Wood, unlike steel or concrete, carries a cultural expectation of warmth and naturalness that no designer can ignore — and precisely because of this it becomes a particularly rich ground for linguistic experimentation: three contemporary architects who have made it the protagonist of their own work have drawn three completely different design conclusions from that same expectation.
Kengo Kuma: the defeat of architecture as a program
Kengo Kuma has explicitly theorized, in writings and interviews, the concept of "architecture defeated": the building must not dominate the landscape with its own volumetric presence, but dissolve into it through a minute, repeated texture that breaks up its perception as a single mass. Wood, worked into thin slats arranged according to patterns derived from traditional Japanese carpentry (kigumi — dry joints without nails or glue, handed down through centuries of temple carpentry), becomes the privileged tool of this strategy: façades that look more like woven fabric than walls, shadows multiplied through dense screens, a perceptual scale that rejects monumentality even in buildings of significant public size. It is no coincidence that Kuma was chosen for the new Japan National Stadium in Tokyo (2019): a sports infrastructure of enormous scale, clad in wood slats sourced from all 47 Japanese prefectures, explicitly designed not to intimidate the city skyline the way an all-steel-and-glass structure would have.
The clearest way to understand Kuma is to look at how he builds his joints. At the GC Prostho research centre in Kasugai (2010) the structure is a three-dimensional grid of small-section Japanese cypress sticks, inspired by the chidori, a traditional toy from the town of Takayama in which wooden sticks lock together by being rotated, without nails or glue. At the Yusuhara Wooden Bridge Museum (2010) Kuma instead takes up the principle of the stacked brackets of temples: small elements piled up, each projecting a little beyond the one below, which carry a volume suspended above the road. At the SunnyHills shop in Tokyo (2013) the whole envelope is a lattice of slats crossing at 30 and 60 degrees, derived from a joint called jigoku-gumi, which turns a commercial building into a kind of wooden cloud.
Technically, this approach has a strength and a cost. The strength is that it uses small-section timber, often from local species such as Japanese cedar and hinoki cypress, which can be cut from young trees and managed forests, and which CNC machining now makes it economical to cut into thousands of identical pieces. The cost is maintenance. A thin slat exposed to rain and sun warps, cracks and greys faster than a solid beam, and a façade made of thousands of slats weathers unevenly, especially between the areas struck by rain and those that are sheltered. Anyone using this language has to design from the outset for projecting roofs, replaceability of individual elements and a schedule of inspections. It is no coincidence that at the Tokyo stadium the timber slats are placed mainly under the eaves, protected by the overhangs of the roof.
Shigeru Ban: structure as a response to emergency
Since the 1990s, Shigeru Ban has developed an entire structural repertoire based on high-strength cardboard tubes (paper tube), often used in post-disaster reconstruction projects where affordability, speed of assembly and local availability of materials matter as much as structural performance in the strict sense. The cardboard tube, technically, is a spiral-wound cellulosic composite treated to resist moisture — not simple packaging cardboard — capable of structural performance sufficient for temporary shelters and even permanent medium-scale buildings, such as the Cardboard Cathedral in Christchurch (New Zealand, 2013), built as a provisional cathedral after the 2011 earthquake. Ban applies the same design rigor of "noble" structural timber to a poor, temporary material, showing that the language of exposed cellulosic fiber — joints, connections, surface texture — can communicate architectural dignity regardless of the building's cost or intended lifespan.
It would, however, be reductive to see Ban only as the architect of cardboard. Much of his work is in genuine structural timber, and here his research concerns the continuity of the structure. For the Japan Pavilion at Expo 2000 in Hanover, designed with Frei Otto, he built a large gridshell vault of paper tubes reinforced by timber arches. For the Centre Pompidou-Metz (2010) he designed a roof of glued laminated timber beams woven in a hexagonal pattern, inspired by a Chinese straw hat, and covered by a textile membrane. For the headquarters of the Tamedia media group in Zurich (2013) he built a seven-storey office building in which the main structural joints are wood against wood, without metal connectors. On these last two projects Ban worked with the Swiss engineer Hermann Blumer, one of the pioneers of CNC timber fabrication. In 2014 Ban received the Pritzker Prize, with a citation that highlighted precisely his work in humanitarian emergencies.
For designers, the paper tube teaches something that also applies to timber: a material's performance depends as much on the detail as on the material itself. A paper tube performs well in compression if its ends are protected from moisture, if it is raised off the ground, if its surface coating is waterproofed and if the building has a roof that shelters it. Take away those precautions and the same tube degrades within a season. With timber the logic is identical: the difference between a structure that lasts a century and one that rots in a few years almost always lies in how water is kept away, not in the species chosen.
Peter Zumthor: wood as a sensory envelope
Zumthor, already encountered for his use of stone at the Therme Vals, develops an equally physical and sensory language of wood in the Saint Benedict chapel in Sumvitg (Switzerland, 1988): a small teardrop-plan chapel, clad externally in untreated larch wood shingles that, exposed to alpine weather, naturally grey over time into a silvery tone — an ageing process accepted, even sought, as part of the design, rather than fought with protective treatments. The interior, entirely in untreated spruce, creates an environment acoustically and visually different from the cold stone of Vals: warm, resonant, almost organic. The common thread with Kuma and Ban, despite the opposite formal outcome, is the refusal to treat wood as a neutral surface interchangeable with any other cladding — all three choose it precisely for what wood, and only wood, can do perceptually.
Two other works complete the picture. The Swiss pavilion at Expo 2000 in Hanover, the Klangkörper ("sound body"), was a maze of walls made of larch and Douglas fir beams simply stacked, with air gaps between the beams to let the air circulate and the green timber dry out. The stacks were held together by steel tie rods and springs, with no glue or nails, so that at the end of the exhibition the timber could be dismantled and sold as building material: a building conceived from the outset as a temporary store of wood. At the opposite extreme is the Bruder Klaus chapel in Mechernich, Germany (2007). There timber is used as formwork: about 112 spruce trunks arranged like a tent, around which concrete is poured in layers; the trunks are then burned slowly for weeks, leaving a blackened cavity. The wood is gone, but its shape and its smell remain imprinted in the concrete.
Leaving timber exposed: what really happens
The greying that Zumthor accepts at Sumvitg is not a poetic effect, it is a precise chemical process. Ultraviolet radiation breaks down lignin, the substance that binds the cellulose fibres together at the surface of the wood; rain washes away the products of this breakdown, and dark fungi and micro-organisms settle on the surface layer, giving it its grey colour. The affected layer is extremely thin, a fraction of a millimetre, and does not compromise the strength of the piece. For larch and cedar it takes a few years for the transformation to be complete. The real problem is not the grey itself but its unevenness: under an eave, behind a downpipe or close to the ground timber weathers differently, and a façade meant to turn a uniform silver can end up streaked. Anyone who wants the Sumvitg effect has to design the exposure, not just choose the species.
The second issue is durability. European standards classify the natural durability of species against fungi (EN 350) and exposure conditions in use classes (EN 335): from timber that is always dry indoors to timber in contact with the ground or with water. Larch and Douglas fir have moderate durability, sufficient for well-ventilated external cladding protected from standing water, but not for elements in contact with the ground; Norway spruce has low durability and must be protected with particular care outdoors. The practical rule shared by the three architects in this article is protection by design: roof overhangs, clearances from the ground, sloping cuts that shed water, joints that allow the wood to dry out. It is the same logic as the shingles at Sumvitg, overlapped so that each element protects the joint of the one below.
Finally there is fire, the first objection raised whenever an exposed timber structure is proposed. Timber burns, but it does so predictably. Beams of adequate section form a layer of char on the surface that insulates the inner part and slows combustion: for softwood glued laminated timber, Eurocode 5 gives a charring rate of about 0.65–0.7 millimetres per minute. This means that a beam can be sized with a sacrificial thickness, so that after thirty, sixty or ninety minutes of fire the residual section can still carry the loads. It is this calculation, more than any aesthetic choice, that has allowed Ban to build multi-storey offices in exposed timber and Kuma to use it in a stadium for tens of thousands of spectators.
Three questions before choosing
Anyone who has to decide whether and how to leave timber exposed on a project can start from three questions, one for each of the three architects. The first comes from Kuma: at what scale do I want the building to be perceived? If the aim is to reduce mass, you need small, repeated elements, and therefore a maintenance plan. The second comes from Ban: how long must the building last, and what happens to the materials when it is taken down? A temporary pavilion, an emergency shelter and an office headquarters need different details, but in every case reversible joints make the timber reusable. The third comes from Zumthor: am I willing to accept that the building will change colour and appearance over the years? If the answer is yes, it pays to design the exposure so that weathering is even; if it is no, you have to budget for periodic treatments, and admit that you are asking timber to behave like another material.