Faculty of law blogs / UNIVERSITY OF OXFORD

The Height of Confusion?

Guest author Jonathan Evans is Chairman and CEO at Ash and Lacy. This post originally appeared on his LinkedIn blog, and is reposted with his permission.


Jonathan Evans


Time to read

5 Minutes


Photo by Simone Hutsch on Unsplash

The Cube fire on November 15th 2019 was a brutal reminder that buildings up and down the country have dangerous materials on their walls nearly two and a half years after the Grenfell tragedy. It exposes the impotence of the government’s Building Safety Program and the futility of derogating the dangers of non-ACM materials such as High-Pressure Laminates.


No lives were lost but many have lost precious belongings and have had their lives and educations traumatically disrupted. Government advice and the Building Safety Program has focused on buildings ‘over 18m’. Initial reports suggested The Cube was ‘under 18m’ which is a significant issue in at least three regards: (a) in assessing whether the scope of the remediation program is adequate; (b) in giving clues to the possible causes of the rapid spread of fire (due to the significant step change in building material performance requirements by Approved Document B around this level); (c) was this building, which without question failed the Building Regulations B4 functional requirement/statement-of-the-obvious, compliant with approved guidance?

Members of the public were vocally surprised that the building was reported as ‘under 18m’ as the seven-storey building clearly appears to be over 18m in height. However, those familiar with our Approved Documents know that nothing is simple. It transpires that The Cube incident highlights at least two further issues with AD B. Firstly, it’s very easy even for professionals and third party certifiers to get the cladding specification wrong on a building of this height. Furthermore, the origin of this confusion dates back to changes made nearly thirty years ago and are reminiscent of those widely discussed in relation to the Grenfell Tower tragedy.

Those who've followed the Grenfell Class 0 cladding debate will be familiar with articles in Inside Housing that describe events and changes in AD B leading up to the tragedy. There was a subtle wording change in the 2006 AD B edition that allowed combustible insulation to be used ‘above 18m’ via the BS8414 + BR135 system testing route. Guidance changes relating to The Cube bear many similarities.

Confusion highlighted by The Cube results from AD B comprising two height thresholds – building height dictates the cladding requirements and storey height dictates the insulation requirements. For the vast majority of (residential) buildings there is no practical difference, but for a building around seven storeys like the Cube, it can be crucial.

AD B 2019 Table 12.1 shows that the reaction-to-fire classification requirements of the external surfaces (i.e. the cladding) vary with respect to 'building height':



However, in Clause 12.6 you can see that the reaction-to-fire classifications of insulation products etc. are defined in relation to whether the building has ‘a storey 18m or more in height’. It then cross references the reader to the associated Diagram D6 in Appendix D. As you can see, the key measurement is to the floor finish level of the top storey which is likely to be around 3m lower (in the case of a flat roof, more if pitched) than the ‘building height’. As a result, a building that is required to have limited combustibility insulation is at least 21m in height, whereas for the cladding to have elevated reaction-to-fire properties, it is only 18m. It's possible that there are a disproportionate number of non-compliant buildings within this 3m zone.



Table 12.1 doesn’t make an equivalent diagrammatic cross reference regarding building height, but "Appendix A: Key Terms" confirms that ‘building height’ as used in Table 12.1 is measured as shown in D4.





This inconsistency is largely inconsequential for residential buildings unless you have a building which is over 18m in height, but does not have a storey over 18m. The Cube is such a building and it will be interesting to see whether it has been built with the requisite fire-retardant cladding above 18m. If you have no storey above 18m it's easy to believe that there are no reaction-to-fire provisions for the cladding materials (especially when a third-party certificate appears to make the same mistake). I think it’s fair to say that most professionals would think of a ‘storey above 18m’ being the determining factor and I believe this is what the Greater Manchester Fire and Rescue Service had in mind when declaring that The Cube was ‘under 18m’.

The key question I attempt to address here is “why do we have two rules if it creates complexity and confusion, rather than just one like in 1985?”

The first Approved Document B was published in 1985 after the introduction of The Building Act 1984 and life appeared relatively simple. There was only one height measurement rule (building height), and it applied to the entire external wall construction. There wasn’t even a diagram. From the outset there were separate clauses 4.7 and 4.13 relating to the reaction-to-fire requirements of the insulation (required to be limited combustibility above 15m) and cladding respectively (required to be Class 0 above 15m and a BS476/6 Fire Propagation Index of <21 below 15m, if the building was over 15m).



As has been the case ever since, there were no cladding provisions for buildings below the relevant threshold of the day. (There is no requirement at any height in any edition of AD B prior to 2019 for cladding to be limited combustibility, despite what @mhclg has claimed since Grenfell).





However, in the very first revision of Approved Document B in 1992 things start to get rather messy.



The ‘building height’ threshold moves up from 15m to 20m (no sign of 18m) and a very familiar diagram appears.

A separate height threshold mechanism is introduced for insulation. The limited combustibility threshold is now a building with “a storey at more than 20 m”. Given a flat roof this is roughly a ‘building height’ of at least 23 m - probably eight or nine storeys. This is approximately three storeys or eight metres higher than before – a 50+% increase.



Fragmenting the overarching clause 4.7 leaves requirements for materials other than cladding or insulation undefined, such as cladding mounting components, sheathing boards and membranes. AD B loses an elegance it will never regain. Combustible membranes (and polyurethane expanding foams?) are yet to appear on the market, but this gap never gets completely addressed (even by the ‘filler ... etc’ wording of 12.7 in 2006). The distinct clause convention that separately defines cladding and insulation requirements through numerous revisions and amendments was established at this point.

The increase in height from 15 m to 20/23 m is an obvious step backwards for fire safety, and so is the uncertainty created for other materials. But why was a second method of height measurement introduced that so confusingly complicates matters?

I suspect that there were at least two factors which are possibly linked. At the time of the first revisions in 1992, we have the United Nations Framework Convention on Climate Change which is later extended to become the Kyoto Protocol. It is clear that climate change, energy efficiency and CO2 emissions were already a political issue.

At this time, we also see the emergence of polyurethane/PIR composite cladding panels. This was an exciting, new and innovative construction technology particularly attractive to non-residential buildings.



The intentional change of wording to ‘a storey above 20m’ meant that combustible insulation such as PIR/PUR composite panels could be used on virtually any non-residential building regardless of height. (Why this threshold was subsequently reduced to 18m is unclear.) This doesn’t appear to be a nefarious attempt to reduce safety standards for commercial purposes given the political context.

The unintended consequence is an increase in complexity and confusion. The Lakanal Coroner called for a simplification of Approved Document B. MHCLG has a track record of minimising the restriction on combustible materials and each time it does so (such as reducing the scope of the combustible ban in Dec 2018), it manages to take us in the opposite direction.

One final question: 'how common is it for buildings to be designed just under the 18m thresholds to benefit from cheap, possibly untested materials?' If this is already common practice, will the 53 Hackitt Report recommendations, tougher regulatory powers, criminal and dutyholder responsibilities over 18 m simply determine her legacy will be a country full of highly combustible 17.9 m buildings?


How to cite this blog post (Harvard style) Evans, Jonathan. (2019). The Height of Confusion?. Available at: https://www.law.ox.ac.uk/housing-after-grenfell/blog/2019/12/height-confusion (Accessed [date])