Mass Timber and Cross-Laminated Timber Explained

Cross laminated timber, usually shortened to CLT, is a solid engineered panel made from layers of sawn lumber glued together with each layer running at right angles to the one beneath it. That simple change of direction turns a stack of boards into a rigid plate that can carry load in two directions, which is why CLT now appears in floors, walls and roofs of buildings that would once have been built only in concrete or steel.

This guide explains what cross laminated timber is, how it sits within the broader mass timber family, how the material actually behaves structurally and in fire, and what a practising engineer weighs up before specifying it — including the particular questions that arise in a hot, humid climate such as Singapore’s.

What is cross laminated timber?

A CLT panel is built up from an odd number of layers — typically three, five or seven — of kiln-dried boards. Adjacent layers are oriented perpendicular to each other and bonded under pressure with a structural adhesive. The result is a large-format solid panel, often two to three metres wide and up to twelve metres or more long, with a thickness usually between roughly 100 and 300 millimetres.

Because the grain alternates, the panel resists bending and in-plane forces along both axes rather than only along the grain. This is the defining feature that separates CLT from ordinary glued laminated timber, where all the laminations run in the same direction. A CLT panel behaves much more like a reinforced concrete slab than like a timber beam: it is a two-way plate, not a one-way member.

The mass timber family

CLT is one member of a group of products collectively called mass timber, all of which build large structural elements from smaller pieces of wood. Understanding the family helps you choose the right product for each part of a frame.

Glulam (glued laminated timber) — laminations bonded in a single direction to form beams, columns and arches. The workhorse for linear members.
CLT — crossed layers forming solid plate-like panels for floors, walls and roofs.
LVL (laminated veneer lumber) — thin peeled veneers bonded to make very strong, dimensionally stable beams and headers.
DLT and NLT (dowel-laminated and nail-laminated timber) — boards stood on edge and joined by dowels or nails, without structural glue, to form decks and panels.

These products are often combined: glulam posts and beams carrying a CLT floor plate is a common arrangement. The wider category of engineered wood products covers these and the smaller-format materials such as plywood and I-joists.

How CLT carries load

Structurally, CLT does two distinct jobs. As a floor or roof, the panel spans between supports as a plate, carrying gravity loads in bending and shear much as a slab does. As a wall, the same panel acts as a vertical load-bearing element and, just as importantly, as a shear wall that resists horizontal forces from wind and, where relevant, seismic action.

The panels also act as diaphragms — stiff horizontal planes that gather lateral load and deliver it to the shear walls. In this respect a CLT building has a clear structural logic that any engineer used to reinforced concrete frames will recognise, even though the material and the connections are entirely different.

Two properties dominate the design. The first is strength-to-weight: timber is light, so foundations and transport loads are reduced. The second is stiffness and vibration — long-span timber floors are often governed not by strength but by deflection and by the comfort criterion for footfall vibration, which frequently sets the panel thickness.

Connections and the role of prefabrication

Mass timber is fabricated off site to tight tolerances and delivered as finished panels with openings and service routes already cut. Erection is fast, quiet and clean, which is one of the strongest commercial arguments for the system. A floor that takes days to form, reinforce, pour and cure in concrete can be installed in hours.

The connections carry the structure and deserve as much attention as the panels. Self-tapping screws, steel brackets, plates and proprietary connectors transfer forces between panels and into the supporting frame. Detailing must account for shrinkage, swelling and the perpendicular-to-grain compression that builds up where many floors stack on bearing walls in a tall building.

Fire behaviour

It surprises people that a heavy timber element can offer good fire resistance. When mass timber burns, the outer surface forms a layer of char that insulates the sound wood beneath, and the charring advances at a slow, predictable rate of roughly 0.6 to 0.8 millimetres per minute. Engineers exploit this by sizing members with a sacrificial allowance so that, after the required fire period, enough uncharred cross-section remains to carry the load.

Where exposed timber would contribute too much fuel, panels are encapsulated behind fire-rated boards. Tall timber buildings that are widely documented — such as Mjøstårnet in Norway and Brock Commons in Canada — combine these strategies, and they have been instrumental in persuading authorities that mass timber can be used safely at height.

Moisture, durability and the Singapore context

Timber is a hygroscopic material: it gains and loses moisture with its surroundings, and it is biodegradable. In a tropical climate these are the central engineering concerns. Sustained high humidity, the risk of wetting during construction, fungal decay and termites all have to be designed out rather than hoped away.

The principle is to keep the timber dry in service and to protect it during construction. That means careful detailing of the building envelope, generous overhangs and flashings, vapour-aware build-ups, and durable connection details that do not trap water. Termite protection and, where appropriate, preservative treatment or naturally durable species become part of the specification. Any of these decisions has structural consequences, so they belong with the engineer, not only the architect.

In Singapore, mass timber is still relatively new, and acceptance is assessed case by case through the Building and Construction Authority’s processes rather than treated as routine. That makes early, evidence-based engagement essential. Before committing to a timber scheme here, it is worth confirming the regulatory route, and a qualified structural engineer should be involved from the outset to keep the design and the approvals on the same track.

Frequently asked questions

Is cross laminated timber as strong as concrete?

CLT is not directly comparable on raw compressive strength, but on a strength-to-weight basis it is very efficient, and a well-designed CLT structure can carry the same floor loads as a concrete equivalent while weighing far less, which reduces foundation demands.

Will a CLT building burn down more easily?

No. Mass timber chars at a slow, predictable rate that protects the core of each member, so engineers can size elements to retain their load capacity for a required fire period, and exposed surfaces can be encapsulated where the fire strategy demands it.

Can mass timber be used in Singapore’s humid climate?

Yes, but only with deliberate moisture, decay and termite control built into the detailing, the envelope and the construction sequence, because sustained humidity and biological attack are the principal risks for timber in the tropics.

What is the difference between CLT and glulam?

Glulam has all its laminations running in one direction and forms beams and columns, whereas CLT crosses its layers and forms plate-like panels that span and resist load in two directions, so they are used for different parts of a structure.

How tall can a timber building go?

Documented mass timber towers have reached the order of eighteen storeys and above, and the height achievable on any given project depends on the structural system, the connections, the fire strategy and the local regulatory acceptance rather than on the material alone.

Considering mass timber or cross laminated timber for a project and want it engineered and approved properly? Talk to our consultancy or get in touch.