Guest author Jonathan Evans is Chairman and CEO at Ash and Lacy. This post originally appeared on his LinkedIn blog on January 14, 2020, and is reposted with his permission.

In a previous post, I discussed the two 18 m thresholds contained within Section 12 of Approved Document B relating to cladding and insulation reaction-to-fire requirements. I showed how in 1992 a straightforward single height threshold was abandoned so that combustible insulation could be used without testing in 18 m+ single storey buildings, whilst retaining enhanced cladding requirements. From that point we have had an 18 m ‘building height’ threshold that determines cladding requirements, and an 18 m ‘storey height’ threshold that determines insulation requirements. The difference between the two is usually about 3-4 m, and so for the vast majority of buildings it’s inconsequential.

The fire on November 15th 2019 in The Cube student accommodation block on Silverwell Street, Bolton highlighted the range of building height that could potentially lead to confusion. Buildings within this range are above one threshold but below the other, and can often exhibit incorrect specification of the cladding materials and non-compliance.

The Cube did not meet the functional requirement of the Building Regulations B4.1 by adequately resisting the external spread of fire, and it was fortunate that there were no serious injuries or deaths. Naturally, there has been much speculation about whether the materials used on the building met the guidance contained within Approved Document B that is provided to ensure compliance with B4.1. The implications of a building design which followed the Guidance but failed the regulatory functional requirement, would be far-reaching.

Initial statements from the fire and rescue services as well as the building architect suggested that the building ‘was under 18 m’. Buildings under 18 m in height have no external wall material provisions outlined in Approved Document B. This is popularly interpreted as meaning that you can use virtually any materials, within reason. However, no official confirmation of the building height was forthcoming.

A written question was therefore submitted to MHCLG on January 7, 2020 as follows:

Which on January 13, 2020, received the following reply:

Let’s look at this prosaic answer from Esther McVey.

The planning drawings for Bolton show the follow elevation on Silverwell Street:


This indicates that the intended finished floor level of the top storey was 17.86 m above Silverwell Street, which corresponds precisely to the 17.8 m 'as-built' BRE Global measurement. The wording of Ms McVey’s response ‘height of the top occupied storey from ground level’ suggests that BRE Global have used the following method of measurement:

This ‘top storey height’ is the threshold measurement method relating to the items defined in Section 12 Clause 12.7 of Approved Document B – most significantly, the insulation. It does not relate to the reaction to fire requirements of the cladding as defined in Clause 12.6 and Diagram 40. This threshold is measured differently, as follows:


Mr Steve Reed MP asked what ‘the height was for fire regulations purposes’. As discussed, Approved Document B describes two thresholds that are relevant for fire regulations purposes. Given the building was over one threshold but apparently not the other, Mr Reed should have been answered with two heights, the first respectively relating to the cladding (i.e. the distance between the top of the roof and the mean ground level adjacent to the building) and the second relating to the insulation (i.e. the distance between the top floor surface above the lowest adjacent point to exterior of the building). If the building’s top storey was over 18 m, then of course only the lower height is necessary as the building height would clearly also be over 18m.

Even taking into account typical construction tolerances, the ‘building height’ of The Cube is likely to be close to 22.0 m, and therefore well in excess of the 18 m threshold required for Diagram 40(e) of Approved Document B to apply. Accordingly, the High-Pressure Laminate cladding at 18 m and above would need to be at least Class 0 or B-s3, d2, and below that at least Class C-s3, d2 or have a BS 476-6 fire propagation index of 20 or less. This means the upper storey essentially would have needed to be a Fire-Retardant grade. This is the storey that was largely destroyed by the fire.

The lower areas below 18 m could possibly have been a standard grade, but this is uncertain. The Trespa BBA Certificate only quotes the standard grade Class D EN13501-1 classification so it must be reliant on its BS 476-6 fire propagation Index being 20 or less. Andrew Chapman has covered this extensively in his articles but has not been able to get definitive confirmation from Trespa that its FPI is indeed 20 or less. Essex University stated that the FPI was 19 which is extremely close to the allowable limit. Was the colour and coating of the product the same as used as this could be significant with such a close result?

The 17.8 m figure provided by BRE Global courtesy of Ms McVey, clearly relates to the insulation threshold measurement, and comes in just under it. Therefore, the building wouldn’t require limited combustibility insulation or the cladding system to have a BS 8414+BR135 classification.

But is the top storey actually below 18 m?

Diagram C6 above suggests that the storey height measurement should be taken at the lowest adjacent point to the building, as opposed to the building height which is taken to the mean surrounding ground level. The planning diagram shows a distance of 17.86 m to Silverwell Street ground level. However, there is a path down the side of the building which was inaccessible after the fire:


The Cube is on the left of the image and Silverwell Street runs perpendicular to this path at the far end of this photograph. As you can see, the ground falls away from Silverwell Street towards you by what appears to be four or five brick courses. Assuming a standard brick vertical pitch of 75mm, this would equate to 300 to 375 mm between the two yellow arrows. If the BRE measurement wasn’t taken down this side elevation, it’s possible the ‘top storey height’ could be slightly over 18 m. These are fine margins and the as-built building may deviate from the drawings by this amount. BRE Global may therefore have measured their 17.8 m correctly, but without being able to measure myself and with limited information, I came away with the impression the building was probably over 18 m in both key respects.

We wait with interest to see how the BRE have measured this building and crucially, given their measurement is under 18 m, why they thought only one dimension was sufficient to answer Mr Reed’s question.

Whatever the explanation, there remain significant issues:

  1. The building height is undoubtedly over 18 m and we’ve established that this engages Diagram 40(e) of the relevant Approved Document B. Does the cladding therefore meet the requirements of Diagram 40(e)? If it does, then a building design has followed the Guidance but failed the Regulations.
  2. What chance do the ‘professionals’ referred to in Advice Note 14 have, if the government’s own advisors can’t get the height measurement of the building correct? Who is leading Hackitt’s competence initiative on cladding? Who is writing the Advice Notes?
  3. Even if the building was under 18 m, this fire demonstrated that having no cladding provisions on such buildings is unacceptable to the public. Either the threshold needs to drop to 11 m or new provisions introduced for buildings below 18 m as the MCRMA have recommended since November 2017.
  4. If the top storey actually turns out to be 1 mm or more above 18m, then did the building have a BS 8414 + BR135 given it appears to have used combustible insulation? I know of no positive classifications of HPL systems with combustible insulation.

In its determination to preserve the application of combustible products, has MHCLG through its decades of successive revisions ended up with an Approved Document that it doesn’t even understand itself ?

... and where does it go from here?


How to cite this blog post (Harvard style) Evans, Jonathan. (2020). The Land of Confusion?. Available at: (Accessed [date])