Timber cladding: dealing with an apparent 60-year mistake

Subsequent legal disputes confirmed the evidence of the real fire, where timber cladding did not “adequately resist the spread of fire over the walls”, to satisfy Requirement B4(1) in the Building Regulations.

Creating specific regulations based on Reaction to Fire

Following the Grenfell Tower fire, the government introduced a specific Building Regulation 7(2), requiring Class A2‑s1,d0 materials to BS EN 13501-1 Reaction to Fire on the external walls of high-rise residential buildings.

This limited the amount of fuel available for fires by restricting the calorific value of materials, with a list of practical exclusions in Regulation 7(3).

Timber cladding, by contrast, has a higher calorific value, is designated as a Class E material and at best may achieve a Class B rating when treated with some fire retardants. Surface-painted fire retardants are less reliable than fire retardant impregnated under vacuum pressure throughout the timber, and not all impregnations on the market are equivalent in effectiveness.

Timber cladding may be fashionable as a ‘green’ material, but fire retardants are often considered unsustainable by environmentalists. In other buildings outside the scope of Regulation 7(4) for 7(2) there is no specific regulation preventing Class E timber cladding.

Why is timber cladding approved?

Approved Documents are government guidance showing ways to comply with the Building Regulations, which include the legal requirements in Schedule 1, Part B concerned with fire safety.

In both volumes of the non-mandatory Approved Document B (AD-B), where the Reaction to Fire of materials is classified, there is a longstanding note which states: “Timber cladding at least 9mm thick is also acceptable.” 

Prior to AD-B in 1985, the first National Building Regulations of 1965 approved 3/8-inch timber cladding. The Real Fires researchers argue this originates not from policy intent, but from a misreading of a 1960 fire test.

The result is many buildings following AD-B do not reflect the real fire performance of timber cladding. Both OFR Consultants and CROSS have confirmed the 9mm note is unreliable yet remains in AD‑B, while continuing to influence design decisions and fire risk assessments.

Correcting the regulations

Since 2022, the Fire Risk Appraisal of the External Walls in PAS 9980 has been used to inform fire risk assessments of existing buildings, when considering external wall construction that is “tolerable”. PAS 9980 does not correct the legacy of timber cladding at least 9mm thick, insufficient for Requirement B4(1).

Fire risk assessors may consider external wall construction as “tolerable” even where emerging research shows otherwise. This creates differences in views of reasonable life safety as the qualification in Building Regulation 8 among lenders or insurers additionally concerned with property protection.

This results in a problem for residents inhabiting combustible‑clad buildings and facing ongoing financial uncertainty.

Litigation highlighted these issues for Samuel Garside House, but the time it takes, the costs involved and the case-by-case results it delivers make it impractical to deal with the number of buildings affected.

A straightforward fix would be to withdraw the AD‑B timber cladding note and to adopt a universal specific Building Regulation, improving Regulation 7(2). This aligns with the approach in the insurance sector, where maximising Class A2-s1,d0 construction is an essential principle for both life safety and property protection.

As the industry awaits updates to Regulation 7(2), or a new specific Building Regulation, our advice to clients is to remain alert to this emerging regulatory context when considering timber cladding. Meanwhile, AD-B is projected to 2029 with amendments, before a further revision is anticipated.

The approval of timber cladding at least 9mm thick in AD-B is likely to be corrected, though it is astonishing it was not addressed for 60 years. Being aware of this trajectory now will help building owners plan proactively and avoid being caught out by future changes.

If new or replacement timber cladding is sought, it should be fire retarded to Class B-s1,d0, using a vacuum impregnated chemical of proven effectiveness, and a minimum thickness of 22mm should be specified.

Replacement of 9mm timber cladding previously thought by AD-B to be sufficient for Requirement B4(1) in the Building Regulations should be considered by building owners. So far as reasonably practicable, of course, timber cladding should be designed out of external wall construction.

A consultation to amend Approved Document B is now open, and any professionals who want to put right loopholes and errors such as those relating to timber cladding, should make their views heard.

Ian Abley, technical designer, TFT