Is Combustible Always Dangerous ?

I'm writing this article to explore the issue about whether combustibility always means dangerous. Obviously in some cases it clearly does. The Grenfell Tower fire was a dreadful example of that and has brought shame on the construction industry for allowing that to happen.

However, there is a question about whether combustibility always means dangerous. This is a very important issue nowadays because there are many people who are dealing with this issue in buildings (especially external walls of buildings) who are firmly of the belief that anything combustible is inherently dangerous and therefore unacceptable. Unfortunately, things aren't that simple and taking that kind of approach is leading to various unpleasant outcomes, particularly when dealing with existing buildings. Unfortunately, the insurance industry seems to be taking that approach, where the presence of anything combustible in the external walls seems to be automatically resulting in high insurance premiums. That is really not the 'evidence based' approach that insurers normally take so I'd really like to explore the issue.

Firstly, I'd like to address the issue of what "combustible" means. There are various dictionary definitions of that word, such as "able to catch fire and burn easily". Unfortunately, in the construction industry the term "combustible" is not defined under any standard fire test.

There are various types of fire test in the construction industry, but the type of test that would most closely relate to "combustibility" would be the ones that are described as "reaction to fire" tests.

Reaction to fire tests assess whether something is likely to burn in a fire, or whether fire might spread over its surface. Under European standards there is a classification standard to cover this, BS EN 13501-1, which rates materials as either A1, A2, B, C, D, E or F, with A1 being the best (i.e. least likely to burn) and F being the worst (i.e. most likely to burn). There's also sub-categories for smoke production and flaming droplets. But that isn't a method for working out whether something is "combustible" or not. It's a grading system.

In the UK we'd refer to Class A1 as "non-combustible". So maybe we could decide that everything that isn't Class A1 (i.e. "non-combustible") is "combustible". Well, I'm afraid it's not that simple, because Class A2 would be referred to as "limited combustibility" which is not quite as high a standard as "non-combustible" but is still pretty strict. Even under the very rigorous standards that have been applied to tall residential buildings after the tragic Grenfell Tower fire, external walls can include materials that are either A1 or A2.

So maybe we should take the approach that "combustible" means everything that's not Class A1 "non-combustible" or A2 "limited combustibility". That makes sense, but already you're hopefully seeing that this is a complex issue. Even within the materials that we're now classing as "combustible", there are then the varying grades of B to F, with gradually increasing levels of combustibility. A material that is Class B will be very different from a material that is Class F, despite the fact that we've decided that they are both "combustible".

The key point I'm trying to make here is that "combustibility" isn't a binary issue where a material is either "combustible" or "not combustible". There's a gradual scale, and some materials are more or less combustible than others.

There are lots of other factors that affect whether a "combustible" material presents a risk. For example, if the "combustible" material is protected from the fire by a layer of brick, then it's pretty unlikely that the material would be affected. It would still be necessary to make sure that the fire couldn't affect the material from other directions (e.g. from the opposite side, or around gaps) but if the material is encased in masonry on all sides, then it really is a very low risk.

Obviously if we're relying on a protective layer, we need to be sure that it's a robust protective layer that won't fail in a fire. Brick is pretty robust, but sometimes protective layers aren't as reliable. Some products include metal sheets over the combustible material. Those metal sheets won't burn, but they may distort or melt (particularly if the metal is aluminium which has a much lower melting point than steel).

The ACM panels that were used on Grenfell Tower are a real example here, where the highly combustible PU core was protected by thin sheets of aluminium on both sides. The aluminium sheets would help protect the PU core against very small flames that are applied to the centre of the panel, but not if the fire were larger, or if the fire were applied to the exposed edges of the panel. Once the fire has bypassed the thin aluminium layer, the highly combustible PU core presents a major risk. PU is also thermoplastic, which means that it melts and creates burning droplets, which make it more dangerous than a material that is thermoset, which does not melt.

So essentially if the combustible material is protected by a fragile protective layer (e.g. a very thin layer of aluminium) it could be a risk, whereas if it were protected by a robust protective layer (e.g. a layer of brick) it would probably not.

The presence of cavities is also a major factor. External wall construction sometimes has cavities hidden within the wall construction. Fires can spread very rapidly within cavities, so if there are cavities present, with combustible surfaces facing into the cavity, that can create a risk of fire spread through those cavities. If there are cavity barriers at regular intervals that can drastically reduce that risk. Unfortunately, cavity barriers are sometimes missing or ineffective (e.g. if they've been incorrectly installed) and intrusive investigations are usually the only way to find out whether that’s happened.

The other main factor that I’ve not mentioned yet is that risk itself isn’t binary. It’s a range, going from very low risk to very high risk. Zero risk is not possible, so the key thing is to ensure that the risk is reduced to a point where the residual risk is low enough. HSE produce various guidance documents on this topic which describe it very clearly.

So, the main points that I'm trying to make here are:

• Combustibility isn't a binary factor of either being combustible or not. It's a range.
• Risk isn’t a binary factor of either being a risk, or not being a risk. It’s a range.
• There are various factors that would affect whether a material is likely to burn in a fire, or present a risk to occupants. This would include the presence (and robustness) of any protective layers as well as checks on cavities and cavity barriers within the external wall system.
• There's also various other factors about the building that would affect the risk, such as height, layout and the presence of fire precautions such as sprinklers.
• A competent fire engineer should be able to make an assessment of whether a particular material presents an unacceptable risk or not.
• In many cases that assessment might conclude that the materials need to be removed. But in other cases, the assessment might decide that there isn't an unacceptable risk of fire spread.
• If someone wants to challenge that fire engineer’s decision, that's fine. Look at the assessment, make sure the fire engineer has carried out a thorough inspection and has assessed the situation properly and if they haven't, they should be challenged. But if there's nothing wrong with the inspection and assessment, then it should be accepted.
• Anyone who is pushing the view that combustible materials always have to be removed (irrespective of what any surveys or inspections conclude) are not helping the situation.

In particular, I'd make a plea to the insurance industry on this. If a competent fire engineer has carried out a thorough inspection of the building and has decided that the building is safe enough for families to live in, then there is no reason to charge high insurance premiums, purely on the basis of combustible materials within the external wall system.

One response from insurers on this issue is that fire engineers are only looking at safety issues, not protection of the building. But for that to justify an increased insurance premium, it would mean that fire engineers are concluding that it is acceptable for the fire to spread as long as the occupants can evacuate faster than the fire is spreading.

Maybe that argument could be made for an office building, where everyone is awake and rapid evacuations can happen. But residential buildings include families who might be asleep. It is not possible to rapidly evacuate a building that includes sleeping families. Changing to a simultaneous evacuation approach (with an alarm and/or waking watch) should only ever be a temporary measure, not permanent. So if we sign off a residential building as being safe, it's because we are convinced that a fire is not going to cause rapid fire spread. The lives of families are far more important than the cost of the building, so if our assessment shows that it's safe for families, then that should also be good enough for the insurers.

I would note one item that I touched on earlier. It would be important that the building is assessed by a "competent" fire engineer. That is a very contentious issue, because the construction industry is riddled with people who either think they're competent and aren't, or who really don't care. The Grenfell Tower Inquiry has highlighted that in the worst possible way.

If the assessment has been carried out by someone who is a Chartered Engineer registered with the Institution of Fire Engineers, then that will at least mean that they have qualifications in fire engineering, and that they've registered with a professional institution. It is equally important that they have the knowledge and experience of external wall systems, and if they are found to have carried out work outside their area of competence, they could be kicked out and lose their career. If the assessment is carried out by someone who doesn't have that kind of qualification, then it's down to the client to work out whether they're competent, and that is a task that many clients may struggle with.

An incompetent assessment would be worthless, whatever its conclusions. But that would be the same for any profession. A medical assessment by someone who wasn't a qualified medical practitioner would be just as dangerous as a fire safety assessment who wasn't a qualified fire engineer.

So, in conclusion, "combustible" doesn't always mean dangerous. Sometimes it does, sometimes it doesn't. An assessment by a competent fire engineer should be able to distinguish between the two. If that's been done competently, and has concluded that it's safe, then that should be fine. And, in particular, insurers should not be using that as a reason to charge higher premiums on the basis of combustible materials within the external wall system.