Cut through a rafter and the load it carried has to go somewhere else. That is why a skylight job comes under the Building Regulations even when planning permission is not in question. The regulations ask three things of a new roof opening: that the roof still carries its loads, that the glazing keeps heat in to the current standard, and that fire safety and escape are not made worse.
Planning permission, which looks at how the outside of the house changes, is a different system with different decision makers, and it has its own page in our planning permission guide. Where the two meet, as they do on a flat-roof lantern, the pages point to one another.
Why a roof opening counts as building work
The Building Regulations apply to building work, and the definition is wide enough to include making a fresh hole in the roof covering and fitting a unit into it. Replacing an existing roof window or rooflight is covered too, because new windows are controlled fittings under the energy rules. A replacement in the same opening can be simpler on structure, since no timber is cut, but the unit that goes in still has to meet today's limit.
Because the work is notifiable, the body that checks it has to be told before the roof is opened. We handle the Building Control notification where the work is notifiable, and the quote shows the checking body's charge as its own line. Different parts of the regulations apply to different parts of the job:
- Part A covers the structure around the opening.
- Part L covers the energy performance of the new glazing.
- Part B covers fire safety, including escape from rooms in the roof.
A small roof window in a back bedroom engages Parts A and L. A loft room with a roof window brings Part B into view as well. A flat-roof lantern on an extension adds a fourth consideration: the extension's own building control approval may already describe the glazing, and the notification then has to fit with that.
Structure around the opening
Rafters and ceiling joists work as a set. Each carries a share of the roof's weight down to the walls, and a rooflight opening interrupts the ones in its path. The members that are cut have to be supported by new timbers fixed across the gap on either side of the hole, known as trimmers, and those in turn pass the load to the uncut members at the edges. A modest opening that sits between two rafters needs very little of this. A wider one cuts several, and the trimming becomes a framed structure in its own right.
Older roofs and wider spans
Not every roof can be treated the same. A roof built with a purlin, a heavy timber that runs along the slope and carries the rafters, needs the opening placed so as to keep the purlin's supports intact. A flat roof over a garage conversion may have joists spanning a wide room with no walls between, and an opening there can ask more of those joists than the room's owner would guess. Where the span or the condition of the existing timber calls for it, we specify deeper timbers, a steel beam or a structural engineer's calculation, and the choice is set out on the quote as its own item.
The same attention goes to the unit's weight and fixing. The kerb or frame a roof lantern sits on has to be fixed to timbers that can take it. We check the timber at the survey from inside the roof space where there is one, so the quote reflects what is really there.
A purlin runs along the slope and can decide where the opening may go.
Energy limits under Part L
Part L of the Building Regulations sets the heat-loss limits for new glazing, and the 2021 edition of Approved Document L holds them in Table 4.2 for existing dwellings. A roof window is treated as a window, so a new or replacement one takes the window limit of 1.4 W/m²K, or a Window Energy Rating that reaches band B. A rooflight, including a roof lantern, takes a limit of 2.2 W/m²K.
The U-value is a measure of how much heat passes through a unit for each degree of temperature difference, and a lower figure means less heat lost. It covers the glass, the spacer and the frame together, so a unit with good glass in a poor frame can miss the limit. The two limits differ because the regulations define the two products differently: the roof window is in plane with the roof, and the rooflight stands out from it on a kerb or upstand. Which product you are fitting therefore decides which number it must meet.
The extra effort in the glass pays back in comfort as well as compliance. A unit with a low U-value keeps the surface of the glass warmer on a winter night, which can reduce condensation on the inner pane. Our explainers on rooflight U-values and on Part L and rooflights unpick the figures further, and they are written to help with the choice between a double and a triple-glazed unit.
| Product | How the regulations describe it | Limit |
|---|---|---|
| Roof window | Set in line with the slope, in the same plane as the roof around it | 1.4 W/m²K, with Window Energy Rating band B as the minimum alternative |
| Rooflight | Glazing raised above the roof surface on a kerb | 2.2 W/m²K |
| Roof lantern | A rooflight, so it takes the rooflight limit | 2.2 W/m²K |
Fire safety and escape from rooms in the roof
Approved Document B covers fire safety, and the part that touches roof windows is escape. An emergency escape window is the Approved Document B route only for a storey no more than 4.5 m above ground level. That decides what a roof window can do in the two loft situations people ask about.
A loft above a two-storey house
Turn the loft of a two-storey house into a room and the new storey sits above 4.5 m. Escape from it normally runs down a protected stairway, with the doors and walls around the stair built to resist fire. That loft room's roof window is not a recognised escape route. It remains a good source of light and air, and it is often the thing that makes the room pleasant to use as a study or a spare bedroom, but the staircase carries the safety case.
A bungalow or chalet bungalow
Bedrooms and studies under the roof of a chalet bungalow or bungalow stay within the 4.5 m height, which allows a roof window to be the escape window, so long as the opening meets these tests:
- the clear opening has an area of 0.33 m² or more;
- its width and its height are each 450 mm or more;
- the lowest edge of the opening is 1,100 mm or less above the floor;
- the window holds itself open without being propped.
These dimensions steer the choice of unit and the position in the slope, so they are settled before the roof is cut. A window that opens wide enough but sits too high up the slope fails on the 1,100 mm rule, and the fix is a lower position or a different roof window.
Two figures to remember
The 4.5 m storey height and the 1,100 mm sill height are the numbers that sort an escape window from an ordinary one. The 4.5 m is measured from ground level, and the 1,100 mm from the floor of the room.
Daylight: a rule that does not exist
For houses in England the Building Regulations contain no daylight requirement at all. A dark landing or a windowless utility passes Building Control without any comment on the light. What the regulations govern is the way a rooflight is built and the heat it loses, and the amount of light is left to the owner.
Without a daylight rule, the size and position of the opening come from the room, the roof and the structure. We start from the dark part of the plan and check that the roof above it can take an opening at that spot. If the rafters or a purlin are in the way, the unit may move along the slope or change size to suit. The cleanest answer is the one where the glass is over the darkest part of the room and the framing is simple, and the survey is where we work that out. Our open the centre page covers why the middle of a plan goes dark in the first place.
For walk-on glass, which is a floor as well as a rooflight, the structural and loading questions carry more weight, and they are covered in walk-on rooflights and Building Regulations.
Who checks the work
Two kinds of body can do the checking. The first is the building control team at your local authority. The second is a registered building control approver, the name given since April 2024 to what used to be called an approved inspector, a private firm that is registered to do the same job. Whichever route is used, that body receives the notification, looks at the work and signs it off. The choice of route depends on the job, and we tell you which one applies at the quote.
The body decides what it wants to see and when. Structural work is the part that ends up hidden once the ceiling is closed, so the timing of any visit around the open roof matters. That is why the notification goes in first, and the date of the work is fixed against what the checking body needs. We arrange that with them as part of the job.
From notification to sign-off
- Notification Sent to the checking body before the roof is opened, with the plan of the opening and the glazing specification.
- Checks as the work goes on The body looks at the work at the stages it asks for, which can include the trimmed structure.
- Sign-off The local authority or registered approver confirms the finished work meets the regulations.
- Guarantee Our 10-year workmanship guarantee is given in writing alongside the finished work.
Where planning fits in
A roof opening can satisfy the Building Regulations in full and still need a planning application, or the other way round. The two are checked by different people against different questions. A roof lantern is the clearest case: it usually stands more than 150 mm proud of a flat roof, which normally takes it outside the permitted development allowance for rooflights, so it tends to need planning permission, and it also has to meet the 2.2 W/m²K limit. The two requirements are covered on one page for lanterns in roof lantern planning and Building Regulations.
For a price that reflects the structural work and the checking fee, start from the costs page. If you would like us to look at a roof, book a free survey or send photos on WhatsApp to 07880 284133, and you will get a fixed written quote afterwards.
