This July, a site in north London turned into a terrifying inferno in the time it takes to make a cup of coffee. Nobody knew why. Now the London Fire Brigade has talked exclusively to Building about what happened and the dangers inherent in multistorey timber-frame sites.

At 3.39pm on 12 July this year, excavator driver Frank Torrance was driving between two timber-framed buildings at the Beaufort Park development in Colindale, north London, when he smelled smoke. He looked up to see flames inside one of the buildings and immediately dialled 999.

CCTV footage from a camera on the Hendon Police College next door shows that, just three minutes later, the six-storey apartment block where the fire started was ablaze.

Eight minutes after Torrance made his call, flames leaped 40m into the air and the block collapsed.

Although the London Fire Brigade had its first pump at the scene four minutes after it was alerted, there was nothing it could do to save the building. Instead, the firefighters concentrated on preventing the fire spreading to a second timber-framed apartment block under construction next door, and to neighbouring, occupied buildings.

Unfortunately, despite the presence of 20 pumps, the blaze not only destroyed the second block but spread to a student halls of residence. Hendon Police College on the other side of a road suffered damage, 30 cars were written off and 2,500 people had to be evacuated.

According to Steve Cracknell, an investigator with the London Fire Brigade, the strength of the fire shocked construction workers. “When I asked them whether they had thought of using an extinguisher they said: ‘Are you joking?’ They didn’t even look round – they just legged it. A lot of the guys said they would never work on a timber-frame development again. They would rather stay at home doing nothing.”

After the blaze was extinguished, London Fire Brigade began an investigation into how it started, and why it grew so quickly and uncontrollably. Building has obtained the first findings of this investigation, and its consequences will extend far beyond Colindale.

Stephen Robinson, the group manager of the fire engineering department at the London Brigade, says the first lesson is that little can be done to save half-built multistorey timber-framed buildings that catch fire. Instead, Robinson and other fire experts agree that a new approach to their construction is needed.

“With timber-framed buildings of this size you need to take a different approach from steel or concrete during construction,” says Peter Bressington, director of fire engineering at multidisciplinary consultant Arup.

“If there is a fire in a concrete or steel building, it wouldn’t be an issue, but in a timber-framed building you lose the lot.”

This blaze was useful in demonstrating the effect of fire on timber-frame development, because the full spectrum of construction, from the initial frame to the completed building, was present on the site.

The fire was probably started by accident. This means it could happen to anyone working with large timber-framed buildings

The first structure was a frame protected by plastic sheeting and capped by a completed roof. There was no plasterboard on the inside or external cladding on the exterior to provide protection. Hardly surprising then that within minutes it went up like a torch. What is more worrying is the effect this conflagration had on adjacent buildings.

Without a protective barrier, the fire was able to radiate enormous amounts of heat – enough to ignite the second block, which was 19m away. The damaged cars were 15m away and the halls of residence were 11m away at the closest point.

The buildings were not particularly close – indeed, they were well beyond the minimum distances set out in Part B of the Building Regulations. The trouble is, these rules apply to finished buildings, which have the benefit of full fire protection.

“If an unfinished building like this catches fire, it will spread more quickly than the fire brigade can intervene and, as we have seen, it may spread to adjacent structures,” says Robinson. “The space separation in Part B is based on the finished product. While the building is unprotected it is a potential fire hazard to nearby structures.”

The second building was at a much more advanced stage. The external, fibre-cement cladding was complete, the windows were in, the roof was on. The structure was divided into two sections. The section nearest to the initial fire was less advanced; it is understood doors hadn’t been fitted. The lower parts of the section furthest away from the initial fire were finished and just days away from occupation. According to the fire brigade, fire stops between the two sections were not fully in place: there was access between the two sections at high level to allow workers free movement.

The fire spread to the second building at 4.26pm. By 5.10pm, it was fully alight. Where the fire separation was in place, it seemed to work; but the fire jumped across the upper levels and took hold. By 5.58pm, 80% of the second block was ablaze.

Once the fire was put out all that was left of the first building was a pile of blackened and twisted scaffolding. The only signs that the second building had ever existed were its scaffolding, which somehow stayed up, and a small area of the building at the point furthest away from where the fire started.

The London Fire Brigade will not say how the fire started – yet. It was unlikely to be an electrical fault, as the power had not been connected. Cracknell says there is no evidence that it was started deliberately, and the Metropolitan police have never treated the blaze as suspicious, so it was probably the kind of fire that can start by accident on any construction site. This means it could happen to anyone working with large timber framed buildings.

What is being done to address this risk? According to Robinson, the legislation is already in place to tackle the problem: the Construction (Design and Management) regulations, which control safety on site, and the Regulatory Reform (Fire Safety) Order, which protects people in the workplace.

For construction sites, both sets of legislation are the responsibility of the Health and Safety Executive. Stephen Williams, the HSE’s chief inspector for construction, says the executive is “working with the industry to help it understand and manage the risks”. However, there is no specific guidance on working with timber frame. There is a general piece of health and safety guidance called HSG 168 Fire Safety in Construction Work but, according to Robinson, “it doesn’t address the risk to construction workers working in high timber-framed buildings, which is disproportionately great”.

Arup’s Bressington thinks the risks need considering early on. “I don’t think you can use the same approach across all forms of construction and this shows you can’t,” he says. “It’s the CDM supervisor’s job to look at risk and work out ways of mitigating it during the early design stages.

If there’s a fire in a concrete or steel building, it wouldn’t be an issue, but in a timber-framed building
you lose the lot

“If it’s flagged up that a development is going to be built from timber, it should trigger a process. There is a programme issue, which should address during the time the building is vulnerable. For example, you should get the fireboarding on as soon as possible. You also need to reduce the fire load and ignition sources, such as welding, as much as possible.”

Bressington’s point about minimising the time a timber-framed building is vulnerable, is particularly relevant here. At Colindale, the frame of the building where the fire started was completed in February. Unusually, it was left in that vulnerable state for five months for technical reasons that are not yet clear.

Martin Goss, the group technical director of specialist offsite consultant Mtech Group, thinks more research is needed. He is acutely aware of this issue as a virtually completed timber-frame hotel down the road from his offices in Shrewsbury burned to the ground in two hours last November.

He cites the Timber Frame 2000 (TF2000) research project, where the timber-frame industry, BRE and the government collaborated on tests conducted on a full-size multistorey structure, including fire testing that resulted in the best practice guide that was published in 2003. “TF2000 produced a lot of good work and it shows the timber-frame industry has tried to address fire issues,” he says. “Unfortunately, with hindsight, it’s clear they didn’t address all of them. The timber-frame industry needs to be similarly responsible now. That team needs to get together again.”

So what is the timber-frame industry doing about the risk of fire? Bryan Woodley, chief executive of trade body the UK Timber Frame Association (UKTFA), does not like the suggestion that timber frame is more vulnerable to fire than other materials. He says it is hard to set fire to a timber-framed building and that no method of construction is without risk. "All building methods have their areas of vulnerability," he says.

Despite this unwillingness to acknowledge the relative flammability of timber, the UKTFA is not completely ignoring the issue. Woodley points to a set of new guidance that will be launched at the end of next February. This includes a best practice pocket handbook for site workers. Unfortunately, the guide will not mention the risk of fire during the construction phase. There is also a new Health and Safety Toolkit, and although this contains lots of useful information, including mitigating fire risk on site, it doesn’t suggest anything beyond standard best practice.

Additionally there is a new single-page pre-contract risk assessment to ascertain the level of security needed in timber-framed developments to mitigate the risk of arson, although it does not address the possibility of an accidental fire.

On a more positive note, Woodley is due to meet up with the Health and Safety Executive and the London Fire Brigade in early December to discuss the Colindale incident and to see what can be done about it. “If Colindale shows extra measures are needed, we will look at that,” says Woodley.

There is some time to do this: the Association of British Insurers says it is looking at the risks associated with modern methods of construction, but will not be putting up premiums in the short term.

Back in Colindale, housebuilder St George is rebuilding the destroyed blocks in concrete. It did not want to comment on the fire, saying it preferred to wait “until all the facts had been collected”.

At least Frank Torrance, the excavator driver, has learned one thing about fire: fierce heat radiates up, not down. According to Cracknell, Torrance returned to the excavator that had been left between the two burned out buildings. “It was undamaged,” says Cracknell. “He got in, found his sunglasses on the dashboard, put them on, started the engine and drove off.”