Flat Rooflight Sizes

Chapter 1

Which measurement a rooflight size actually describes

Flat rooflight sizes are almost always quoted as the internal dimensions of the kerb, which is the hole you see from the room below, not the size of the glass or the overall unit. Getting that one point straight avoids most sizing mistakes.

A flat rooflight sits on an upstand, or kerb, built on the roof around the opening. When a maker lists a "1000 x 1000" unit, they normally mean the kerb measures 1000 mm by 1000 mm on the inside. That is also the size of the ceiling opening, so it is the number that decides how big the light well looks from the kitchen or the landing. The glass and frame are larger, because they have to sit on or over the kerb walls.

Three sizes are in play on every job, and they are worth keeping separate:

• The builder's opening, or kerb internal size. The clear hole through the roof and ceiling. This is the size we plan the light around.
• The kerb external size. The internal size plus the thickness of the kerb walls on each side. With a timber kerb and insulation this can add 150 mm to 250 mm in each direction, and it decides how much roof covering has to be turned up and how close the unit can sit to a wall or parapet.
• The overall glass or frame size. A framed unit's frame laps over the kerb; a frameless unit's glass oversails it so rain drips clear of the joint. Each maker publishes its own offset, so the outside dimension of two units sold as the same size can differ.

There is also the visible glass, the daylight opening, which is what actually lets light through. On a framed unit the frame covers part of the kerb and can eat into the view. On a frameless unit the glass is bonded on top, so almost the whole kerb opening is glazed. Our guide to flat rooflight upstands explains how the kerb is built; here the point is simply that the number on the quote should say which size it means.

Chapter 2

The standard sizes you will be offered

Off-the-shelf flat skylight sizes run in steps of 100 mm to 500 mm, from small squares for a landing or shower room up to long rectangles for a kitchen extension. Standard sizes are quicker to supply and have tested seals and fixings for exactly that pane.

Ranges differ between manufacturers, but the same family of sizes turns up across the market. The table below groups them by the kind of space they tend to suit. Treat it as a starting point for a conversation, not a prescription: the room below always has the final say.

Many ranges also include 600 x 900, 800 x 1200, 900 x 900 and 1200 x 1800, and a few go up in 50 mm steps. Opening versions usually come in a narrower band of sizes than fixed ones, because the hinge, the actuator and the weight of the moving sash limit what is practical. If you want ventilation as well as light, check the opening range first, then plan the room around it. Our guide to fixed and opening flat rooflights covers which rooms need that.

Standard sizes have one quiet advantage in Cambridge extensions. Flat roof joists are usually at 400 mm or 600 mm centres, and a designer who knows the unit early can set out the joists so that one side of the opening lands on an existing joist. That reduces the amount of trimming and keeps the ceiling line tidy.

Chapter 3

When made-to-measure is the better answer

Made-to-measure units are built to the kerb you have or the kerb you need, in almost any rectangle within the maker's glass limits. They cost more and take longer to arrive, but they are sometimes the only way to get the light where it belongs.

There are four situations where we would usually steer towards a made-to-measure flat rooflight rather than forcing a standard size to fit.

1. Replacing an older unit on its existing kerb. Older rooflights and domed units were often fitted to kerbs of odd sizes. If the kerb, trimming and ceiling are sound, a unit made to that exact opening avoids cutting the plaster and joists again. If the kerb is too low or poorly insulated, it is usually better to rebuild it and then choose the size.
2. A narrow side return. On a Romsey or Mill Road terrace, the side return roof may be only 1.8 m to 2.5 m wide once the walls are counted. A standard 1000 mm wide unit can leave awkward margins, while a made-to-measure 1200 mm or 1300 mm width fills the space properly.
3. Lining up with the kitchen plan. If the island is 2400 mm long, a 2400 mm rooflight above it reads as one designed piece. A 2000 mm unit looks almost right, which is often worse than obviously different.
4. A long run along a wall. When you want a band of light down a party wall or a rear wall, a single long pane or a set of joined modules is usually made to the length of the room. The guide to linear and modular rooflights goes into how those systems join.

Made-to-measure does not mean unlimited. Every maker sets a minimum and maximum size for each product, and a maximum ratio between the long and short sides, because a very long, thin pane flexes differently from a squarer one. We check those limits against the room before anything goes on the quote.

Chapter 4

How big a single pane can go

The largest single flat rooflight panes are limited by glass weight, deflection, the glass maker's production sizes and how the unit will be lifted onto the roof. Fixed frameless units go biggest; opening units and walk-on glass have tighter limits.

As a guide to what the market offers, many fixed frameless ranges run up to around 3000 mm by 1500 mm as standard, and specialist structural glazing makers go well beyond that, into panes of four metres or more, with engineered support. Framed aluminium units with glazing bars can cover larger areas again, but the bars split the view. Opening units are usually smaller, because the sash and its motor have to lift the whole weight of the glass, and the heaviest opening units need powerful actuators and a strong, square kerb.

Glass weight is worth understanding because it drives everything else. Float glass weighs about 2.5 kg per square metre for every millimetre of thickness. A typical double glazed rooflight with a toughened outer pane and a laminated inner pane has around 14 mm to 17 mm of glass, so roughly 35 kg to 45 kg per square metre. Triple glazing adds another pane and pushes that towards 50 kg or more. A 1000 x 2000 unit can therefore carry 70 kg to 100 kg of glass before the frame is counted, and a 3000 x 1500 pane several hundred kilos. That weight has to reach the roof, be set down on the kerb without twisting it, and be carried by the structure for the life of the building.

Deflection is the other limit. A long pane spanning a wide opening bends slightly under its own weight, snow and wind. The glass is thickened as the span grows so the bending stays within what the edge seals will accept. That is also why the overall depth of a very large unit is greater; the answer on how thick a flat rooflight is covers the knock-on effect on the upstand and reveal.

Chapter 5

Sizing to the room below, not to the roof above

A rooflight is the right size when it lights the part of the room you use, at the level of light the room needs, without overheating it. Floor area gives the first figure; ceiling height, depth of plan and existing windows adjust it.

A commonly used starting point is that roof glazing should equal somewhere around 10% to 20% of the floor area it serves, at the upper end for deep rooms with little other glazing and at the lower end for rooms with big garden doors. So a 4 m by 5 m kitchen extension of 20 square metres might take 2 to 4 square metres of rooflight: one 1000 x 2000 unit, or a pair of 1000 x 1500 units. The answer on what size rooflight a kitchen extension needs works through that arithmetic in more detail.

That rule of thumb is only the first pass. Four things move it, and they are the things we spend the survey on:

• How deep the plan runs. A long terrace kitchen with glass doors at one end is bright near the garden and dim at the back. A modest rooflight in the dark half does more than a large one near the doors.
• Ceiling height and reveal depth. Light spreads out as it drops. In a room with a 2.4 m ceiling a 1000 mm square rooflight lights a patch of floor noticeably larger than itself; with a deep warm deck roof and a tall upstand the light well is deeper and the spread narrower. Splaying the reveal widens the spread again without a larger unit.
• Which way the room faces and what shades the roof. Cambridge is flat and open, so a rooflight here sees a lot of sky. That is good for daylight but means a big pane admits a lot of summer sun, which is why size and glass are chosen together.
• How the room is used. A home office needs even, glare-free light on a desk; a dining table can take a pool of sun; a TV wall wants none on the screen.

Where one large pane would put all the light in one place, two or three smaller units spread it more evenly. Our guide to planning multiple flat rooflights covers spacing and rhythm, and the guide to positioning a flat roof skylight covers where each one should sit.

Chapter 6

Bigger glass and the summer sun

Doubling the area of a rooflight roughly doubles the solar heat it lets in on a clear day, and a near-horizontal pane faces the high summer sun almost square on. Size is the first heat control, before any coating or blind.

On a flat roof in Cambridge, the midsummer noon sun stands about 61 degrees above the horizon and strikes the glass at a steep angle for much of the day. In winter the noon sun is only around 14 degrees up, so the same rooflight admits much less direct sun when you would actually welcome it. A large clear pane can therefore be pleasant from October to March and hard work in June and July. The way we plan the light sets out how we weigh that for each room.

Glass choice then fine-tunes what size has set. Solar control coatings cut the share of solar energy that passes through while keeping most of the visible light, and triple glazing improves the insulation on a winter night. The guide to flat rooflight glazing choices goes through the options for near-level glass.

Chapter 7

What size means for the roof structure and the rules

Size decides how many joists are cut, how the opening is trimmed, whether an engineer is needed, and how the rooflight counts towards the Building Regulations glazing allowance for an extension.

Joists at 600 mm centres leave only about 550 mm clear between them, and at 400 mm centres about 350 mm. Almost every useful rooflight is wider than that, so joists are cut and the opening is trimmed with doubled joists and trimmers carried on hangers. A modest opening can usually be trimmed to a standard detail. A wide opening across a long span, or several openings close together, generally needs a structural engineer to size the new members. The answer on whether a flat roof needs strengthening for a rooflight sets out where that line usually falls.

Size also bumps into three regulatory limits:

• Glazing area in extensions. Approved Document L sets a limit on the total area of windows, roof windows, rooflights and doors in an extension, generally 25% of the extension's floor area plus the area of any openings the extension covers up. Go over it and the design has to be shown, by calculation, to perform no worse overall. A generous rooflight plus wide sliding doors can reach that limit faster than people expect.
• Distance from a party wall. Fire guidance keeps rooflights out of a 1500 mm band either side of a compartment or party wall. On a narrow terrace roof this can set the maximum width before anything else does. See how close a rooflight can be to a party wall.
• Height above the roof. Where a rooflight is added to an existing roof under permitted development, it should not project more than 150 mm above the roof plane. A larger unit is usually deeper, so the upstand and glass together need checking against that figure.

We install to current Building Regulations and handle the Building Control notification where the work is notifiable, so these checks are part of setting the size, not an afterthought.

Chapter 8

Measuring for a replacement flat skylight

When you replace an older unit, measure the inside of the existing kerb, not the old dome or frame. That internal size, plus the kerb height and condition, tells you whether a standard unit fits, a made-to-measure one is needed, or the kerb should be rebuilt.

From inside the room you can measure the ceiling opening, which is usually close to the kerb internal size once any plaster reveal is allowed for. From the roof, lift the edge of the old unit's flashing only if it is safe to do so; otherwise leave it for the survey. What matters is:

• the internal width and length of the kerb, in millimetres, measured at the top;
• whether the kerb is square, which you check by measuring both diagonals;
• the kerb height above the finished roof surface, since anything under 150 mm is usually rebuilt to current guidance;
• whether the kerb is insulated, because an uninsulated timber kerb is a cold bridge around the new glass.

Older domes and plastic units were often fitted on low, bare timber kerbs. Replacing only the top leaves the weak part in place. In that case we usually rebuild the kerb to the new unit's size and a proper height, which also gives the chance to adjust the size slightly if the light plan calls for it. The parent guide to flat roof rooflight installation in Cambridge covers what the whole replacement involves.

Chapter 9

Questions people ask

Is the size on the quote the glass size or the hole size?

On a well-written quote it is the kerb internal size, which is the hole in the roof and ceiling. Our quotes state which dimension is meant, and the written light plan that comes with every quote shows the size against the room.

What is the smallest flat rooflight worth fitting?

Standard ranges start at around 600 mm square. That is enough to lift a small shower room or WC, but in a larger room a unit that small tends to light a narrow column rather than the space. For a hallway or landing, 800 mm to 1000 mm square is a more useful starting point.

Can a flat rooflight be bigger than the gap between two joists?

Yes, and almost all of them are. The joists are cut and the opening is trimmed so the load is carried round it. The larger the opening, the heavier the trimming, and above a certain size an engineer specifies it.

Is one large rooflight better than two smaller ones?

Not by default. One large pane gives a strong, open view of the sky; two smaller units spread the light more evenly, admit heat in two smaller patches and are easier to lift and trim. Which suits you depends on the room's depth, orientation and use. If you would like that worked out for your own room, book a light survey and we will set the size, glass and position out in writing before anything is cut.