By Neil Summers | All images by the American Hardwood Export Council

Cross-laminated timber or CLT has proven to be a game-changer in the construction sector in recent years.

From small beginnings in the mid-1980s, building in CLT is now a widely accepted and a hugely popular method of construction, especially for larger commercial developments. The versatility of CLT is leading to new and innovative timber buildings.

The MultiPly project is a three storey, three-dimensional maze structure made from square CLT panels.

The MultiPly project is a three storey, three-dimensional maze structure made from square CLT panels.

CLT defined and associated benefits

There are many benefits to building in CLT: the speed of build is much quicker; the build process is less noisy; and timber construction is much lighter – meaning reduced foundation requirements and the prospect of extra storeys. The environmental advantages are considerable too, with the timber panels acting as long-term carbon stores. The health and wellbeing offered by timber buildings is also now well-documented and the insulation and thermal performance of the building is considerably improved.
CLT consists of planks (or lamellas) of sawn, glued, and layered wood, where each layer is oriented perpendicular to the previous one. By joining layers of wood at perpendicular angles, structural rigidity for the panel is obtained in both directions, similar to plywood but with thicker components. In this way, the panel has great tensile and compressive strength. These panels can be used to form wall, floor and roof panels. The panels are quick to produce in a factory environment and they can be machined to very high tolerances. Panel sizes are typically 16 metres by 2.95 metres but can be larger. The majority of CLT producing factories are located in Europe, where the raw material softwood timber resource is located. This softwood resource is plentiful and relatively inexpensive.

The Smile project represented the first-ever use of industrial-sized hardwood CLT panels.

The Smile project represented the first-ever use of industrial-sized hardwood CLT panels.

Hardwood as an alternative to softwood

In the last few years the American Hardwood Export Council (AHEC) has explored the possibility of using hardwood as an alternative fibre resource to softwood for CLT manufacturing. AHEC has collaborated on a number of landmark CLT projects utilising American tulipwood and each project has been undertaken with an element of research and development to expand the knowledge around American tulipwood as a raw material for potential commercial CLT production.

The first demonstration of the laminating potential of American tulipwood was the Endless Stair in 2013. This structurally challenging installation designed by dRMM architects with the help of Arup structural engineers was a three-storey network of stairs based on the concept by the Dutch graphic artist M. C. Escher. Initial testing at Trento University prior to the Endless Stair project revealed significant strength benefits of American tulipwood, including being three times stronger in rolling shear than softwood.

The knowledge gained from the Endless Stair project was taken further with The Smile, a project designed by Alison Brooks Architects in collaboration with Arup for the London Design Festival in 2016. This showcased the enormous structural potential for tulipwood CLT, emphasising its strength-to-weight ratio by reducing the amount of fibre required to achieve the same strength performance as the equivalent in softwood timber. The Smile represented the first ever use of industrial-sized hardwood CLT panels.

The year 2017 saw the completion of the first permanent building made from American tulipwood CLT. Maggie’s Oldham, designed by dRMM Architects, selected American tulipwood for its potential to deliver large panels and, considering the function of the building, for its visual beauty and warmth. Maggie’s Oldham represented a milestone in the development of hardwood CLT, showing that American tulipwood CLT could be manufactured on a commercial scale for a project with a very tight budget.

The most recent demonstration of the structural capability of tulipwood CLT was in the 2018 London Design Festival project MultiPly – a three storey, three-dimensional maze structure designed by Waugh Thistleton Architects. The complete structure comprised 17 interconnecting modules made from a total of 10 260mm and 100mm thick x 2.6m square CLT panels. This modular structure with its flexible design and connection detail allowed it to be re-erected in a different configuration as part of 2019’s Milan Design Week.

During the MultiPly project a comprehensive testing regime was conducted at Edinburgh Napier University in order to examine all of the structural properties of American tulipwood CLT and to enable a comparison with published data for softwood CLT. This information will be published in a Tulipwood CLT Technical document which will cover all aspects and findings of the production of the MultiPly project.

The Endless Stair project revealed significant strength benefits of American tulipwood.

The Endless Stair project revealed significant strength benefits of American tulipwood.

Thermally modified timber

Thermal modification is now globally recognised as a low environmental impact method of preserving timber for use in outdoor environments. Originally developed in Scandinavia as a method of preserving softwoods without the need for chemicals, it soon became apparent that some American hardwood species have the structure to enable them to withstand the intense kilning process that is employed to modify the timber. The North American temperate hardwood forest resource does not offer durable timber species in any real commercial volume and so to be able to thermally modify hardwood species such as ash, tulipwood, soft maple, yellow birch and red oak, allows North American hardwoods to be utilised for external applications with confidence. The three main benefits of thermally modified timber are increased durability, increased dimensional stability and a low thermal gain, meaning it is comfortable to touch or walk on in hot temperatures. The modification process also treats the wood through the whole cross section of the timber.

Europe is still very much the hub for thermal modification of timber with the majority of large producers in this region. However, increasing investment in the technology is being made in North America and elsewhere across the globe, as the demand for sustainable outdoor timber products increases.

AHEC has pioneered a number of showcase projects using thermally modified hardwoods in recent years, beginning with Martino Gamper’s Infinity Bench for the London Design Festival in 2011. Since then many other global projects have shown just how versatile thermally modified American hardwoods can be. From David Trubridge’s Aleni reclining chair in thermally modified American ash in New Zealand to the Houtlander’s extraordinarily complex thermally modified red oak bench project in South Africa. These projects are helping to develop awareness of these products and are, undoubtedly, having a positive impact on the market for thermally modified American hardwoods in these regions.

Two permanent examples of the use of thermally modified American tulipwood can be seen in the United Kingdom. These are Asif Khan’s Room on a Hill – an outdoor play and learning environment made from thermally-modified tulipwood slats in a galvanised steel frame, which has stood the rigours of a succession of enthusiastic schoolchildren since 2015 – and the acclaimed fluted tulipwood cladding that envelops the exterior of Maggie’s Oldham and is weathering well in the harsh climate of northwest of England. Both projects are being monitored for their long-term performance.