Do rooflights meet current building regulations?

The short answer

Yes. Any current rooflight or roof window from a mainstream manufacturer sits comfortably inside the limits the regulations set for the fitting itself, and it arrives with a document that proves it. What that document says, and which lines of it matter, is the useful part of the question. A unit is a construction product with declared performance: a thermal transmittance, a classification for the safety glass, a water tightness and wind resistance class, often an equivalent ventilation area and sometimes a security rating. Compliance of the product is read off those declared figures. This page is about the unit as an object on an order. The Part L position for a replacement fitting is set out in full on rooflights and Part L for a replacement, and the thermal specification behind the numbers on U-values and glazing specification.

The declaration of performance is where the answer lives

Manufactured roof windows and rooflights are covered by a harmonised product standard, BS EN 14351-1 for windows and external pedestrian doorsets including roof windows, with BS EN 1873 covering individually made plastic rooflights on kerbs and BS EN 14963 covering continuous rooflights. A unit placed on the market under those standards carries a UKCA or CE mark and a declaration of performance listing the tested and calculated values.

That document is short, it is available from any supplier, and it is what an inspector or a buyer’s solicitor is content to see. Where a made-to-size unit is produced outside a harmonised standard, the equivalent evidence is a set of test reports and calculations from the fabricator covering the same properties. Ask for it before ordering rather than afterwards.

The thermal limits a fitting has to sit inside

Approved Document L sets limiting standards for glazed fittings, and there are two figures rather than one because roof windows and rooflights are declared in different ways. A roof window is measured in the plane it is installed in and carries a limiting standard of 1.4 W/m2K. A rooflight is conventionally declared in the vertical plane, which produces a higher number for the same product, and carries a limiting standard of 2.2 W/m2K.

Fitting	Limiting standard	Typical current product
Roof window, double glazed	1.4 W/m2K	1.0 to 1.4 whole unit
Roof window, triple glazed	1.4 W/m2K	0.8 to 1.1 whole unit
Flat rooflight, double glazed, declared vertically	2.2 W/m2K	1.1 to 1.6
Flat rooflight, triple glazed, declared vertically	2.2 W/m2K	0.7 to 1.0
Roof lantern, whole unit	2.2 W/m2K	1.2 to 1.8, depending on bar count

The trap is not the limit, which every current product clears. It is comparing two quotations where one figure is a whole-unit value at the real size and the other is a centre-pane value, which flatters the product by ignoring the frame and the edge of the glass entirely. Ask both suppliers for the whole-unit figure at the size being bought. Which build-ups reach which figures is set out on the energy efficient rooflights hub.

Safety glazing overhead, which is not the same as in a window

Approved Document K deals with glazing in critical locations, and glass in a roof is treated as overhead glazing because anything that fails will fall on somebody. Two rules follow, and they are the ones to check on an order.

The outer pane is toughened, classified to BS EN 12600, so that if it breaks it goes to small blunt fragments rather than shards. The inner pane over any habitable space, stairwell, bath or shower is laminated, so that if it breaks the interlayer holds the glass in the frame rather than letting it drop. A unit with two toughened panes and no laminate is specified wrongly for a room, whatever its U-value is. Where a rooflight sits over a bath or a wet room the requirement is firmer again, and that case is on flat rooflights over a bathroom or wet room.

Non-fragility, and the classification for a roof somebody may walk on

A rooflight is a hole in a roof surface, and roofs get walked on by people cleaning gutters, servicing aerials and clearing moss. The relevant classification is ACR[M]001, the red book test, which sorts assemblies into non-fragile classes and a fragile category on the basis of a drop test.

This is a product property, not an installation one, and it is declared. Older plastic domes and wired cast glass are typically fragile and become more so with age as ultraviolet degrades the plastic. Modern glass units on a proper kerb are commonly declared non-fragile in class B or C. Where a unit will be walked on deliberately rather than accidentally, that is a different product again with a rated load, described on what is a walk-on rooflight.

Weather performance, stated as three classes

The declaration also carries the weather tightness classes from the product standard, and they are worth reading on an exposed Gloucestershire site. Water tightness is given as a class from the pressure at which water passes the unit in the test rig. Wind load resistance is given as a class covering both deflection and safety. Air permeability is given as a class from 1 to 4, with 4 the tightest.

For an exposed Forest of Dean or wolds ridge site, where driving rain arrives from the south west with force behind it, a higher water tightness class and a tighter air permeability class are worth specifying rather than accepting a base offering. They cost little at order stage and they are not changeable afterwards.

Background ventilation, declared in square centimetres

Approved Document F is met partly through the fitting, because a roof window with a background vent flap has a declared equivalent area, measured in square centimetres, that counts towards the background ventilation the room needs. A fixed unit with no flap contributes nothing to that figure, so the room needs its ventilation from somewhere else.

This matters at specification stage rather than at installation, because it decides whether the unit ordered is a fixed one, a manual opener or an opener with a filtered vent flap. The equivalent area is on the datasheet next to the U-value. The wider question of vents is on do rooflights need trickle vents, and the flap itself on roof window ventilation flaps and filters.

Escape openings and security, both of which are product dimensions

Where a roof window is the escape opening from a room in the roof, Approved Document B sets dimensions the unit itself has to satisfy: a clear openable area of at least 0.33 square metres with no dimension less than 450mm, and the bottom of that opening between 800 and 1100mm above the floor. That is a size code and an opening type on an order, and choosing a fixed unit or a small one in that position is a specification error rather than a fitting error. The detail is on roof window escape openings and the rules.

Approved Document Q applies to new dwellings and asks that easily accessible windows and rooflights are made to a security standard, in practice tested to PAS 24. Manufacturers offer PAS 24 versions of the common sizes and declare them as such. On a ground floor extension roof reachable from a garden wall, it is worth asking for even where Part Q does not formally bite. Security build-ups are on flat rooflight security and laminated glass.

Conservation rooflights, and the flexibility that exists for historic buildings

A slim cast or pressed steel conservation rooflight is a genuinely worse thermal product than a modern composite-framed unit, because the frame is metal and it is deliberately thin. Whole-unit figures around 1.5 to 1.9 W/m2K are normal, and a very slim single-glazed heritage pattern is worse again.

Approved Document L recognises this. Where a building is listed, in a conservation area or of traditional construction, the requirement is the best that can reasonably be achieved without unacceptably altering the character or appearance of the building. In practice that means specifying the best glass the frame will take, a warm-edge spacer and a well insulated perimeter, and recording what was chosen and why. Given how many Cotswold stone slate roofs across Painswick, Bibury, Northleach and Chipping Campden fall into this category, it is a conversation this county has more often than most. The product itself is on conservation rooflight installation.

What a compliant specification looks like, line by line

Eight entries make an order that satisfies the fitting side of the regulations and can be evidenced afterwards.

Manufacturer, model and size code, with the declaration of performance attached.
Whole-unit U-value at the actual unit size, with the plane of declaration stated.
Glass build-up written surface by surface: toughened outer, cavity and gas fill, laminated inner.
Warm-edge spacer named, with its psi value, as described on what is a warm-edge spacer.
Solar factor, for any slope facing between east and west.
Equivalent ventilation area in square centimetres, or a note that ventilation is provided elsewhere.
Non-fragility class where the roof is accessible, and PAS 24 where security applies.
Weather tightness, wind load and air permeability classes for exposed sites.

Severn records each of those at survey and puts them on the order, and the paperwork goes to the homeowner with the completion documents so it is on file when a buyer’s solicitor asks years later.