is built

The cheapest moment to change a rooflight size is on a drawing. The most expensive is after the deck is down.

Two decisions want fixing early. The kerb size, because the carpenter frames to it, and the position along the roof, because it interacts with joist runs, any steel, and where the light actually lands in the room below. Both are settled at survey, before an order goes anywhere. Everything after that is a matter of glass specification and a delivery date, and neither of those needs the roof to be open while it is decided. Typical ranges and what moves them are covered under costs.