How big can a flat rooflight be?

Manufacturers publish a range of standard sizes for each model, and a bespoke maker will go beyond those. So the limit for a flat roof rooflight is not set by the word flat or by the building rules. It comes from four practical things.

• The glass. Large panes are heavy and need to be lifted onto the roof. A glazed unit weighs a good deal more than the same area of roof, and the heavier it is, the stronger the frame and the kerb must be.
• The span. Glass and frames can only cross a certain distance without support. Beyond that, the rooflight is made in several panes with a bar between them, or in a rigid frame with extra bearing.
• The roof structure. The opening removes part of the deck and some joists, and the rest must carry the load around it.
• The access. A big unit has to get to the site, up to the roof and into place. Narrow side passages and terraces make this harder, and some large units are craned on.

For a given size, a maker can tell you whether the unit is a standard product or made to order, and how long that takes. We confirm that on the survey and put it in the quote.

A fixed rooflight can go biggest, because it has no moving parts to limit the span. An opening one is smaller, because the sash has to be lifted by hand or by a motor, and the weight of the sash sets how large it can be. Beyond a certain size, the sash is lighter in glass and the surrounding frame stays fixed.

If you want a large opening that you can walk through, such as a sliding or hinged access to a roof terrace, that is a different product with its own size range. See our fixed or opening flat rooflights guide for the trade-offs between the two.

Glass type matters too. Triple glazing is heavier than double, and toughened or laminated panes weigh more than plain. A rooflight that needs laminated glass for safety above a seating area weighs more than the same size in plain glass. So safety and energy choices can bring the maximum size down, and the quote shows the final specification so you know what you are getting.

The deck and joists under a large rooflight carry more than the weight of the glass. They also carry the kerb, the finishes, snow when it falls and the people who go onto the roof for maintenance. The wider the opening, the more joists are interrupted, and the harder the remaining ones work.

For a small opening, a pair of trimmers across the gap is enough. For a wide one, the new trimmer beams may need to be larger, sit in hangers or bear on a wall plate, and in some cases a structural engineer calculates the sizes. This is part of why cutting a roof opening is controlled work, and we handle the notification where it applies.

Walls under the roof matter as well. A large opening near the edge of the roof loads the wall below it. On older houses and extensions, the lintels and the wall itself are checked before a big opening goes in.

In practice, the structure often settles the size. A rooflight the width of two joist bays is a modest job. One that crosses four or five needs engineered beams, and the price and the programme change. That is better known in advance, which is why the survey looks above the ceiling and not only at the glass.

There is no daylight minimum for a house in the Building Regulations, so size is a design choice. Bigger lets in more light, and under a grey sky that is useful, because the sky is soft and comes from overhead. A wide rooflight over the deepest part of the room gathers more of it.

The cost of size is heat. Glass loses heat faster than an insulated roof, so a very large rooflight in winter means a colder surface above your head, more risk of condensation on the inner pane and more care with the glazing. The limit for rooflights in existing homes in Approved Document L is 2.2 W/m²K, and good triple glazing sits well inside it. Summer brings the opposite, which is solar gain, so coated glass or blinds help.

A sensible rule is to size the glass to the area you want lit and the budget for the roof beneath it, and not to the biggest unit the maker sells. Two moderate rooflights in a row often light a long room more evenly than one huge one, and each is easier to build.

For a house, permitted development rules set limits on how far a rooflight may stand above the roof, and large units on tall kerbs are where that is tested. Flats, listed buildings and homes under an Article 4 direction are handled differently, and in a conservation area we first check whether an Article 4 direction applies. Our planning permission for rooflights page goes through those cases.

Cutting a new opening in a roof is also controlled work under the Building Regulations, and a large one is more likely to need engineer's details. We handle the Building Control notification, whether it goes through the local authority or a registered building control approver.

To get a size agreed, the survey is the place to start. We mark the opening on the ceiling so you can judge the size in the room, check the structure above and give a fixed written quote that names the exact unit. Read about upstands and kerbs for what a large unit stands on.