Mass Timber and CLT: When Tall Buildings Are Built of Wood Again

Mass timber is a collective name for building components of solid, glued wood that are strong enough to carry large and tall buildings. The most important are cross-laminated timber, CLT, in which boards are glued in layers with the grain in alternating directions into stiff panels for walls and floors, and glulam, in which laminations are glued in the same direction into long beams and columns. The elements are made in a factory and assembled quickly on site. Since the building regulations were changed in the 1990s, multi-storey timber buildings are again being built in Sweden, and Sara Cultural Centre in Skellefteå from 2021 is one of the tallest.

Facts

What Is Mass Timber?

An ordinary Swedish detached house is built with timber framing: a skeleton of slender, upright studs clad with boards and panelling, with insulation in between. Mass timber is something different. Here walls, floors and roofs are made of thick panels or heavy beams of solid wood, so that the timber itself carries, without being divided into thin parts.

The most important product is cross-laminated timber, often abbreviated CLT (KL-trä in Swedish). Boards are laid in three, five or more layers, and each layer is turned so that the grain runs across the layer beneath. The layers are glued and pressed together into large, stiff panels. Because the wood no longer has a single weak direction, the panel can carry in several directions and function both as wall and as floor.

Glulam is the older relative. There the laminations are glued in the same direction, and the result is beams and columns that can be made much longer and stronger than a single tree allows. Glulam has long been used in sports halls, bridges and large roofs. In many modern timber buildings the two are combined: a frame of glulam columns and beams and floors of CLT.

Why Wood Again?

Wood is Sweden's oldest building material. Log houses of horizontal logs were built from the Viking Age into the 19th century, and churches, farms and whole towns were of timber. But the timber towns burned time and again, and so in the 17th and 18th centuries the towns required that buildings in the town centre be built of stone or brick. In the 20th century the fire regulations prevented multi-storey buildings from being built of timber, and until 1994 timber buildings in Sweden could not have more than two storeys.

The new rule set requirements for how a building must cope with a fire, not for what material it is built of. At the same time glued timber products became stronger and cheaper, and the climate question gave wood a new argument. A growing tree absorbs carbon dioxide, and the carbon stays in the wood as long as the building stands. Producing timber components also requires less energy than producing cement and steel. Sweden has large forests and a strong timber industry, and since the 2000s multi-storey timber buildings have been built in many Swedish towns.

Mass timber also has practical advantages. The elements are light compared with concrete, which means smaller foundations and makes it possible to build on top of existing buildings. They are made with high precision in a factory, and on the building site they are lifted into place by crane and screwed together, without concrete having to be poured and dry. Construction becomes faster and quieter. The drawback is that the timber must be protected from rain during construction and that sound between floors requires careful solutions.

Doesn't a Timber Building Burn?

Wood burns, but a thick piece of solid wood burns slowly and predictably. When the surface chars, a layer of carbon forms that insulates the wood inside, and the core retains its load-bearing capacity for a long time. The structural engineer calculates how fast the wood chars and makes beams and panels so much thicker that what remains after a given time still carries. Steel, by contrast, can soften and give way when it gets hot enough, unless it is protected.

In practice part of the timber is often clad with plasterboard, and tall timber buildings get sprinklers and fire compartments like other buildings. How much of the timber may be visible is a balance between fire protection and architecture. In the public spaces of Sara Cultural Centre the glulam frame is openly visible.

Sara Cultural Centre and Other Tall Timber Buildings

Skellefteå municipality wanted to gather the town's library, theatre and art in a new cultural centre and chose to build it in timber, citing the forest and the sawmill tradition of Västerbotten. White Arkitekter won the competition in 2016 with a proposal by Oskar Norelius and Robert Schmitz, and the building opened in 2021. It was named after the author Sara Lidman, who came from the area. The timber came from a sawmill nearby.

Sara Cultural Centre has a lower part with a library, stages for the Västerbotten Theatre, an art gallery and a museum, with columns and beams of glulam and floors and walls of CLT. Above rises a twenty-storey hotel, built of prefabricated modules of cross-laminated timber that were made in a factory and stacked on site. Even the lift shafts are of timber, which was unusual when the building was built. When it opened it was counted among the tallest timber buildings in the world.

In Norway the Mjøstårnet in Brumunddal was completed in 2019, with eighteen storeys and a glulam frame. Tall timber buildings have since been built in several countries, and the records change quickly. What is new about mass timber is not the material but the scale: timber can now do what for a hundred years was reserved for steel and concrete. Sara Cultural Centre also appears in Swedish history under the present day and among the buildings of contemporary architecture.

Load-Bearing Wall or Skeleton

A building can carry in two ways. Either the walls carry, as in a masonry building, or a skeleton of columns and beams carries, and the walls are merely a shell. The drawing of the structure below shows the difference. Mass timber can do both: CLT panels can form load-bearing walls that build the building into a box, and glulam can form a skeleton with free plans and large windows. Often they are combined in the same building.

The history of mass timber in Sweden is thus a return. From the vertical planks of the stave churches and the farmers' log houses to today's factory-made panels, wood has carried Swedish buildings, and the guide to Swedish timber houses tells the older part. How the materials have succeeded one another is shown by the timeline in the basics on materials, and the word mass timber is in the glossary.

How to recognise mass timber

Exercise

What distinguishes cross-laminated timber from glulam?