Flat Rooflight Sizes: Getting the Opening Right

Three measurements, one rooflight

First, the glass. This is the clear area you see from inside, and it decides how much daylight arrives. Second, the outer frame, which is wider than the glass because it includes the frame profile and the lid. Third, the structural opening, the hole formed in the roof deck and ceiling. It must be the right size for the kerb the rooflight sits on, not simply the glass.

When we survey, we write all five on the quote, so the supplier, the carpenter and you are looking at the same numbers.

Standard or made to measure

Most makers offer a range of fixed sizes in squares and rectangles, plus made-to-measure sizes within limits. Standard sizes are quicker to supply and easier to price, and where the existing hole is close to a standard size it is usually sensible to adjust the hole rather than order something special.

Made-to-measure rooflights have their place. A long, narrow opening in a hallway ceiling, or a unit sized to sit centrally over a kitchen island, may be worth the extra wait. The glass is cut to order and the lead time is longer, so decide early, and measure twice before the order goes in.

Choosing a size for the room

A common starting point is to think of the rooflight as a window you are laying flat. Daylight through a roof is stronger than through a wall, because the sky is brightest overhead. A rooflight can therefore be smaller than a wall window and still light a room well.

• A narrow hallway or landing is helped by a modest unit placed to light the middle of the space.
• A utility room or cloakroom rarely needs more than one small unit.
• A kitchen extension deepens a long way from the garden doors, and a rooflight placed at the back lights the gloomy half.
• A living space with a large rooflight should consider a blind or solar control glass.

The key point is position. A rooflight over the spot where people sit, cook or work gets used every day. One placed over a corner or a built-in unit looks bright from outside and does little within.

What the joists allow

The roof structure sets practical limits. Joists in a flat roof run in one direction, usually from the back wall of the house across to the outer wall. An opening that fits between two joists needs little structural work. A wider opening cuts one or more joists, and the cut ends must be carried by trimmers, short beams fixed across the gap.

The result is that a rooflight sized to the joist spacing is quicker and tidier to fit. Even a modest change, such as a unit slightly narrower to sit between joists rather than cutting one, can make the structural work simpler. We tell you where this applies when we survey, with a note of what a small change in size would save.

If the structure is unclear, the survey looks at it from inside the room and, where there is access, from the roof. Cutting a fresh hole in the roof is a matter for Building Regulations, which look at the framing and at heat loss, and the dimensions we write on the quote are the ones the drawings use.

How big can a flat rooflight go

The limit is not just the glass. A large rooflight is heavy, and the structure around it must carry both the weight and the wind load that lifts at the edges. Larger units need deeper kerbs, stronger trimmers and sometimes a steel beam. For a very wide opening, the roof deck may need to be supported differently.

Where several rooflights are wanted, a pair of mid-sized units is often easier to build in than one very large one, and the beam between them keeps the roof stiff. That also gives you the option of opening one and leaving the other fixed. If the plan calls for something much larger than ordinary, such as a long run of glass, it moves from a rooflight job towards a structural one, and we say so at the survey rather than after the order.

Size and heat

A bigger rooflight gives more light, and in summer it also gives more heat. In winter it loses more heat than the insulated roof beside it, because glass is never as warm as a well-insulated roof. The glazing specification can offset some of that, with triple glazing for warmth or solar control coating for sun. Our flat rooflight glazing options guide compares them.

Under Part L 2021 for existing homes, rooflights should achieve a U-value of 2.2 W/m²K. Roof windows, which are the pitched kind, are held to 1.4 W/m²K or Window Energy Rating Band B. It is the glazing specification that meets these figures, not the size, but a larger unit makes the specification matter more.

North-facing rooms rarely overheat, and can take generous glass. South and west-facing rooms are the ones that reward a more considered size.

Sizing for a kitchen, hall or bathroom

Rooms ask different things of a rooflight, so a useful way to settle a size is to start from the room's job. A kitchen worktop wants even, shadow-free light, and a rooflight whose long side runs parallel to the run of units tends to give it. Over a dining table, a squarer unit centred on the table feels natural, and the light falls on faces rather than on the floor beside them.

A bathroom or shower room is the opposite case. The space is small, the light is wanted but the privacy is too, and a single compact unit with frosted glass is generally enough. In a hallway, where the aim is to pull daylight down a dark corridor, a long and narrow opening placed along the line of travel does more than a square one of the same area.

Think too about what will be seen from the room above, if there is one. A rooflight on an extension roof below a bedroom window is looked down on every morning, so the upstand, the covering and the glass edge are part of the view, and a neat, plain unit is better than a large one that dominates it.

Allowing for blinds, vents and lead times

A size is not final until the extras are settled. A blind made for the rooflight is cut to the glass dimensions, so the blind is ordered with the unit and not afterwards. If you want the rooflight to open, the frame and the motor or hand mechanism need a little more room inside the opening, and the hinge side has to be chosen so the open lid does not catch a parapet or a neighbouring wall.

Lead time depends on the size. Standard units are often held by suppliers, while larger or made-to-measure glass is built after the order, and it is worth asking at the survey how long your specific size will take so the building work can be timed around it. A hole formed in a roof should never wait weeks for glass, so the order is placed first and the roof is opened when the delivery date is firm.

Size and the planning rules

On a house, a rooflight usually needs no application when it stays within the permitted development limits, which concern how far it stands proud of the roof and how high it sits. The area of glass is not the test. The height above the roof plane is. Flats and maisonettes have no permitted development right for this, and a listed building needs consent, whatever the size.

The skylight planning permission guide gives the conditions in one place, and if your street falls inside a conservation area, ask the council whether an Article 4 direction takes the right away there.

Measuring a roof for yourself

You can gather enough for a first conversation without a ladder. Inside, measure the room and mark with masking tape on the ceiling where you imagine the light falling. Stand beneath it and look at how the furniture sits. Note where the ceiling lights are, and whether there are pipes or ducts above.

Outside, from the ground or an upstairs window, look at the roof edge, the parapet or the way the roof meets the wall. Note any trees overhanging the roof, as leaf and sap collect on glass beneath them. Take a few photos in daylight.

Please do not climb onto a flat roof to measure it. Felt, rubber and fibreglass are slippery when wet, and the edge is often hidden. The survey is free, we have the right equipment and access, and the measurements that matter are the ones taken carefully at the opening itself.

Common sizing mistakes

Several errors come up often enough to be worth naming. The first is ordering from the glass size alone and discovering the frame needs a bigger kerb. The second is choosing a size on a drawing without checking where the joists fall, which turns an easy opening into one needing a new beam. The third is forgetting the ceiling below: a rooflight that looks well on the roof can land across a lighting circuit, a boiler flue or a stud wall in the room beneath.

Smaller mistakes are about proportion. A unit that is too big for the room can dazzle, and one that is too small is a disappointment you live with for years. If you cannot decide, tape a rectangle to the ceiling at the size you are considering and look at it for a day, in morning and evening light.

What the survey settles

At the survey we check the joists, the roof build-up, the position of services and the height available inside. We agree the size with you against the room, then put it on the quote, priced line by line, beside a plan for each day of the fit. We also say where a small change in size would save structural work.

If you are comparing sizes against price, see our flat rooflight cost breakdown. If the roof is already built and you are adding a rooflight to it, adding a rooflight to an existing flat roof covers the sequence. Everything about the service lives on the flat roof rooflights page, and you can book the survey with the form below.