What an oversized skylight feels like from inside
An oversized skylight rarely looks wrong on the drawing. The trouble shows up once the room is lived in, on a bright afternoon in June or a clear morning in February.
The signs are fairly consistent. The room is pleasant at breakfast and uncomfortable by mid afternoon, because a large horizontal pane keeps collecting sun for hours. A bright square of sunlight sits on the floor or the table and moves across it, so someone is always squinting or shifting a laptop. The kitchen reflects that brightness off gloss worktops and cabinet doors. At night the glass turns into a large dark panel overhead, and if an upstairs window next door looks down onto the roof, the room can feel on show in both directions. In winter a very large pane can also feel cool to sit under, because even good double glazing loses more heat than the insulated roof it replaced.
None of this means large glass is a mistake. It means the size has to be chosen alongside the direction the roof faces, the use of the room and the glass specification, rather than on its own.
Why flat roof glass gathers so much summer heat
A flat rooflight faces straight up, so it meets the high summer sun almost square on, while a wall window in the same room sees that sun at a glancing angle.
Cambridge sits at about 52 degrees north on flat land with very little to shade a roof. The table gives the approximate height of the sun above the horizon at solar noon over the city through the year.
| Date | Noon sun height | What a flat skylight gets |
|---|---|---|
| Mid December | About 14° | Low, weak sun; mostly sky light |
| March and September | About 38° | Moderate sun, low enough to glare |
| Mid June | About 61° | High sun striking the glass nearly head on |
That June figure is why a big flat skylight over a south-facing kitchen extension in, say, a Romsey terrace or a newer home at Trumpington Meadows can overheat, while the same area of glass in the rear wall does not. Heat arrives as sunlight falling on the floor, worktops and furniture, which then warm the air. The larger the pane, the more surface gets that direct sun, and the longer it lasts through the day. Our guide to which way a skylight should face covers how north, east, south and west slopes change this picture.
Glare and privacy grow with the glass
Glare is a contrast problem: a very bright area in your line of sight next to something much darker. A bigger opening makes that bright area bigger.
Under a long skylight in a side-return extension the sun patch can cross the dining table, the sofa and a television wall in a single afternoon. In a home office, a large pane above the desk puts light straight onto the screen. The fix is often to move the glass rather than shrink it, so the direct sun lands on a floor or a wall no one faces. Our answer on how to stop glare from a skylight goes through placement, glass and blinds in more detail.
Privacy works the same way. On tight Cambridge plots, where neighbouring bedrooms can look down onto a single-storey roof, more glass means more of the room is visible from above. See our guide to rooflight privacy and overlooking for glass and layout choices that help.
Rules of thumb, and why they are only a start
Sizing rules of thumb are useful for a first sketch, but they are not regulations and they do not know which way your roof faces.
Roof lantern suppliers commonly suggest a lantern of roughly 15 to 20 per cent of the flat roof area as a starting point. Treat that as a rule of thumb from the trade, not a standard. It says nothing about orientation, ceiling height, how deep the room is or whether the space is a kitchen, a playroom or a bedroom. A north-facing room in a deep terrace may well take more glass comfortably; a south-facing garden room with glazed doors on two sides may need less. Our page on how much daylight a room needs explains what different rooms ask for.
The better question is what the light will do in the room at different times of year. That is the basis of the light plan that comes with every quote: position, size, glass and shading, with the reason for each.
What the Building Regulations say about large glazing
Building Regulations do not set a maximum skylight size for a room, but two parts are relevant when the glass gets large.
Part L deals with heat loss. A new or replacement rooflight in an existing home has to meet a limiting U-value, currently 2.2 W/m²K measured in the horizontal plane. For extensions, the Approved Document also gives a reference amount of glazing relative to the extension's floor area, and larger glazed areas can usually still be accepted if the designer shows by calculation that the extension performs no worse overall. Your designer or Building Control will confirm how that applies to your plans.
Part O deals with overheating, but it applies to new dwellings, not to most extensions to existing homes. So an extension on a 1930s semi in Cherry Hinton will not normally be assessed for overheating under Part O, even though the heat risk from a large south-facing rooflight is just as real. It is sensible to design against overheating whether or not the regulations require it. Our guide to Building Regulations for rooflights covers Parts L, K, B, F and A, and we handle the Building Control notification where the work is notifiable.
How to keep a large rooflight comfortable
If a room genuinely needs a lot of daylight, there are ways to have large glass without the heat and glare.
- Solar control glass with a lower g-value lets through daylight while reflecting more of the sun's heat. See our energy efficient rooflights for the specifications.
- Two smaller units instead of one large one spread the light more evenly and let each sit over the part of the room that needs it.
- An opening rooflight lets warm air out at ceiling level, where it collects on a hot day.
- A blind, ideally one that stops heat before it reaches the room. Our answer on whether a skylight needs a blind sets out where one helps most.
For a lantern over a kitchen or dining space, our roof lantern installation page shows how size, glass and position are chosen together.
Questions people also ask
Is a small skylight a safer choice?
Not always. A rooflight that is too small for a deep room leaves the far end dark and can create its own contrast problem, with one bright square on a dim ceiling. The aim is the right size for the room and the slope, not the smallest possible opening.
Can I swap an oversized skylight for a smaller one?
Usually, though reducing an opening means reforming the roof structure and finish around it, so it is a larger job than a like-for-like swap. Changing the glass to a solar control specification can sometimes solve the problem without changing the size.
Does a bigger rooflight make a room colder in winter?
It can make the space under it feel cooler, because glazing loses more heat than an insulated roof. Modern double or triple glazing with a warm edge and a well insulated reveal keeps that effect small.
What if I want to go bigger when replacing an old unit?
Upsizing is often possible, with the structure, flashing and approval to check first. Our answer on replacing a skylight with a bigger one explains what is involved, and the Vision Skylights homepage sets out how we plan the light before any roof is cut. To see what size suits your room, book a light survey.
