Flat rooflight sizes and standard openings

Start here, because almost every ordering mistake traces back to this. When a manufacturer lists a flat rooflight as “1000 by 1500”, that number could refer to any of three things depending on whose catalogue you are reading.

The first is the structural opening, sometimes shortened to SO. This is the hole framed into the roof deck, measured as the clear gap between the timbers or the internal face of the builder’s upstand. It is the number that matters most, because it is the one the roofer builds to and the one most reputable makers ask you to quote when you order.

The second is the kerb or upstand size. Flat rooflights do not sit flush in the roof. They sit on a raised timber kerb that lifts the glass above the water line, and the rooflight either sleeves over the outside of that kerb or drops inside it. So the external kerb dimension can be 150mm larger or smaller than the structural opening, depending on the detail.

The third is the daylight opening, also called the clear glass or visible glass size. This is the pane you actually see light through once the frame is in. It is always the smallest of the three, because the frame eats into the edge. On a typical flat glass rooflight the frame borrows somewhere between 80mm and 130mm off each side, so a unit ordered to a 1000 by 1500 structural opening might give you a visible glass area closer to 800 by 1300.

None of this is exotic. It is the same logic as a door, where the frame, the lining and the leaf are all different widths. But rooflight catalogues are inconsistent about which number they lead with, so the safe habit is to ask one question every time: is that figure the structural opening, the kerb, or the glass?

Most manufacturers hold a range of stock sizes that ship in days rather than weeks and cost a good deal less than a bespoke unit. The exact grid varies from one maker to the next, but the market clusters around a recognisable set of dimensions. The figures below are structural opening sizes in millimetres, which is the way the bulk of the industry quotes flat rooflights.

Read the glass area column rather than the dimensions, because area is what governs both the daylight you gain and, on a sunny plane, the heat. A 1000 by 1500 opening lets in roughly six times the light of a 600 by 600, and on a south-facing roof it collects roughly six times the solar energy as well. Doubling the length of one side does not double the effect. Doubling both sides quadruples it.

Above about 1500 by 2500 in a single fixed pane, weight and handling start to dictate the answer. Double-glazed rooflight units weigh in the region of 25 to 30 kilograms per square metre, so a 1500 by 2500 pane is carrying something over 100 kilograms of glass before you add the frame. Beyond that size the sensible route is either two units side by side with a slim structural bar between them, or a stepped or pitched glass lantern that spreads the load, rather than one enormous flat sheet.

If there is already a hole in the roof, or the roofer has framed one, measure the clear internal gap in both directions at more than one point. Roof timbers are rarely perfectly square, and a 5mm difference corner to corner is enough to make a tight-tolerance unit sit badly. Take the smallest reading in each direction as your structural opening.

If you are working from a room that does not yet have an opening, decide the glass area you want first, then work outwards. Add the frame allowance to reach the daylight opening you need, then confirm the structural opening the maker wants for that model. Do not measure the ceiling aperture and assume it equals the roof opening. On a flat roof the ceiling hole and the roof hole are usually the same, but where there is a deep void or a splayed reveal the two can differ, and the roof opening is the one that counts for ordering.

One practical point specific to the flat roofs common on south Essex rear extensions. A great many of the 1930s semis across the patch have had a flat or shallow single-storey extension added at the back, and the roof build-up on those is often warm-deck construction with insulation above the deck. That raises the finished roof level, which affects how tall the kerb needs to be and how far the rooflight will project. Measure to the finished roof surface, not the structural deck, or the kerb height will come out wrong.

A flat rooflight is never truly flat to the roof. It sits on a kerb, and understanding the kerb is half of understanding the size.

The kerb is an upstand, usually built in timber, that raises the glass above the surrounding roof surface so that rainwater draining across the roof cannot reach the glass seal. A standard builder’s kerb is around 150mm tall. Some manufacturers supply the rooflight with its own insulated kerb built in, ready to fasten straight to the roof deck, which takes the guesswork out of the height and the insulation detail. Others expect the roofer to form the timber kerb on site, and the rooflight is then ordered to sleeve over it.

This is where the structural opening and the ordering size diverge, so it pays to be precise about the model. On a sleeve-over unit, the rooflight is made larger than the kerb so it can drop over the outside, and you order to the external kerb dimension. On a sit-in unit, the rooflight fits within the opening and you order to the internal size. Ordering a sleeve-over unit to an internal dimension, or the reverse, is one of the more expensive ways to get sizing wrong, because the glass is bonded into the frame and the unit cannot be trimmed on site the way a timber window sometimes can.

Kerb height also feeds into the permitted-development question. A rooflight on a house is generally permitted development only if it projects no more than 150mm beyond the plane of the roof. On a flat roof the entire kerb plus rooflight height counts as projection, so a tall kerb with a domed unit on top can push past that limit and, in some cases, tip a job into needing a planning application. This is general guidance and the local authority is the place to confirm it, but it is worth knowing before you specify a deep kerb for the sake of extra headroom in the glass.

What the rooflight does affects how big it can sensibly be.

Fixed flat rooflights are the simplest and go the largest, because there is no mechanism to carry the weight of moving glass. This is the type that reaches the 1500 by 2500 single-pane sizes in the table above.

Opening rooflights, whether hinged, sliding or electric, have a practical ceiling well below that. Once glass has to move, the actuator, the hinges and the frame all have to carry it, so most opening flat rooflights top out around 1200 by 1200 to 1200 by 2000 as a single leaf. Manual opening units, worked by a winder or a pole, sit at the smaller end because a person has to physically shift the weight. Where a room has a high ceiling or a large opening rooflight is wanted for ventilation, an electric opening unit carries the glass on a motor and removes the reach problem entirely.

Walk-on and access rooflights are their own category. Walk-on glass, designed to be stood on as part of a terrace or a light well, is thicker, heavier and made to a tighter set of standard sizes, commonly stepping up from around 600 by 600 to 1200 by 2000. Access hatches that a person climbs through to reach a flat roof follow yet another size grid, driven by the clear opening a body needs rather than by daylight.

If ventilation matters to you, size the opening for the airflow rather than the view. An opening rooflight at the top of a room clears warm stratified air far faster than a window at head height, because hot air collects at the ceiling and the rooflight sits exactly where it wants to escape. A smaller opening unit paired with a larger fixed pane often serves a room better than one big unit trying to do both jobs.

The temptation is to make the rooflight as large as the roof allows. Sometimes that is right. Often it is more than the room needs and more heat than the room can shed.

A useful rule of thumb for a habitable room is a glazed area of somewhere between 15 and 20 per cent of the floor area for good daylight, and a rooflight punches well above a wall window at the same size. Because a flat rooflight faces straight up at the whole sky dome, it collects more daylight per square metre than a vertical window, so you generally need less rooflight than you would guess. A 1000 by 1500 unit, a little over a square metre of glass, transforms the light in a room of 12 to 15 square metres. Two of those over a larger open-plan kitchen-diner usually does more, and more evenly, than one oversized pane in the middle.

Spreading the light matters as much as the total area. Two medium rooflights placed apart wash a room in even daylight, where a single large one throws a bright pool under itself and leaves the corners darker by contrast. For a long galley layout, a run of three smaller units down the length beats one big square in the centre.

Position also decides how the light lands. A rooflight set towards the outer wall of an extension throws light back into the depth of the room, reaching the older part of the house behind. One set in the middle of the roof lights the middle of the room and can leave the back wall, often the one shared with the original building, in shadow.

On a north-facing flat roof, size is almost a free choice: more glass means more welcome daylight and very little heat to manage. On a south-facing flat roof in Southend, size is a decision with consequences, and it is the reason we specify the glass before we agree the dimensions.

Southend-on-Sea faces south across the Thames Estuary, with an open horizon and reflected light coming off the water onto every south-facing roof plane along the coast. A flat rooflight faces straight up, which means at midsummer it collects close to twice the solar energy per square metre of a vertical south-facing window. Combine the aspect, the open sky and the upward-facing geometry, and a large flat rooflight over a south-facing rear extension is, in physical terms, a solar collector with a dining table under it.

This does not mean fitting a small one. It means matching the glass to the area. The larger the pane on a sunny plane, the more the G-value of the glass matters, because the G-value is the share of the sun’s energy the glass lets through, and it is multiplied by every square metre you install. Clear double glazing runs a G-value around 0.6. A good solar-control unit brings that down to roughly 0.3 while keeping the room bright, so it takes out more than half the heat without dimming the daylight. On a 1.5 square metre south-facing rooflight, that difference is the heat of several people, arriving all afternoon at ceiling height. Size and glass are one decision, not two.

For roofs that want more glass than a single flat pane sensibly allows on a sunny aspect, a pitched glass roof lantern spreads the area across sloping faces that catch the sun at kinder angles through more of the day, which can be an easier plane to manage than one large flat sheet pointing straight at the noon sun.

A stock size is cheaper, quicker and easier to replace years later, so if a standard opening suits the room there is rarely a reason to pay for bespoke. The case for a made-to-measure unit is when the structural opening is fixed by the building, when you want a glass area no stock size reaches, or when a slim sightline or a particular frame finish matters to the design. Bespoke flat rooflights are made to your exact structural opening, so the measuring has to be right first time.

Whichever route you take, the size interacts with the regulations. New and enlarged rooflights are notifiable under Building Regulations. Part L sets the thermal standard the glazing has to meet, and Part K governs safety glazing, which for anything overhead means a laminated inner pane so that a broken unit cannot fall into the room. A larger opening also means a larger hole cut in the roof structure, which is where the structural question comes in: spanning a wide opening can call for a trimming beam or additional support, and that has to be worked out before the roof is cut, not after. Every installation we carry out is built to current Building Regulations, and we make the Building Control notification to Southend-on-Sea City Council, or the relevant local authority for your address, on your behalf.

If you are weighing up sizes for a flat roof, the practical order is: settle the glass area the room needs, check the aspect and pick the glass to match, confirm the structural opening the roof can carry, then choose a standard size if one fits and go bespoke only if none does. The full picture of what the work involves is on the flat roof skylight installation page, and the fastest way to get the right numbers for your own roof is to ask for a specification with the structural opening, the glass area and the G-value written down for each unit.