Timber is a natural, renewable material, easy to fabricate, and with low-carbon emissions. As a construction material, however, when put under enough directional force along its grain, sawn timber is structurally unstable, so deemed unsuitable under higher loads. In comparison, the manufacture of cross-laminated timber (CLT) involves simply gluing multiple layers of timber together at right angles. By crossing the direction of the grains, CLT achieves a far higher level of structural rigidity along both axes. CLT boards start with a minimum of three layers but can be strengthened further with the addition of more. Simply put, due to the complex physics involved in the perpendicular lamination, the strength of CLT board is similar to that of reinforced concrete, and has proven performance under seismic forces.
So what’s new? Wood’s been around for long enough now, and we’ve been using it as a building material for centuries. Surely this isn’t the first time someone’s realized it gets stronger the more you use it? Well… as you’d expect, the changing popularity of cross-laminated timber in construction does coincide with a greater understanding and focus on environmental causes, but the relationship hasn’t always been positive.
50-20 years ago, the environmental crisis was based on deforestation, not carbon footprints, meaning wood and wood products in all their forms were demonized, with campaigners arguing that wood belonged in the ground where it could continue to do good, rather than cut down and used to make buildings. The preferred alternative materials, however, were carbon-intensive steel and concrete. In the early 21st century, as a more sustainable and responsible reforestation-focused wood industry grew, CLT started to become the material du jour for sustainable and, indeed, circular building projects, with organizations like Circular CLT dedicated to reducing CLT production waste and finding solutions for it such as biomass-fuelled hydrogen power. But how did CLT use in construction begin? And what does it mean for the future?
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The pioneers of cross-laminated timber (CLT) construction
In September 2016, Alison Brooks Architects, in collaboration with the American Hardwood Export Council (AHEC), Arup, and the London Design Festival, built The Smile at the Chelsea College of Art, London, UK, ‘showcasing the structural and spatial potential of cross-laminated American tulipwood,’ explains the architects, as the project uses CLT’s structural capacity to cantilever visitors in its 34m-long curve, up to three meters above the ground, without additional support.
The year before the CLT-build Smile was revealed, however, architects Hawkins/Brown had already begun to present the structural capacity of CLT with The Cube, a ten-story apartment block down the road in Hackney, London. In the building, ‘CLT panels are set into a steel frame, bracing it to form an integral part of the structure,’ explains Hawkins/Brown. Around the same time, meanwhile, the primary goal of Albina Yard in Portland, USA, by LEVER Architecture, the first US building to utilize CLT, was to promote the use of the domestic wood product by combining a glue-laminated timber frame with CLT panels. And more recently, the Mass is More Installation from IAAC + Bauhaus Earth references Mies van der Rohe’s German Pavilion at the 1929 Barcelona Exposition, by reflecting the formal grid of the original pavilion, using CLT.
Simplicity and circularity of CLT
After extensive field experience, the fabled three little pigs found that straw and wood are two of the least structurally sound materials with which to build. Thankfully, however, architecture office Kollektiiv didn’t include old nursery rhymes in their material research for the Straw Chapel in Tallinn, Estonia, which holds up a wall and ceiling of straw bales with CLT arches. With all its components prefabricated, the pavilion was built both for, and by, the local community.
The possibilities of prefabrication, meanwhile, make even complex architectural solutions more achievable, like CRAB Studio’s design of an Innovation Center at the Arts University Bournemouth, UK, which uses CLT to form irregular sections of wall, floor, and roof, and playfully combines the material’s colorful or natural wood finish with voids of light. Alternatively, the Voxel Quarantine Cabin in Barcelona, Spain, took waste material created during the CLT-production process, and turned it ‘into a facade that showcases the organic complexity of the tree,’ as the architects, Valledaura Labs, explains.
The rise of the plyscraper
Along with its structural integrity, there are many other assumed challenges for buildings made from wood to overcome, namely its inherent flammability and warping under humidity. Studies have shown, however, that although cross-laminated timber is highly flammable, it also has a Resistance to Fire rating of REI 90 (meaning it retains the sufficient load-bearing capacity for up to 90 minutes), compared to unprotected steel’s REI 15 rating. Hakwins/Brown’s decision to reconstruct the Freemen’s School Swimming Pool with structural CLT, after the original building was destroyed in a fire, exemplifies this, and also proves the material performs when dealing with the challenges of a pool environment, too.
It’s thanks to this structural load-bearing capacity, even under extreme conditions such as humidity and fire, that means in 2021 the International Code Council ruled that CLT buildings could come under the International Building Code IV-A, meaning the maximum height of a CLT ‘plyscraper’ could reach up to 270 feet. At over 260 feet tall, the Sara Kulturhus Center in Sweden is currently one of the world’s tallest high-rise timber structures, with a ‘load-bearing structure built entirely without concrete, speeding up construction time and drastically reducing the carbon footprint,’ explains the architects White Arkitekter.
With CLT research, innovation, regulations, trust, and, indeed, timber-frame buildings themselves growing all the time, the only way to go is up.
Cross-Laminated-Timber Cottage / Kariouk Associates
WoodTek HQ / Origin
Puukuokka Housing Block / OOPEAA
The Cube / Hawkins\Brown
The Smile / Alison Brooks Architects
The CLT House / KAWA DESIGN GROUP ARCHITECTURE
CLT Multi Confort Office Building / Tecto
Albina Yard / LEVER Architecture
Moholt Timber Towers / MDH Arkitekter
Freemen’s School Swimming Pool / Hawkins\Brown
Santo CLT Office / Junichi Kato & Associates
"KITERASU" Model CLT Building at Kuse Station / ofa
University of Massachusetts Amherst Design Building / Leers Weinzapfel Associates
Indiana Hardwood CLT / IKD
The Seed House / fitzpatrick+partners
Timber School in Kuhmo / ALT Architects
Flatiron Office Building / Works Partnership Architecture
Arimunani School / Aulets Arquitectes + Aixopluc
Villa Wood / NORD Architects
95PLA Refurbishment / Vallribera Arquitectes
CLT House / FMD Architects
Nordtvet Farm Kindergarten / MORFEUS arkitekter
Community Center / NORD Architects
The Voxel Quarantine Cabin / Valldaura Labs
Smartware CLT Office Building / Vertical Studio
55 Southbank Boulevard / Bates Smart
Innovation Center for the Arts University Bournemouth / CRAB Studio
The Float / Studio RAP
Speehuis House / Spee Architecten
Lipno Lakeside Cabin / Les Archinautes
1721 House / HARQUITECTES
m.o.r.e. Cabin / Kariouk Architects
NIOA Timber Tower / KIRK Studio
Wintringham Primary Academy / dRMM
Vélizy Morane Saulnier Apartments / DREAM
Sara Kulturhus Center / White Arkitekter
B&O Wooden Car Park / HK Architekten, Hermann Kaufmann + Partner ZT
CLT House / Unknown Works
Biv Punakaiki Cabin / Fabric
Straw Chapel / Kollektiiv
Bocaina-Paraty House / Cicero Ferraz + Fábio Mosaner
Octothorpe House / Mork-Ulnes Architects
Mass is More Installation / IAAC + Bauhaus Earth
Ziggurat and Rhomboid Buildings London Design District / Mole Architects
Fuggerei NEXT500 Pavilion / MVRDV
St. Olavsvei 18 Residential Building / Oslotre
Houston Endowment Headquarters / Kevin Daly Architects + PRODUCTORA
6 Housing Units in Paris / mobile architectural office
High Street Apartments / Gardiner Architects
HAUT Amsterdam Residential Building / Team V Architecture
Find more projects that use Cross-Laminated Timber in this My ArchDaily folder created by the author.
This article is part of the ArchDaily Topics: Circular Economy. Every month we explore a topic in-depth through articles, interviews, news, and architecture projects. We invite you to learn more about our ArchDaily Topics. And, as always, at ArchDaily we welcome the contributions of our readers; if you want to submit an article or project, contact us.
Editor's Note: This article was originally published on February 15, 2023.
