CLT and mass timber: what architects need to know

What mass timber is

Mass timber is a family of engineered products made from many pieces of wood glued, nailed or doweled together into large, strong elements.

• CLT (cross-laminated timber) is made of layers of boards, each layer at right angles to the one below, glued into solid panels. It can be used for floors, walls and roofs.
• Glulam (glued laminated timber) is made of layers of timber glued together to form long beams and columns, including curved shapes.
• LVL (laminated veneer lumber) is made of thin veneers glued together into strong beams and panels.
• Dowel-laminated and nail-laminated timber are built up without glue.

What it offers

• Lower carbon, potentially. Timber from responsibly managed forests stores carbon, and timber structures can have a lower embodied carbon than concrete or steel alternatives. The result depends on the whole building and its life, so it should be compared carefully. See Embodied carbon in plain language.
• Speed. Elements are made accurately off site and assembled quickly, which can shorten construction time and reduce noise and traffic on site.
• Light weight. Timber is lighter than concrete, which can reduce foundations.
• Warmth. Exposed timber gives interiors a warm, natural character, as in our concept for Masjid Al Noor's timber details and many of our schools and houses.

Mass timber is a family of engineered products made from many pieces of wood glued, nailed or doweled together into large, strong elements.

What to be careful about

• Fire. Mass timber chars at a predictable rate, and structures can be designed to keep their strength during a fire by allowing for the char. Rules differ between countries and building types, and some places restrict exposed timber in taller buildings. A fire engineer must be involved.
• Moisture. Timber must be protected from rain during construction and from moisture in use. Design for drying, and plan the programme to keep the structure covered.
• Sound. Timber floors can transmit footfall and noise. Floors and walls need additional layers to meet acoustic requirements.
• Movement and shrinkage. Timber moves with changes in moisture, so details and finishes must allow for it.
• Connections. Joints and fixings are important and often a source of cost and complexity.
• Supply and certification. Make sure the timber comes from responsible, certified sources, and check lead times.
• Insurance and approvals. Insurers and authorities may have specific requirements for timber structures.

Where it fits

Mass timber suits offices, schools, housing, and cultural buildings, especially when speed, lightness and a warm interior are valued. It often works best in a hybrid with concrete or steel in the right places, such as cores, foundations and long spans.

In short

Mass timber offers a lower-carbon, faster way to build large structures, with careful design for fire, moisture and sound. It is not right for every project, and specialist advice is essential.

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