Building Regs

Changes to part L

Part L of the Building Regulations are set to change in June 2022, as the UK Government continues its drive towards greater thermal efficiency in house building. Find out how this impacts the rooflight industry

Rooflights and Building Regulations

A rooflight is a window that is installed within a pitched or flat roof normally to give more light to rooms or spaces within a dwelling or other building. Often known as skylights.

Rooflights offer opportunities for savings in CO2 emissions by reducing the use of electric lighting systems and the subsequent reduction in energy demand.

Approval under the Building Regulations will generally be necessary for the installation of a new rooflight although some exemptions may apply.

With all building work, the owner of the property (or land) in question is ultimately responsible for complying with the relevant planning rules and building regulations (regardless of the need to apply for planning permission and/or building regulations approval or not).

Therefore, failure to comply with the relevant rules will result in the owner being liable for any remedial action (which could go as far as demolition and/or restoration). The general advice is to always discuss your proposals with the relevant local planning authority and building control service before starting work.

The following are some of the relevant considerations when installing a new bespoke rooflight:

  • Roof structure alterations to create an opening for a rooflight
  • The roof will need to carry the weight of the new rooflight
  • Any rooflight requires sufficient insulation, i.e. U-Value rating
  • Close proximity to boundaries or escape routes
  • Potential exposure to impact from falling objects
  • Conservation area
While planning permission isn’t often necessary for rooflight installation, you will usually have to seek approval under Building Regulations. This is to ensure that the installation of a rooflight doesn’t compromise the integrity of the roof & cause a health & safety hazard, for example, if the rooflight is too heavy. The thermal performance of the new rooflight is also a factor.

There is no size limit to the rooflight from a planning aspect, providing it doesn’t occupy an area of your roof beyond what is considered a ‘reasonable size’ based on historic precedent. A central mullion (glazing bar) can help to reduce the perceived size and in the case of traditional properties may also be a stipulation of the local authority planning officer.

Part B - Fire Safety

Consideration for enhanced glazing/fire integrity, minimum unobstructed openable area for emergency escape, use for smoke ventilation (BS EN 12101-2), rooflight lining Class rating.

Rooflights should meet the following classifications, according to material. No guidance for European fire test performance is currently available, because there is no generally accepted test and classification procedure.

Non-plastic rooflights should meet the relevant classification in Table as shown below:
Location Classification
Small rooms of maximum internal floor area of 4m² Garages (as part of a dwellinghouse) of maximum internal floor area of 40m² D-s3, d2
Other rooms (including garages) Circulation spaces within a dwelling Other circulation spaces (including the common areas of blocks of flats) C-s3, d2 B-s3, d2Pl
Note: Wallcoverings which conform to BS EN 15102, achieving at least class C-s3, d2 and bonded to a class A2-s3, ds substrate, will also be acceptable.

In the Secretary of State’s view, requirement B2 is met by achieving a restricted spread of flame over internal linings. The building fabric should make a limited contribution to fire growth, including a low rate of heat release.

All Stella rooflights are manufactured using grade 316L stainless steel as this material offers a number of benefits over many other materials used to manufacture rooflights.

Stainless steel has many qualities that make it ideal as a construction material. One of these is its inherent oxidation resistance and continued strength at elevated tempe­ratures. Stainless steels do not have an intrinsic ‘fire rating’ and so their fire resistance is tested on specific fabri­cations. These are carried out under extremely precise conditions and are covered fully by BS476 parts 20, 21 (load-bearing elements) and 22 (non-load-bearing elements).

There is no possibility of ignition of stainless steels and flame spread is minimal, reducing the propagation of fire. The surface of the metal is inert and stable under oxidizing conditions, which are found in almost all flames and heat sources.

Most stainless steel grades considered for building applications i.e. 304 and 316 have useful, long-term oxidation resistance at temperatures over 800°C and do not begin to melt until temperatures over 1375°C are reached. It is unlikely that uniform, sustained high temperatures like these would be reached in short term ‘transient’ fire conditions.

There is a great deal of research comparing carbon steel, aluminium and galvanised steel to stainless steel. The overwhelming conclusion is that stainless steel outweighs these other metals in corrosion resistance, durability, life-cycle cost savings, and even fire prevention.

Approved Document B of schedule 1 of the Building Regulations covers the requirements with respect to Fire safety.

Visit the website for more information from the Planning Portal website and to download the Part B document.

Part F – Means of Ventilation

Requirements with respect to ventilation.

The key aim of the requirement of Part F1(1) is that a ventilation system is provided that, under normal conditions, is capable of limiting the accumulation of moisture, which could lead to mould growth, and pollutants originating within a building which would otherwise become a hazard to the health of the people in the building.

Stella rooflights can be manufactured as fixed and opening designs. For smaller casements it is possible to have a manually operated casement which opens using a cill mounted screwjack winding mechanism.

These typically open the casement to around 250mm. For landscape or larger casements it is often necessary to use electric actuators to manage the safe and effective operation of the rooflight. The type and application of these actuators are often as bespoke as the rooflight itself and there are a wide range of options depending on the requirements of the opening casement.

The electrically operated rooflights we produce are also supported by a range of accessories to aid compliance with Part F of the Building Regulations. This includes, rain, wind and temperature sensors, connectivity to smoke systems along with switch or remote control options.

Our electric ventilation solutions deliver on every front using the latest technically advanced control systems. Our natural ventilation systems are manufactured in Hamburg by D+H Mechatronic. For 50 years, D+H has repeatedly set the standard for safe, reliable ventilation systems improving comfort, safety and energy efficiency in buildings around the world.

One important element to note is that whilst our rooflights can offer a means of controlled ventilation, our rooflights do not have trickle vents. While more modern chunky framed rooflights have the framework to include a trickle vent, our flush fitting low profile design means that there is just nowhere to incorporate trickle ventilation without the risk of water ingress.

A good source of information concerning trickle ventilators can be found at the Glass & Glazing Federation here

Visit the Planning Portal website for more information from the Planning Portal website and to download the Part F document.

Part K – Protection from Falling, Collision & Impact

Consideration for non-fragile classification to protect from impact of falling objects or other risks.

It is important to note that there is no legal requirement for rooflights to be non-fragile; it is only relevant if the customer has specified that requirement. A risk assessment should be undertaken to control the risks around any roof work, both during construction and for subsequent maintenance activities, with relevant measures put in place depending on the roof type.

All overhead glass must be a type of safety glass – toughened, laminated or wired glass. Heat-strengthened glass is not a safety glass. Glass used in rooflights that are installed on a roof that is accessible, but not designed for regular foot traffic, should be specified as “non-fragile”.

Depending on the anticipated use of the roof, a non-fragile rooflight is designed to prevent people or objects falling through it in the event of an accident on the roof. To that end, roof access requirements should be assessed at design stage and specified carefully, helped by appropriate risk assessments.

Stella conservation rooflights are not suitable for walk-on applications and should be classed as ‘fragile’.

To avoid any ambiguity it is fairly essential that the architect, structural engineer and rooflight manufacturer discuss large rooflight installations, likely weights, load bearings, site access, and lifting capacity at an early stage to avoid any problems down the line. It is not advisable to leave this for the builder to deal with at the last minute.

Another area for consideration should be the safety of large areas of glazing situated high up in a roof structure. Regardless of size your rooflight should meet the BS 5516-2: 2004 patent glazing and sloping glazing for buildings standard. This code of practice for sloping glazing defines that inner panes must always be laminated wherever rooflights are more than 5 metres above floor level (increased to 13 metres for panes less than 3 square metres) or are located over water (e.g. swimming pools).

The Standard permits the use of toughened inner panes in other applications (for example where rooflights are less than 5 metres from floor level), but only where a stringent risk assessment for the particular application has been completed and has concluded that the use of toughened glass does not give any additional risk to those below the rooflight.

It is important to understand when specifying glass for use in rooflights that the combination of interlayer and glass panes that make up the complete section is critical in achieving your desired performance. Laminated glass is two or more sheets of glass bonded together with a plastic interlayer.

This interlayer holds the glass together and will be retained in the frame if either or both of the glass sheets should break. It is therefore a safety glass and can be used in overhead glazing.

Annealed glass is not as strong as toughened glass and is susceptible to thermal fracture. This is caused when there is a significant temperature difference within the same piece of glass and is likely to occur where adjacent structure or trees, for instance, could cast a shadow across the glass, with part of it in sunshine and part in shade. It can also be caused by other finishes being located close to the underside of the glass, where heat coming in through the glass is reflected back to the underside.

As all Stella rooflights are bespoke it provides our customers with the opportunity to discuss their requirements to ensure that their rooflight glazing is the right specification for their project.

For more information on large rooflights and laminated glazing read our blog.

Part L – Conservation of Fuel & Power

Consists of four parts:

  1. Conservation of fuel & power in new dwellings
  2. Conservation of fuel & power in existing dwellings
  3. Conservation of fuel & power in new buildings other than dwellings
  4. Conservation of fuel & power in existing buildings other than dwellings

For domestic applications, the minimum acceptable standards for the thermal performance of rooflights in new build work are defined in Building Regulations Approved Document L1A and for all other work in Building Regulations Approved Document L1B.

One of the main reasons for installing rooflights in a building is to provide a source of bright, natural light which reduces the requirement for artificial lighting. Natural daylight has significant advantages over artificial light. Apart from being free, it is an unlimited natural resource, which rooflights make good use of. Using artificial light requires energy, so reducing your dependency on this type of lighting will also cut the CO₂ emissions which result from using artificial light.

Typically the total CO₂ emissions associated with all aspects of operating a building without rooflights can be over 50% higher than for a building with 12% rooflights. Rooflight area of 15% or more of floor area is a good approximation of the optimum rooflight area.

Building regulations require roof windows or rooflights to achieve a maximum thermal performance of WER Band C or better or U-value 1.6 W/(m2.K).

As there is currently no test specifically for roof windows or rooflights, they should be tested in accordance with BS EN ISO 10077-1:2017 for Thermal Performance of Windows, Doors & Shutters as a means to determine and provide a calculation for thermal transmittance.

There are some exceptions which include where the replacement windows are unable to meet the requirements because of the need to maintain the external appearance of the façade or the character of the building, replacement windows should therefore meet a centre pane U-value of 1.2 W/m2K. There are also additional exemptions for Listed buildings and those in designated conservation areas and we would advise that further guidance is sought from your local authority concerning any requirements which specifically relate to your project.

What this test will provide is a thermal performance (u-value) figure based on installation at both horizontal and vertical inclination. This is somewhat problematic for most in-plane rooflights as their design does not allow for their use in either horizontal or vertical settings, so it is always advisable to discuss your roof pitch with your rooflight supplier.

There are also other factors which need to be taken into account as most rooflight companies will provide their highest specification as the test sample. This means that any changes to glazing specification and addition of things such as glazing bars are likely to alter the u-value as much as the pitch of the roof.

​For more on rooflights and U-values, read our blog.

Another important factor in the conservation of fuel and power is the extent to which air is lost through uncontrolled leakage. Sometimes referred to as the weather performance test, this is a crucial indicator of rooflight energy efficiency. Part L compliance of the Building Regulations requires a rooflight to meet the standards of air permeability, water-tightness and wind resistance.

Air permeability is a measure of how draughty the rooflight is with the product achieving a ‘class’ based on testing performance. Class 4 is the highest performance and Class 1 the lowest. The Stella Rooflight conservation design has achieved the Class 4 rating for air permeability.

Water-tightness is as you would expect, a test of the rooflights ability to keep water out. Most rooflights are tested to Class 8A, 450 Pa or Class 9A, 600 Pa. Above Class 9A the performance figure is shown as an E rating followed by the maximum pressure (Pa) that the rooflight achieved. The Stella Rooflight conservation design has achieved the Class 8A, 450 Pa classification.

Wind resistance is an important test as nobody wants their rooflight to be damaged or collapse in high winds. The test simulates changes in high winds and measures any deflection of the rooflight by subjecting it to both positive and negative pressures. The Stella Rooflight conservation design achieved resistance to wind loads of 2100 Pa causing deflections less than 1/150 of the span of a frame member. It is resistant to repeated pressure cycles of 1050 Pa.

There is also a safety element to the test where a much higher pressure is applied and deflections are not taken into consideration. While snow might be a more seasonal consideration, depending on the severity of any given winter it can linger and increase over an extended period of time. The ability of roof glazing to withstand snow loading is tested in terms of long-duration loading, and any deflection in the centre of the glass measured accordingly. Site location is a significant determining factor in the wind and snow loads to which roof glazing is exposed, and should be considered accordingly.

The Stella Rooflight conservation design was tested to both positive and negatives pressures as high as 3150 Pa and no parts became detached and the design remained closed.

  • New rooflight installations will not normally require an application for planning permission providing:
  • They do not protrude more than 150mm beyond the plane of the roof slope
  • They are not higher than the highest part of the roof
  • If they are in a side elevation roof slope they must be obscure-glazed & either non opening or more than 1.7m above the floor level

There is no need to change the position of chimneys, flues & pipes

If you live in a listed building you will need listed building consent for any significant works, internal or external. If your house is in a conservation area or a designated area of outstanding natural beauty then there may be additional restrictions in place and we would advise that you speak to your local authority for further guidance.

Navigating through the various requirements of Building Regulation can be a minefield and we would always recommend that you seek advice from a qualified professional to ensure that your build complies with the current standards.

When it comes to your rooflights, Stella offers a completely bespoke product so we are able to work with you to ensure that you get the right rooflights for your project.

We are always happy to discuss your requirements and provide guidance concerning the framing materials, glass, thermal performance and safety; so to find out more about how a Stella rooflight can enhance your property, email