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External wall systems in existing multi-storey residential buildings - fire risk: advice note - version 2

Version 2.0 of this Scottish Advice Note provides advice for those responsible for fire safety in residential buildings including building owners/managers/residents groups responsible for determining the fire risk posed by external wall systems on existing multi-storey residential buildings.


Part 2: Technical Advice

This part focuses on the process of determining fire risk. Sections 2.1 and 2.2 establish benchmarks to assist in the determination of risk and provide relevant background information. Sections 2.3 and 2.4 provide risk-based guidance on cladding systems and the UK Government Fire Test results respectively, which can inform the determination. Annex 1 contains information relevant to other external wall system features.

2.1 Benchmarks for Assessing Risk

Fire risk assessors often use guidance from a variety of sources as a means to inform their professional judgement in the assessment of risk. Guidance within the Technical Handbooks that support Building Regulations can be used as a benchmark to assist with the fire safety risk assessment process for existing buildings. These benchmarks should not be treated as prescriptive or minimum standards or even recommendations. It would be unreasonable to expect existing buildings which were previously deemed compliant with Building Regulations to be upgraded to current standards, unless justified by a significant risk to life. Benchmarks should be regarded as comparators to assess how far removed existing provision is from current standards. The assessor must then decide whether or not deviations from the benchmarks result in unacceptable risk and, if so, what reasonable measures may be required to reduce that risk to an acceptable level. This does not mean the risk is eradicated, but it should be reduced to a level where life safety is ensured so far as is reasonably practicable.

In June 2022, the Building (Scotland) Amendment Regulations 2022 came into force. External wall cladding systems of Relevant Buildings and Specified Attachments in buildings having a storey at a height of 11m or more above the ground must be constructed of materials achieving European Classification A1 (non-combustible) or A2 (will not significantly contribute to fire load and fire growth). The large scale fire test BS 8414 and BR 135 is not permitted as an alternative approach for Relevant Buildings. See Annex 4 for more background information on the BS 8414 test and the performance criteria in BR 135.

Relevant Building means a building having a storey, or creating a storey (not including roof-top plant areas or any storey consisting exclusively of plant rooms) at a height of 11 metres or more above the ground and which contains a -

a. dwelling,

b. building used as a place of assembly, or as a place of entertainment or recreation,

c. hospital,

d. residential care building or sheltered housing complex,

e. shared multi-occupancy residential building.

Specified attachment means -

a. a balcony,

b. solar shading,

c. a solar panel,

attached to an external wall.

For all other residential buildings BS 8414 and BR 135 may be used as an alternative means of complying with the mandatory standards. In such cases, local authority verifiers are requested to notify the Scottish Government Building Standards Division of any building warrant applications made citing BS 8414 as a route to compliance.

The European Classification standard BS EN 13501-1 'Fire classification of construction products and building elements. Classification using data from reaction to fire tests' may be used for external wall systems. This classification system has seven classes A1, A2, B, C, D, E and F which denotes fire performance: A1 being the highest performance and F being no defined performance. Some classes could have sub-indices s1, s2 or s3 to indicate smoke development, and sub-indices d0, d1, or d2 to indicate potential for flaming droplets. An example with sub-indices is B-s3,d2, where s3 indicated emissions with high volume smoke intensity (whereas s1 would indicate absent or very little smoke emission) and d2 indicated high/intense dripping droplets (whereas d0 would indicate no burning droplets).

More information on current benchmarks can be found in the Technical Handbooks on the Scottish Government Building Standards website.

The sections which follow give an overview of the common types of external wall system available and explore further how the benchmarks are used to inform the risk assessment process for existing residential buildings. The results of large and intermediate scale fire tests are also provided in Section 4 and should be referred to when assessing external wall systems that do not meet the benchmarks.

2.2 Common Types of External Wall System

Metal Composite Material (MCM) cladding is popular because of their precise flatness, variety of surface finishes and colours, lightweight and formability. They are often used as rainscreen panels which prevent significant amounts of water from penetrating into the wall construction. Thermal insulation, airtightness and structural stability are provided by the second, inner part of the wall construction. However, fire performance varies greatly according to the composition of the core material and the panels can melt, warp, disband or delaminate in a fire.

Highly combustible MCM cladding means any panel or sheet, having a thickness of no more than 10 mm, which consists of a number of layers, two or more of which are made of metal, alloy, or metal compound and one or more substantial layer of which is made of material having a gross calorific value of more than 35 MJ/kg when tested in accordance with BS EN ISO 1716:2018 entitled "Reaction to fire tests for products – Determination of the gross heat of combustion (calorific value)" published by the British Standards Institution. For the purposes of this definition a substantial layer means a layer which is at least 1 mm thick or has a mass per unit area of at least 1 kg per m².

Three cores materials are commonly available, each with a different fire performance. Table 1 shows information on different categories relative to core type used by the UK Government screening tests following the Grenfell Tower fire. The type of cladding on Grenfell Tower was ACM (Category 3) with an unmodified polyethylene core.

Table 1 – Summary characteristics of MCM cladding by core type
Category of MCM cladding Common core composition Calorific Value MJ/Kg [1]
1(non-combustible or will not significantly contribute to fire load and fire growth) [2] Largely of mineral composition affording a high standard of fire performance ≤ 3
2 (fire retardant - FR) A mixture of polyethylene and other, inorganic additives to enhance fire performance >3 and ≤ 35
3 (non-FR - highly combustible) Unmodified polyethylene (PE); > 35

Notes

[1] Screen test based on BS EN ISO 1716 'Reaction to fire tests for products. Determination of the gross heat of combustion (calorific value)'.

[2] European Classification A1 (non-combustible) or A2 (will not significantly contribute to fire load and fire growth) as defined in BS EN 13501-1: 2018.

Appraisal of external wall systems

There are many other common types of external wall system including but not limited to: rendered systems with mineral wool, expanded polystyrene (EPS), polyurethane (PUR) and phenolic foams (PF); masonry slips; timber; masonry; tile; slate; stone; metal sheeting; insulated core sandwich panels, High Pressure Laminate (HPL) panels etc.

HPL panels are usually made by taking sheets of wood or paper fibre, layering them with a resin and bonding them under heat and pressure. The fire classification can vary depending on its material properties, thickness and whether or not it incorporates fire retardant additives. HPL panels with fire retardant chemicals added are sometimes referred to as FR grade and may achieve European Classification B-s1,d0 in accordance with BS EN 13501-1. Panels manufactured without fire retardant can be European Classification C, D or even lower depending on the thickness and make-up of the panel (e.g. fibre and resin used).

All components of the external wall system should be assessed.

There is a procedure by which certain construction products can be assigned a fire classification without the need for testing (known as 'Classified Without Testing'). These products have well established reaction to fire performance and has been agreed by the European Commission's Standing Committee on Construction. Products belonging to European Classification A1 (non-combustible) are listed in Commission Decision 96/603/EC of 4th October 1996 as amended by 2000/605/EC and 2003/424/EC. This means that the products listed do not require to be tested provided they do not contain more than 1.0% by weight or volume (whichever is the more onerous) of homogeneously distributed organic material. Materials listed include, expanded clay, concrete, metal products, natural stone and slate products, clay bricks, terrazzo and clay tiles, glass, ceramics, mineral wool etc.

Many buildings in Scotland have external wall systems where the cladding is constructed from these products and hence achieve European classification A1 as per EC decision 96/603/EC as amended and not considered to represent a risk of vertical fire spread.

Table 2 provides some examples of common materials used as external wall cladding of multi-storey residential buildings in Scotland. Whilst some of these materials achieve European Classification A1 (non-combustible) or A2 (will not significantly contribute to fire load and fire growth) and may be considered low risk, the fire performance of the entire external wall system should be assessed. Each system will vary depending on the combustibility of materials that make up the external wall system including the presence of any combustible insulation material behind the outer cladding. This may require further investigation in the substrate is not known following the desk top review and site walkover. The presence of fire resisting fire barriers and/or cavity barriers located within the external wall system are also critical to inhibit vertical fire spread. These barriers may be present within any cavity or within the EWS construction. For example, it is common to install fire/cavity barriers in the cavity or within the external thermal insulation composite system (ETICS) on the same line as separating/compartment walls and floors.

A list of exemptions to European Classification A1 and A2 components that form part of a non-loadbearing external wall cladding system is provided in the Technical Handbooks and includes fire barriers, cavity barriers, membranes, seals, gaskets, fixings, sealants and backer rods etc.

Table 2 - Examples of typical external wall cladding materials used on multi-storey residential buildings
Materials Low risk European Classification [1] Medium risk European Classification [1] High risk European Classification [1]
Masonry units Clay bricks (A1) Concrete blocks and bricks (A1) Natural stone (A1) Ceramic blocks (A1) [1] [1]
Cladding panels Homogeneous metal (A1) [1] [1]
Metal composite material (A2) Cat 1 Metal composite material - Fire Retardant (FR) Cat 2 Metal composite material (non-FR) Cat 3
Natural stone (A1) Reconstituted stone (A1) Composite stone (FR) [1]
Precast concrete panels (A1) Precast brick panels (A1) Terracotta tiles (A1) Glass-fibre reinforced concrete panels (A1/A2) Fibre cement cladding (A1/A2) Basalt mineral fibre (A1/A2) Glass-fibre reinforced concrete panels (FR) Fibre cement cladding (FR) Basalt mineral fibre (FR) [1]
Honeycomb bonded (A2) Honeycomb bonded (FR) Honeycomb bonded (Non-FR)
Not Applicable High Pressure Laminate (FR) High Pressure Laminate (Non-FR)
Not Applicable Acrylic polymer Polymer composite PVC cladding boards
[1] Glass composite Glass-fibre reinforced Glass Reinforced Plastic (GRP)
Not Applicable [1] Natural wood Laminated wood
Masonryslips Masonry slips with non-combustible backing Masonry slips with combustible backing GRP slips
Insulated rendersystems Render on mineral wool Render on combustible insulation [1]
Insulated panels Metal insulated panels with mineral wool core Metal insulated panels with combustible insulation core GRP panels with combustible insulation core
Insulationmaterials behind outer cladding Mineral wool insulation Combustible insulation including Polyisocyanurate (PIR) foam, Phenolic foam (PF) and Polyurethane (PUR) foam Expanded Polystyrene (EPS) and Extruded Polystyrene (XPS)

Notes

[1] The European Classification standard BS EN 13501-1 'Fire classification of construction products and building elements. Classification using data from reaction to fire tests' may be used for external wall systems. This classification system has seven classes A1, A2, B, C, D, E and F which denotes fire performance: A1 being the highest performance and F being no defined performance. Due to variances in composition, some types of products may meet the parameters for either low, medium or high risk. Additional technical information is contained within the Technical Handbooks.

2.3 Risk-based Guidance for External Wall Systems

Fire safety risk assessment does not routinely involve opening up construction. However, as explained in Part 1, an intrusive investigation may be required to identify the type and thickness of materials used or to investigate for suspected deficiencies in structural fire protection, such as inadequate provision of cavity/fire barriers or fire stopping. An intrusive investigation requires a contractor to open up construction and make good after the inspection.

When determining the risk, a range of risk factors and related issues must be considered. The guidance provided is split into 3 sections, as follows:

  • All residential buildings regardless of height;
  • Buildings with any storey at a height of 11m or more; and
  • Buildings with any storey at a height less than 11m.

(a) All residential buildings regardless of height

(i) Highly Combustible MCM (Category 3) cladding

Extensively clad - In light of the Grenfell Tower tragedy and evidence from subsequent UK Government fire tests, any building extensively clad with an external wall system incorporating highly combustible MCM (Category 3) cladding should have the panels removed without delay. Interim measures may need to be put in place until this is done.

Partially clad - Buildings that are partially clad in highly combustible MCM (Category 3) cladding would also be expected to require remediation especially if located above egress and access routes to the building. Any decision not to remove highly combustible MCM (Category 3) cladding from a partially clad building must be robustly justified in the fire safety risk assessment and any accompanying appraisal.

(ii) Risk factors and other considerations for all external wall systems

The fire risk assessor will need to consider a range of risk factors to fully determine the risk posed by external wall systems, such as:

  • The height, use and complexity of the building;
  • The number, location and arrangement of escape routes;
  • Fire safety measures within the building such as fire separation, compartmentation, automatic fire suppression, automatic fire detection and warning (although internal measures may not by themselves prevent a fire involving the envelope having consequences);
  • The ability of occupants to recognise and respond to a fire or a warning of fire without assistance;
  • The type of evacuation strategy in use, whether 'stay put', delayed or simultaneous and the anticipated evacuation time, should evacuation become necessary;
  • Staffing levels, where required for the type of evacuation method employed;
  • The type, position and extent of the external wall system and its potential for fire spread;
  • The risk of ignition of the external wall system both from external sources and from inside the building e.g. via any unprotected window reveals;
  • The quality of construction or presence of building defects;
  • The combustibility of other aspects of the building construction;
  • General management and maintenance arrangements e.g. golden thread of information including a current fire safety risk assessment and any external wall system appraisal and condition of the building structure;
  • The suitability of facilities for firefighting, including site access and water supplies and other operational considerations e.g. dry or wet risers, smoke ventilation systems and the shielding effect of cladding and other attachments or features which could impede the effectiveness of water jets on a fire within the cavity behind.

Further considerations for combustible external wall systems on both extensively and partially clad buildings:

  • An internal fire can break out of the building envelope via unprotected openings such as windows or doors. Cavity/fire barriers should be provided around the openings to inhibit fire spread directly into the cavity. Other apertures in the building envelope may also pose a risk, including areas around balconies and other features such as solar shading and shutters, as well as unprotected service penetrations which may be present such as vents, pipes, ducts etc. Cladding should be removed if a fire could spread and compromise external features provided for life safety, such as automatic opening vents.
  • Cladding which crosses any vertical or horizontal fire separation or compartmentation lines or cavity/fire barriers may enable a fire to spread beyond those lines. A vertically aligned partial band of cladding creates a greater risk of rapid fire spread vertically up the building than a partial horizontal band. Where panels are not directly located adjacent to each other, an appraisal specialist should be able to calculate the likely radiant heat flux, size of any flame, as well as how it might behave in differing weather conditions, and whether or not it could ignite another panel.
  • Cladding around entrances, exits and fire escapes may impede escape and access for firefighting due to the potential for falling debris.
  • Cladding located at or near to ground level may be vulnerable to accidental or deliberate exposure to fire e.g. parked cars and bin stores. Cladding may require to be removed or protected against ignition if it could be vulnerable to either accidental or deliberate ignition which could compromise safe access to and exit from the building.
  • The potential for fire spread to/from a neighbouring building should be considered. Cladding may need to be removed if presenting an impingement/radiative hazard. If thought to be a concern, hand calculations or computer modelling of the radiant heat flux on the facade may be required.
  • The potential for falling debris or burning droplets to contribute to downward fire spread should be considered. This could involve other cladding panels or combustible materials in cavities or stored on balconies.
  • Where there are concerns about the quality of construction and installation, including method of fixing or regarding the presence or specification of components such as cavity/fire barriers, an appraisal of the external wall system will be necessary.

(b) Buildings with any storey at a height 11m or more

From 1 June, building regulations require new building work involving external wall systems on relevant buildings with any storey at a height of 11 m or more above the adjoining ground to be constructed of products achieving European Classification A1 (non-combustible) or A2 (will not significantly contribute to fire load and fire growth). This is to ensure that external walls in taller buildings do not contribute to the development of fire or to vertical fire spread up the facade of the building. See Benchmarks for Assessing Risk above.

The 11 m storey height threshold is based on the reach capability of a fire and rescue service ground mounted water jet where there is sufficient pressure and flow in the water main. In circumstances where there is insufficient pressure and flow in the water main, additional SFRS resources may be required and assistance sought from Scottish Water. In addition, external rescue by SFRS above this height would depend on the availability of specialist height appliances and adequate site access around the perimeter of the building. Cavity/fire barriers and/or fire stopping between flats/compartments are intended to prevent unrestricted fire spread behind the external wall system.

Failure to comply with current building regulation guidance does not automatically make an existing building unsafe, so there needs to be flexibility when using current benchmarks and each risk assessment or appraisal must be building specific. It is possible that some higher risk buildings under 11m may require remediation to reduce risk. Equally, it is possible for lower risk buildings above 11m to fall short of the benchmark, without it posing an unacceptable risk to life. At all times, the key question should be whether the presence of the external wall system poses an unacceptable risk to life and the full range of risk factors in the preceding section are considered.

Due to changing requirements over time, some external wall systems in existing residential buildings will not achieve the benchmark and may be European Classification B-s3,d2 or lower (or may have been considered safe in terms of surface spread of flame and fire propagation i.e. the now obsolete British Standard Class 0). In such cases, the combustibility of the material beyond the direct surface of the product should be considered in the risk assessment and appraisal, as this can contribute to fire spread over or within the external walls of buildings. Subject to supporting test evidence, it may be appropriate to retain cladding panels achieving European Classification B-s3,d2 or Class 0 if any core material within the products and any insulation material achieves European Classification A2-s3,d2 or better.

Where BS 8414 and BR 135 have been used to achieve compliance under Building Regulations, reference may require to be made to BS 9414 'Fire performance of external cladding systems' to ensure that the results of those tests have been applied appropriately. Assessments carried out before the publication of BS 9414 may be acceptable but should be considered on a case by case basis.

(c) Buildings with any storey at a height less than 11m

For buildings with any storey at a height of less than 11m above the adjoining ground, the fire safety risk assessment must also take into account the full range of risk factors in part 3(a) above. It is recognised that there may not always be the same degree of risk to life in a building which has a storey height less than 11m, compared to taller buildings with more occupants, longer escape routes, larger fire loading, impact of building height on firefighting operations etc.

European Classification B, C, D or E external wall systems is generally allowed under building regulation guidance (other than hospitals and care homes). It should be the guiding principle of the fire safety risk assessment process that occupants can escape safely without an over-reliance on fire service intervention.

Some buildings are considered as being higher risk due to the vulnerability of occupants and other factors. For example, building regulation guidance requires that, in general, all new hospitals and care homes regardless of height should achieve European Classification A1 (non-combustible) or A2 (will not significantly contribute to fire load and fire growth) or satisfy BR135 when tested in accordance with BS 8414 as an alternative means of compliance. For existing buildings less than 11m, remediation, if required at all, may only be necessary for those buildings which are higher risk e.g. premises with vulnerable occupants and delayed evacuation.

2.4 UK Government Fire Test Results

This section contains information on the UK Government's fire test results which should also be considered by fire risk assessors and appraisal specialists when assessing combustible external wall systems that do not meet the European Classification A1 or A2 benchmark, particularly for buildings with a storey at a height of 11m or more and higher risk buildings with vulnerable occupants.

(a) UK Government Large Scale Tests

(i) Aluminium Composite Material (ACM) tests

In 2017, the UK government commissioned seven BS 8414 tests on three different types of ACM cladding in combination with different types of insulation. The full test result reports and advice for building owners are summarised in Table 3.

Table 3 – Summary of 2017 BS 8414 test results for ACM systems
ACM filler Insulation
Polyisocyanurate (PIR) Foam Phenolic Foam (PF) Stone Wool
Unmodified polyethylene (Category 3) fail not tested fail
Fire retardant polyethylene (Category 2) fail fail pass
Non-combustible (A1) or will not significantly contribute to fire load and fire growth (A2) (Category 1) pass not tested pass

A pass indicated that the external wall system resisted the spread of fire in line with the criteria set out in BR 135. These results showed ways in which compliance could be achieved and offer an indication of how remedial works could be specified for those buildings were been found to have problems.

(ii) High Pressure Laminate (HPL) tests

HPL (FR) panels (European Classification B-s1,d0) with stone wool insulation, in the specific configuration tested, also successfully met the performance criteria set out in BR 135. These panels used in combination with combustible insulation may require remediation in residential buildings with any storey at a height of 11m or more or higher risk buildings of any height with vulnerable occupants, unless fully justified in the fire safety risk assessment and any available BS8414/BR135 test evidence.

Residential buildings with any storey at a height 11m or more or higher risk buildings of any height with vulnerable occupants with European Classification C or D HPL panels are likely to require remediation unless fully justified in the fire safety risk assessment and any available BS8414/BR135 test evidence.

(b) UK Government Intermediate Scale Tests

In April 2020, the UK Government published the test and analysis report 'Fire performance of cladding materials research'. The aim was to improve understanding of fire behaviour of cladding. The research showed that none of the materials tested (including HPL), had the same fire performance characteristics of fire growth, fire spread or potential fire breakthrough as the ACM (PE) cladding on Grenfell Tower.

(c) Other Considerations

There are many possible combinations of cladding and insulation and it is possible that products from different manufacturers will perform differently in a fire. In order to carry out an assessment of the fire risk posed by external wall systems, it is important that the detailed fire tests results are made available to fire risk assessors or appraisal specialists. Confidentiality agreements may need to be entered into with the test sponsor in order to obtain the detailed information required.

Test results are highly sensitive to variations in design, construction and the parameters used in the test including the location of cavity/fire barriers and fire stopping. The external wall system installed must accurately reflect the system as tested however, variations may be acceptable provided the variations are within direct and extended field of application rules as defined in BS EN 13501-1. For example, BS 9414 provides defined rules for variations in the application of BS 8414 test results (See Annex 4). Assessment of test results should be carried out by those with appropriate skills, knowledge and experience in fire testing e.g. Chartered/ Incorporated Engineers or UKAS accredited fire test houses.

After the Grenfell Tower fire, a small number of existing BS 8414 test reports have been withdrawn due to errors and omissions in the undertaking and reporting of these tests. This includes the associated Assessments-in-Lieu of Tests (AILOT) carried out by fire test houses. Fire risk assessors or appraisal specialists should satisfy themselves that the system installed accurately reflects the system as tested (including products used in those tests/assessments).

Should the external wall system rely on a BS 8414 test for its compliance and satisfaction of risk, the installation of the components should be evidenced and mirror the installation of the components on the test rig. Any variation from the installation of the external wall as observed on the test rig will need to be assessed by a fire engineer with appropriate qualifications, competence, skills and knowledge of fire safety including the use of BS 9414.

BRE provide a list of some external wall cladding systems which achieved the BR 135 criteria when tested to BS 8414. The list is not comprehensive and does not provide any detailed fire test information. Cladding systems not on the list, but tested to BS 8414, may satisfy the BR 135 performance criteria. Reports from other UKAS accredited fire test laboratories or members of the European Group of Organisations for Fire Testing, Inspection and Certification with specific accreditation for BS 8414 testing may be also considered.

2.5 Fire Risk Rating of External Wall Systems

In order to simplify the decision making process, Table 4 provides two categories of risk to identify those buildings that require remediation and those which do not. The appraisal specialist is required to categorise the building as either 'Low Risk' or 'High Risk'. Where the appraisal specialist is unsure of the risk rating outcome, further investigation should be carried. This may include gathering more documentation / evidence, an intrusive investigation and/or testing of materials or systems to establish the final risk rating outcome.

Table 4 – Risk Rating Outcome

Low Risk

External wall systems in this category are considered safe and require no remediation

High Risk

External wall systems in this category are considered unsafe and require remediation

Contact

Email: BuildingStandards@gov.scot

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