Testing retardant-treated American tulipwood thermally modified timber (TMT) to Euroclass B fire classification would have been an important step in any event.

Tulipwood TMT has been extensively used in cladding in the US for over a decade and has already featured in two high profile projects in the UK.

It was selected by designer Asif Khan for a striking play structure at Chisenhale Primary School in East London and architects dRMM specified it as cladding for the Maggie’s Cancer Care Centre in Oldham, a pioneering building with a tulipwood CLT structure. So it was already seen as having real potential for exterior use in construction. The fire testing of retardant-treated timber was expected to further broaden its range of applications and appeal across the UK and the wider European market. However, coming, as it has, in the wake of the Grenfell disaster and the subsequent Hackitt Review of Building Regulations and Fire Safety, the move assumes more significance still.

The testing was the culmination of collaboration over the last year between the American Hardwood Export Council (AHEC) and London architects Waugh Thistleton.

Their aim was to establish that tulipwood TMT can be effectively treated with fire retardant for use in multi-storey buildings.

Waugh Thistleton had it in mind for a specific project, the Black and White building in Shoreditch. This is a new office block replacing an existing Victorian building and featuring a CLT and LVL structure, with the exterior wrapped in a timber-louvred ‘curtain’. It was for the latter’s fins that the architect wanted to use tulipwood TMT.

“Neither the tulipwood TMT used for the Maggie’s Centre or Chisenhale needed to be fire retardant treated, as Maggie’s is a standalone building and Chisenhale a restricted access, single storey-structure,” said AHEC technical consultant Neil Summers. “But with the interest in the material these two created, we knew it wouldn’t be long before a project came up where it would be required, so we thought we’d better get on with testing. We saw the Waugh Thistleton building as ideal for a collaborative project.”

The treated and tested timber was 250mm by 44mm in section and 1500mm long, or “pretty big lumps of wood”, as Mr Summers described them. The test results consequently apply to any length of the same profile up to 250mm, provided it is the same thickness.

PTG Treatments was chosen to treat the material. “It used Kopper’s pressure impregnated, leach resistant Fire-X, which is a benign non-biocidal treatment – and it’s lucky we got in with PTG when we did as, post the Hackitt review, its waiting list has really increased,” said Mr Summers.

While thermo treatment alters the structure of the wood, it does not change its fire performance in its non-retardant treated state.

“Asif Khan did his own fire testing of the tulipwood TMT for the Chisenhale project and found no difference,” said Mr Summers.

There was also no reason to believe the TMT material would be less suited to retardant pressure impregnation than tulipwood in its natural form, and the latter had previously been treated with Fire-X and passed a single burning item (SBI) indicative test. But only evaluation of the retardant treated TMT would tell.

The testing was undertaken by Warringtonfire.

“The process involves two types of test,” said Mr Summers. “In the first a naked flame is held to the material to see if it ignites.

“In the second, the timber is set up in the same vertical orientation as it will be used in the building and a propane gas burner is set in the corner to replicate a fire,” he continued.

“It’s a pretty severe test, with the temperature reaching 600°C. You have to supply sufficient material for five tests and pass three. The tulipwood TMT passed the first three it was subjected to, so successfully achieving a Euroclass B rating.”

With today’s more stringent rules on fire resistance, different timber product profiles must undergo individual testing, albeit that they are the same material as previously tested products.

“But, while it will not be in their final test report, Warringtonfire also provided a statement saying that, as the values of the test were “very comfortable”, while they could not say for certain without further testing, there was a good possibility that other treated tulipwood products would also achieve class B,” said Mr Summers.

A recent evaluation of the cladding at the Oldham Maggie’s Centre has shown that tulipwood TMT also stands the test of time. “It was given no additional protective treatment or finish, but after three-and-a-half years shows no signs of deterioration,” said Mr Summers.

“There is zero cracking or movement over the entire building and that’s despite the fact it’s in a very exposed setting on the side of a hill in an area of high rainfall. It has weathered naturally, but done so uniformly, developing an even silver-grey patina, which both Maggie’s and the architects like.”

There is now a threat of new restrictions in the UK on use of combustible materials in construction, which the timber sector and architects see as potentially having a major impact on use of wood in building.

Following the Hackitt report, new regulations banned the use of combustible materials in and on external walls in buildings with a top storey over 18m.

Subsequently the UK government is proposing, via its combustible materials review, to go further and bring that height limit down to 11m.

In 2018 Waugh Thistleton produced a book, called 100 UK CLT Projects. If this lower height restriction had applied at the time, 56% of the projects featured could not have gone ahead.

The UK Timber Trade Federation (TTF) and the Architects Climate Action Network (ACAN) have come together to produce a ‘robust technical response’ to the consultation on the ban, which has just been extended to May 25. They oppose such a blanket height constraint and say it threatens the UK’s international leadership in low carbon, energy efficient medium to high-rise timber based buildings at a time when the building industry is facing ever greater pressure to cut its climate and wider environmental impacts.

“As the government acknowledges, this change in regulations will have an impact on the continued innovation and development of low carbon building, and hence the rate at which construction can tackle climate change,” said Anthony Thistleton of Waugh Thistleton.

The TTF/ACAN lobby maintain instead that any new regulation should “take a science-based approach”, with BS8414 used as the basis for fire safety compliance for timber used structurally. At the same time, they accept that the 11m height restriction should apply to cladding. That may not in itself significantly affect the potential size of the market for wood cladding, but it is felt increases the onus on the timber industry to ensure the fire safety performance of products used under that limit.

Mr Summers agrees. “The added threat of any restriction is that the UK building industry and, importantly, the insurance sector will become risk averse to the use of timber per se, when in other countries they are actually looking to build more, bigger and higher in wood,” he said.

“It’s important that as many of the timber and timber building industry as possible state the case for wood in construction generally, but also, as we have done, demonstrate that it can additionally be processed and treated to significantly reduce the risk of fire.”