National Association of Rooflight Manufacturers

Legal Requirements – Thermal Performance
The Government is committed to the reduction of greenhouse gases as a result of the Kyoto accord on Climate Change. The Building Regulations for England, Wales and Scotland, which cover Conservation of fuel and power, have been updated accordingly. The revised Regulations require buildings to:

Have more insulation in the building envelope.
To limit heat loss from pipes and ducts.
To provide more energy efficient lighting, heating, cooling and ventilation systems.

In the drive for energy efficiency the revised Regulations set minimum acceptable levels for natural daylighting and refer to CIBSE LG10 for additional guidance. That publication explains the value of natural daylight on human performance and thus on energy efficiency in its widest sense. Widespread research links natural daylighting to tangible work place benefits: improved retail sales, lower staff absenteeism, faster hospital recovery rates, and improved school exam results.

Natural lighting should be provided in all buildings. Windows can provide daylight to areas within 6 metres of a window, but rooflights are the only practical means of introducing daylight to any wider buildings. An appropriate area of rooflights – see Design Support – should be included on all roofs, including curved and flat roofs, standing seam and any other steel and fibre cement roofs.

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The Building Regulations – Part L
The Building Regulations 2000 (England and Wales) Approved Document L (2002 edition) came into force on 1st April 2002 in England and Wales, in two parts:

L1: Conservation of fuel and power in dwellings.
L2: Conservation of fuel and power in buildings other than dwellings.

The above documents are referred to simply as Part L in this document.

The Building Standards (Scotland) Regulations 2001 Part J: Conservation of fuel and power, which has similar requirements, came into force in Scotland on 4th March 2002. In this document all general comments and any specific reference to Part L will equally apply to Part J unless indicated otherwise. Paragraph references are specific to Part L unless otherwise stated.

Daylighting
Workplace (Health Safety and Welfare) Regulations 1992 state, “Every workplace shall have suitable and sufficient lighting which shall, so far as is reasonably practicable, be by natural light”. These comments are restated in HSG 38 – Lighting at Work.

The most effective method of providing even, consistent daylight particularly in large buildings, is through rooflighting – up to three times more efficient than windows of similar area. Diffusing materials should be used wherever possible to provide even light distribution and avoid glare. Wall glazing is less effective and can create internal shadows and dark corners. However it does offer good psychological benefits and must not be ignored.

The existing regulatory requirements are now reflected in the revisions to Part L. Para 1.14 states that “special care needs to be given to confirm that levels of daylight are adequate” and Para 1.55 states “where it is practical, the aim of lighting control should be to encourage the maximum use of daylight and to avoid unnecessary artificial lighting during the time when spaces are unoccupied”.

Daylight Levels
It is clear that there is a regulatory requirement for natural daylight but Part L does not include definitive guidance on how to determine adequate daylight levels. It does define minimum glazing levels; para 1.45 says:
“… a daylit space is defined as any space within 6m of a window wall provided that the glazing area is at least 20%… Alternatively it can be roof-lit, with a glazing area at least 10% of the floor area… "

This means that for any building space which is more than 6m from a window, roof lights should be provided to a minimum of 10% of the floor area. Greater areas may be required in many applications.

No further guidance is given in Part L about the absolute daylight levels needed, but information is provided on this web-site in the Design Support section, Daylight Design – Rooflight Areas to Achieve Adequate Natural Lighting Levels.

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General Design
There are three alternative methods in the Regulations to demonstrate compliance with the requirement for the conservation of fuel and power.

1. Elemental Method
This method considers the performance of each element of the building envelope individually. To comply, a minimum level of thermal performance should be achieved in each of the elements. This is stated as U-value; a lower U-value indicates less heat transfer per square metre, i.e. better insulation.

The simplest method for the designer to demonstrate compliance is to ensure that the U-value of all exposed elements meets the minimum level of thermal performance required and does not exceed the maximum allowable area for any element.

Some flexibility is also provided for trading off between elements of the construction. For example, a designer may choose to use less than the permitted maximum rooflight area (provided it can be demonstrated that daylight levels will remain adequate) which will result in a heat credit which can be used to justify use of less well insulated rooflights, or traded off against performance of other elements such as doors, windows or thermal bridges.

Compliance would be achieved providing that the overall heat loss from the proposed building does not exceed that from a notional building of the same size and shape.

2. The Whole Building Method
This considers the performance of the whole building, and applies to offices, schools and hospitals but not to industrial or storage buildings. Environmental design guidance issued from other authorities is also referenced. Performance of all environmental systems is considered – including heating, lighting, air conditioning and ventilation.

For example, for schools DfEE Building Bulletin 87 is the referenced source. The bulletin provides a holistic approach to school design encompassing acoustic, lighting, ventilation, heating, and thermal performance standards. An energy rating method is defined.

Building Bulletin 87 refers back to Part L for minimum acceptable rooflight U-values and glazing areas, so rooflight limits as defined in the elemental method above still apply. As in Part L the recommendation for daylight provision also applies: “Priority should be given to daylight as the main source of light in working areas, except in special circumstances. Wherever possible a daylight space should have an average daylight factor of 4-5%”.

3. Carbon Emissions Calculated Method
This method also considers the whole building performance including building services. To comply the annual carbon emissions from the building should be no greater than the notional building that meets the compliance criteria of the Elemental Method. A variety of software modelling tools are available – since these are complicated it will be normal for most industrial buildings to be designed to the Elemental Method. In Scotland the calculations are done by the Heat Loss Method.

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Elemental Method
The simplest method for a designer to demonstrate that rooflights comply with Part L is to ensure that:

1. The rooflights achieve a U-value of 2.2 W/m²K or lower.
2. The rooflight area on the building is no more than 20% of the roof area.

However these values may be varied by the designer using:

3. Heat credit trade off calculations.

Other requirements relate to:

4. Air leakage.
5. Solar overheating.
6. Thermal bridging.

Well designed triple skin rooflights will normally meet the standard U-value requirement of 2.2 W/m²K and are available from most rooflight manufacturers. They offer a significant reduction in heat loss through the rooflights compared with double skin rooflights and is a recommended method of fulfilling the requirements of the new legislation.

1. Standard U-values
To show compliance the building envelope has to provide certain minimum levels of insulation. Part L defines standard U-values for each constructional element as given in Table D.

Table D: Standard U-values of Construction Elements


Exposed Element		U-Value (W/m²K)

Flat Roof, Pitched Roof below 10° All roofs with integral insulation (composite or site assembled)		0.25

Walls		0.35

Walls Part J Scotland		0.30

Floors		0.25

Rooflights*		2.20

Windows translucent wall areas		2.00

* For barrel lights and dome lights the standard applies only to the performance of the unit excluding any upstands. Reasonable provision would be to insulate any upstand or otherwise isolate it from the internal environment.

U-values should be determined by physical test or
by finite element analysis in accordance with BS EN ISO 10211 Part1: 1996 as specified in L2 Para 0.15.

2. Maximum Glazed Areas
To show compliance the total area of windows, doors and rooflights should not exceed the values given in Table E, unless compensated for in some other way.

Table E: Maximum Areas of Openings Unless Compensating Measures are Taken


Building Type	Windows* Doors as % area of wall	Rooflights as % area of roof

Residential Buildings	30	20**

Places of assembly, offices and shops	40	20**

Industrial and storage buildings	15	20**

* For the purposes of this calculation dormer windows in a roof may be included in the rooflight area.
** L2 Para 1.14 states that if the rooflight area is to be reduced below 20% of the roof, the designer must ensure that the natural daylight levels are sufficient for the building purpose – see Rooflight Areas to Achieve Adequate Natural Lighting Levels in the Design Support section.

Rooflights that achieve a U-value of 2.2 W/m²K, at areas up to 20% of the roof, thus fully meet the requirements of the new Regulations.

3. Trade Off
Area of rooflights, doors and windows, and their respective U-values, can be varied from standard values given in Tables D and E so long as total heat loss meets allowable limits. A notional building is used to demonstrate that a design meets the energy conservation objectives of Part L whilst allowing these elements to deviate from standard allowable U-values or areas.

A notional building is a theoretical building of the same size as the building under design, with maximum areas of rooflights, doors and windows and with each element meeting its standard U-value. To achieve compliance, the total heat loss from this notional building must not be exceeded by the building being designed.

Care should always be taken that rooflight area is sufficient to provide adequate daylight – see Rooflight Areas to Achieve Adequate Natural Lighting Levels in the Design Support section – but this may be less than the maximum permitted area of 20%. If rooflights with a U-value of 2.2W/m²K are fitted to a reduced area, there will be a heat credit, which can be traded off against the performance of other elements such as doors, windows and thermal bridges.

This heat credit can also be used to compensate for the use of less well insulated rooflights, and Table F shows the maximum rooflight U-value at different areas, if the entire heat credit is traded against rooflight U-value. However, it should be noted that use of rooflights with a U-value of at least 2.2W/m²K is encouraged by ODPM and supported by NARM.

Table F: Maximum Rooflight U-values per Roof Area


Rooflight area (% of roof)		Rooflight U-Value (W/m²K)

30%		1.5

20%		2.2

15%		2.8

12%		3.5

10%		4.1

Constraints to Trade Off
L2 para 1.14 states that care must be taken to confirm that levels of daylight are adequate. If the rooflight area is to be reduced below 20% the designer must ensure that natural daylight levels are sufficient for the building purpose.

L2 Para 1.16(c) states that no more than half of the allowable rooflight area can be converted into increased areas of windows (vertical) and doors. There is no provision for converting vertical openings into increased rooflight areas in the roof.

L2 Para 1.16(b) states that if the area of rooflights is less than the values shown in Table E the respective U-values of roof, wall and floor cannot exceed the appropriate values given in Table D by more than 0.02 W/m²K. Thus however much notional heat loss saving is made by reducing the rooflight area the U-value of the insulated roof cannot exceed 0.27W/m²K.

4. Air Leakage
Air leakage requirements do not apply to Building Standards (Scotland): Part J and for Part L1 – Dwellings – only reasonable provision is required.

Under Part L2 buildings should be reasonably airtight to avoid unnecessary space heating and cooling demand. There is a requirement that the permeability of the envelope (which includes the total area of the perimeter walls, roofs and ground floor area) should be no worse than 10 m3/h/m² at an applied pressure of 50 Pascals. All buildings that exceed 1000m² of gross floor area must be air tested on building completion to show compliance.

Buildings of less than 1000m² gross floor area will be deemed compliant providing evidence that appropriate design detail and construction techniques have been used.

Provided that rooflights are correctly fixed and sealed, rooflight assemblies will usually comply easily with this requirement. Association members strongly recommend that Contractors take care in the fixing and sealing of rooflights in accordance with their recommendations, since subsequent air test failure will generally require extensive remedial work which could prove to be expensive.

5. Solar Overheating
Solar Overheating legislation is not included in the Building Standards (Scotland): Part J.

There is an important distinction between solar overheating and solar gain. Most windows and rooflights are likely to generate solar gain under normal daylight conditions. Solar gain is beneficial in that it reduces heating requirement during daytime hours when buildings are usually occupied. In some cases, such as use of unheated atria, solar gain may play a major role in energy efficiency strategies by reducing the effective exposed wall area of the building and offering a buffer zone between external and internal climates.

However improperly designed glazing may result in solar overheating and the workplace environment becomes unpleasant.

Buildings should be constructed so that occupied spaces are not likely to overheat when subject to a moderate level of internal heat gain, and so that excessive cooling plant is not required to maintain the required conditions.

There are various ways of achieving this:

Appropriate specification of glazing performance.
Incorporation of passive measures such as sun-shading.
Mechanical ventilation without excessive use of cooling plant.
Use of exposed thermal capacity combined with night ventilation.
By calculation ref Part L para 1.23.

If none of the above are suitable alternatives then:

By limiting the area of glazing facing only one orientation to the opening areas shown in Table G.

Table G: Maximum Allowable Area of Glazing


Orientation of opening		Maximum allowable area of opening (%)

N		50

NE/NW/S		40

E/SE/W/SW		32

Horizontal		12

NARM has obtained an interpretation from BRE that Horizontal will be applied to all roofs below 75º pitch.

Thus in England & Wales compliance is only demonstrated if at least one of the above six requirements to minimise solar overheating is adopted. If rooflight area at 12% is the chosen method to achieve compliance, then the designer needs to ensure that the daylight levels are adequate for the purposes of the building. If they are not, then the designer could chose to obtain compliance by adopting the Calculation Method (Part L para 1.23).

6. Avoidance of Solar Overheating by Calculation Method
Part L Regulations state that where the rooflight area is less than 12%, solar overheating will not be a problem and no further action is required. Wherever higher rooflight areas are specified, it must be shown that solar overheating will not occur by calculation or adoption of alternative measures.

Independent research carried out by the Institute of Energy and Sustainable Development, De Montfort University, has been submitted to the Building Regulation Advisory Council and being considered for inclusion in future revisions to the Building Regulations, has predicted the levels of solar overheating which will occur inside typical large span buildings using the latest computer modelling techniques.

The Part L Regulations are met if the overall internal gain does not exceed 40W/m²; they assume an internal gain of 15W/m²; thus allowing a maximum solar load of 25W/m²; but the research demonstrates this assumption does not apply to many large span buildings, depending on the building use.

For typical activities in large span buildings, the heat emitted per person (male) ranges from 140W (seated light work) to 256W (medium bench work). Standing, light work or walking produces about 160W of heat.

Table H (extracted from the independent research) shows the maximum rooflight area which will avoid solar overheating, from various levels of internal gain.

Table H: Maximum Rooflight Area to Avoid Overheating


Internal gain (W/m²)		Max rooflight area (% of floor area)

0		23

5		20

10		17

15		14

20		11

This table shows that where internal gains are 15W/m², rooflight area can be up to 14% of floor area without risk of causing solar overheating; where internal gains are lower then rooflight area can be higher.

For example, in storage buildings, occupant densities are generally very low and can often be ignored; the main gains are from artificial lighting, typically only 5W/m². It can be seen from Table H that rooflight areas up to 20% will not cause solar overheating.

Any large plant or process facility may produce considerable local heat gains. Where these are envisaged, it is recommended that localised heat extraction/removal and/or cooling is used to prevent overheating. Where these are known to be effective in eliminating the localised heat gain, the sources can be excluded from the internal heat gains for the assessment of overheating.

In retail outlets occupant density can be significant (typically around 4W/m²), and retail outlets are usually well lit, with internal gains due to lighting around 15-20W/m². However, the period of highest solar gain is simultaneous with highest daylight illuminance, and provided rooflight area is sufficient, the internal gains due to electric lighting can be greatly reduced or eliminated by switching off the lights either manually or more reliably, by daylight-linked controls. Total internal gains may therefore be around 4W/m² and Table H again shows that rooflight areas up to 20% will not cause solar overheating.

7. Thermal Bridging
The building fabric should be constructed so that there are no significant thermal bridges or gaps in the insulation layer within any elements, at the joints between elements and at the edges of elements such as those around rooflights or windows (para 1.9).

Part L2 refers to BRE IP17/01 and MCRMA Technical Report No 14, which details how thermal bridges should be assessed, and the limiting factors. Thermal bridges have two effects: there is a heat loss per linear metre (measured by the Y-value), and an increased risk of surface condensation due to localised cold spots (measured by the f-factor).

In practice heat loss through thermal bridges can be treated in the same way as that through any other element of the building. The maximum allowed heat loss through thermal bridges is a further 10% of the total heat loss allowed from the notional building described in 3 above.

In addition, the trade off principle can be used so any heat credits (e.g. a rooflight with a higher insulation specification) can be traded off, and thus compensate for correspondingly greater heat loss through thermal bridges in other areas of the building.

To avoid condensation risk in different building types, IP17/01 specifies that the f-factor for every detail must always be greater than a minimum permissible value, as shown in Table J.

Table I: Minimum Permissible f-factors


Types of areas		Minimum f-factors

Storage Buildings		0.3

Office, Retail premises		0.5

Sports Halls, Kitchens, Canteens, etc		0.8

Swimming Pools, Laundries etc.		0.9

In plane rooflights do not usually give any additional cold bridge compared to surrounding metal cladding; the same insulated support brackets and thermal barriers should be used as necessary. Whilst there is no direct thermal bridge, these rooflights still have a defined f-factor associated with the insulation value. Double skin rooflights will usually not have an f-factor higher than 0.7, whilst well insulated rooflights could have an f-factor of 0.8 to 0.9.

Individual and continuous out of plane rooflights may have cold bridges via aluminium or steel frames and this should be factored into the thermal bridging calculation to demonstrate compliance. These may also affect the f-factor of these products, which should be confirmed with the manufacturer.

Exempt Buildings
Buildings or parts of buildings with low levels of heating or unheated buildings do not require measures to limit heat transfer through the fabric of the building and are exempt from these Regulations. For such buildings single skin rooflights are acceptable.

A low-level heated building with a heating requirement no more than 25W/m² could typically be a warehouse used for storing goods to protect them from condensation or frost.

A cold store building is one where insulation is required to a level that will be determined by operational needs.

Roof Refurbishment
The Regulations apply to both new buildings and refurbishing old buildings, however if a single component is defective and needs replacing it is exempt from the Regulations.

Thus if a roof is being stripped and replaced the new roof would need to comply with the new Standard U-values described above. However where only the rooflights are deemed to require replacement a direct replacement will be allowable.

Where the old roof is insulated and rooflights are in place but only single skin and deemed to be defective, it would be advisable to replace the rooflights with rooflights that comply with the requirements if it is feasible to do so.

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Legal Requirements – Non-Fragility and CDM
When specifying rooflights, designers should consider carefully the potential to eliminate or reduce known or predictable hazards. The decision on how best to specify rooflights should take account of the risks associated with temporary gaps during construction, and the risks when access to the roof is needed later e.g. during maintenance or cleaning.

As in all building work good safety standards are essential to prevent accidents. In accordance with the Health and Safety at Work Act and the Construction (Design and Management) or CDM Regulations 1995, the building should now be designed with safety in mind, not only for the construction period but throughout the normal life of the building. This must include considering the safety of people involved in maintenance and repair, and even demolition. It might mean providing permanent access to the roof, walkways and parapets, for example. The HSE document HSG 33 Safety in Roof Work refers specifically to fragile rooflights as an example of a potential hazard to be considered and to be avoided as far as possible.

Construction of the roof is one of the most hazardous operations because of the potential for falls or material dropping onto people below. The roofing contractor must plan and document a safe system of work before starting construction. This must take the fragility of the cladding systems into account. Whilst fully fixed metal sheeting is generally regarded as non-fragile, many rooflights and metal lining panels must be treated with more care.

Where specifying rooflights designers should consider the following options:

Specifying in plane rooflights that are non-fragile.
Fitting rooflights designed to project above the plane of the roof, and which cannot be walked on (these reduce the risk but they should still be capable of withstanding a person falling onto them).
Protecting rooflight openings e.g. by means of mesh or grids fitted below the rooflight or between the layers of a built-up rooflight.
Specify rooflights with a design life which matches that of the roof, taking into account the likely deterioration due to ultraviolet exposure, environmental pollution, and the internal and external building environment.

When properly fixed, most GRP and polycarbonate double skin in plane rooflights can be classified as non-fragile (usually Class B), using the industry accepted test procedure ACR[M]001:2000. (for more information refer to NARM Guidance Note:2003/1). All in plane units (even non-fragile) should be identifiable when installed, (for example by the use of poppy red fixing heads) to identify the rooflight location.

PVC, which is an inherently brittle material, always requires extra safety reinforcement. However, even non-fragile rooflights are likely to be damaged by impact; they are usually not intended to support foot traffic and crawling boards must be used at all times.

Out of plane rooflights (including modular rooflight units, barrel vault and patent glazing derivatives, etc.) should also be classified to the requirements of ACR[M]001:2000. Consideration should also be given to the requirements of prEN1873 using an energy rating of 1200 joules.

On completion of the building, designers should provide a Health and Safety File to the building owner. The following information should be included in respect of the roof and rooflights:

No person should have access to the roof, unless under the direct supervision of a competent person who is to assess and take action to minimise risks.
Access to the roof should be avoided when it is wet or in slippery conditions.
The rooflight specification, including the weight (thickness) of the rooflights, the non-fragile test method and classification when new, and the expected non-fragile life of the roof and rooflights.
A schedule for cleaning and maintenance for both performance and longevity of the specific rooflights.
Never walk on rooflights, irrespective of their non-fragility classification. Even rooflights that are designed to be non-fragile for the life of the roof could be damaged by foot traffic, and this may affect both the non-fragility performance and the light transmitting quality of the rooflight in the long term.

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Legal Requirements – Fire Performance
Existing UK Building Regulations
Building Regulations Approved Document B (2000 edition) sets out the rules for fire safety of buildings. Section B2 covers internal fire spread, and applies to the linings of both the roof and walls of buildings. In general these are surface spread of flame requirements to BS476 Part 7 (typically Class 1 and Class 3). Section B4 covers external fire spread and applies to external coverings or roofs and walls; in general these are fire resistance requirements to BS476 Part 3 (typically AA and AB).

Thermosetting materials (GRP) can be tested to BS476 Parts 3 and 7, and a variety of grades are usually available offering alternative fire ratings to meet the main requirements.

Themoplastic materials cannot be tested to BS476 Part 3, as the material melts during the test. Building Regulations define an alternative classification method for these materials:

Polycarbonate at least 3mm thick, PVC (any thickness), and any thermoplastic materials, which are rated Class 1 to BS476 Part 7, are given the rating Tp(a).
Other thermoplastic materials can be tested to BS2782, and given ratings of Tp(a) or Tp(b).
Polycarbonate or PVC which achieve Class 1 when tested to BS476 Part 7, can also be regarded as having AA designation.

For the majority of industrial buildings, the requirements can be summarised as follows:

The lining of a roof or wall should normally be rated Class 1 to BS476 Part 7 or Tp(a).
A concession allows the lining to be rated Class 3 or Tp(b) if the area of each rooflight is less than 5m2, and there is a clear space of 1.8 metres in all directions between each rooflight.
There are no restrictions on use of roof outer sheets rated at least AC to BS476 Part 3. Rooflights with outer skin fire ratings less than AC should not be used within 6 metres of a boundary.
A single skin sheet must meet the requirements for both the inner ceiling and outer roof surfaces.
The only requirement for greater protection of wall outer sheets is where the building is within 1 metre of a boundary or is over 20 metres tall or is a building to which the public have access, when some areas will require sheets rated Class 0.

Forthcoming European Regulations
New European classification systems are not directly comparable to existing UK tests. They will measure reaction to fire and resistance to fire. Reaction to fire is measured by a classification system giving rating A to F. It is unlikely that any plastic rooflight materials would ever achieve an A classification. Ratings B to F are determined by a small flame test and the SBI test. Resistance to fire may be measured by one of four tests (based on original French, Nordic, UK and German tests) to EN1187.

A European supplement to Approved Document B, will detail which new European tests and ratings will be required to replace existing UK tests in various applications. At the time of writing, the latest version of this supplement specifically excludes rooflights. Existing UK tests, as detailed above, are currently the only means of complying with Building Regulations.

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