Guidance

Abiding principle

A wall that complies with Building Regulation 7(2) may not meet Requirement B4. This is because regulation 7(3) exempts certain materials and components within an external wall from achieving A2-s1, d0 or better. Once a wall is considered Regulation 7(2) compliant, the design team should consider the impact of all the combustible components included in the specification to ensure that walls ability to meet B4 has not been compromised.

Membranes

Sealing membranes

These are the membranes that are typically used around windows, but they can also be used along other interfaces. Membranes are exempt from the provision of Regulation 7(2) through Regulation 7(3) and do not need to be A2-s1, d0 or better. However, some inconsistency exists around whether sealing membranes are considered membranes or seals, both of which are exempt under Regulation 7(3) but treated differently in the guidance in ADB.

ADB suggests that membranes should be B-s3,d0 or better, this guidance does not apply to seals.

The Society of Façade Engineers (SFE) / Centre for Window & Cladding Technology (CWCT) guidance suggests that sealing membranes around openings when no wider than 250mm are seals not membranes, and as such they do not need to follow the guidance in ADB to be B-s3,d0 or better. The SFE/CWT guidance was first published in September 2020 and at the time there was less availability of sealing membranes that achieved B-s3,d0 or better. These are now widely available. Also, the 250mm limit is often not practical, especially in recladding projects where you might need more.

This SFE/CWCT guidance was updated in 2023 but the updates did not impact on the view above”

Sweco do not agree with the SFE/CWCT 2020 guidance. Sweco believe that sealing membranes around windows are like any other membranes, they are not seals and should be B-s3,d0 or better as suggested in ADB.

It is recognised that ADB is guidance only and a Fire Engineer could submit justification for the use of membranes that are worse than B-s3,d0 as part of an application. However, Sweco would not recommend this.

Note – EPDM is a combustible sealing membrane. Due to the fact that this was the only sealing membrane used before the introduction of Regulation 7(2), the name “EPDM” is still being used to indicate any type of sealing membrane. The term “sealing membrane” should be used instead.

Breather membranes

These are the membranes that are typically installed over sheathing boards. As a membrane they are exempt under Building Regulation 7(3) from being A2-s1,d0 or better. However, ADB suggests that they should be at least           B-s3,d0. Once again it is important to understand ADB gives guidance on how to comply with requirement B4 and is not prescriptive, unlike Regulation 7(2). This means the design team can use an expert to substantiate the use of a membrane not achieving B-s3,d0, but Sweco policy is to advise against this.

Any other membrane including Vapour Control Layers (VCL)

VCL is a membrane used to mitigate the risk of interstitial condensation in an external wall build up. As a membrane it is exempt from Regulation 7(2). Similar to other membranes in external walls serving a relevant building ADB suggests that, although exempt under Regulation 7(3), it should achieve B-s3,d0 or better. Sweco once again would advise against the design team deviating from this guidance and seeking to substantiate compliance with Requirement B4 through expert justification.

There could be more nuanced applications where a membrane could be considered a seal. These would need to be looked at on a case-by-case basis.

Specified Attachments

Balconies come within the definition of specified attachments and as such Regulations 7(2) and 7(3) apply. Membranes specified in a balcony build up are exempt from 7(2) through Regulation 7(3). Sweco’s view is the guidance in paragraph 10.21 in AD B only applies to external walls and not specified attachments. This means there is no guidance on a minimum standard for balcony membranes in terms of reaction to fire as classified through EN13501.

However, it should be noted Regulation 7(3) infers that the top surface of the balcony should be A2fl-s1 or better. If a membrane is proposed of lesser performance it would need to be housed between the top surface material and the structural deck of the balcony.

Once again, a competent person should assess the cumulative impact of any combustible materials permitted by Regulation 7(3) in the balcony construction, to ensure that requirement B4 is still met.

Roofs and terraces

Roof membranes are not controlled under Building Regulation 7(2). This regulation is limited to external walls and specified attachments serving a relevant building.

For roofing systems guidance on how requirement B4 is met, can be found in the appropriate section of ADB.

Roof intersection with relevant building external walls

Regulation 7(3) was amended to clarify that roof upstands do not become part of an external wall at their intersection.

To perform its waterproofing function, any waterproofing roof membrane that continues up the external wall, Sweco’s view is that the element of the roof continued vertically up the face of the wall to form the weather seal is part of the roof and therefore exempt from Regulations 7(2) & 7(3). Even though the “continuation of the roof” is exempt, the reasonableness of the junction detail still needs to be considered in terms of compliance with B4. Often the detail may include an upstand of combustible insulation as well as the continuation of the combustible roof membrane. This insulation may be considered part of the roof.

The design team should firstly substantiate the extent and size of the detail technically e.g. as a weather seal or prevent cold bridging etc.

Then its compliance with B4 in terms of size, number, position and orientation of the combustible materials in the proposed design should be considered by a competent specialist.

A number of bodies have given “rules of thumb” dimensions for what would be an acceptable limit for the height the combustible roofing membrane extends up the wall. Sweco believe each intersection should be considered on its own merits, considering the nature of the materials involved and their size, number, position, and orientation.

Sweco consider parapet details on external walls to relevant buildings to be part of the external wall and controlled by Regulation 7(2).

Relevant Building Definition

“Relevant building” means a building with a storey (not including roof-top plant areas or any storey consisting exclusively of plant rooms) at least 18 metres above ground level and which:

• contains one or more dwellings;
• contains an institution; or
• contains a room for residential purposes

Reaction to fire classification

Reaction to Fire Classification is set down in BS EN 13501 Part 1 using data from the following test standards.

• Non-combustibility test (EN ISO 1182)
• Heat of combustion test (EN ISO 1716)
• Single burning item test (EN 13823)
• Ignitability test (EN ISO 11925-2)
• Determination of the burning behaviour of floorings, using a radiant heat source (EN ISO 9239-1) (Used for A2fl-s… Bfl-s…etc)
• EN 13823 (used for Class BL, CL DL, EL, FL. L – Linear Pipes)