The 4+1 structural systems that make a Mass Timber building efficient

The module and the grid, defined at schematic design, shape cost, schedule, erection and the building’s future capacity to change. There is no universal ratio: how much this decision weighs depends on spans, loads, the architecture and the structural system chosen.

Four families of structural organisation and one additional option: composite action between materials. They are not mutually exclusive; they can be combined in a single project.

1Post & Beam: the classic grid

Column, beam and slab. It is one of the most widespread configurations in engineered timber buildings.

A grid of columns and beams, usually GLT, supporting CLT floor panels. It can be one-way, with beams in a single direction, or two-way, with primary and secondary beams.

The panels carry gravity loads and can form a horizontal diaphragm, provided their joints, anchors and collectors transfer forces to the lateral system. Thickness is set by strength, deflection, vibration and fire performance, as applicable.

• Where it appears. Offices, institutional buildings, universities, cultural centres, mid and high-rise housing.
• The upside. It is modular and repetitive, and adapts to almost any programme.
• The cost. Coordinating beams with services and ceilings, and resolving transitions to stiff cores or zones that break the module.

That last point looks minor and is not. In a repetitive system, the exception is always what costs.

2Post & Plate: taking the beams out

This option drops downstand beams and the slab transfers loads to the columns through a purpose-designed system. Unlike a solid concrete slab, a conventional CLT panel is orthotropic: two-way behaviour cannot be assumed without checking layer orientation, continuity, joints, spans and local reinforcement.

Without downstand beams, services can run more freely and the total floor depth can drop, although thicker panels, reinforcement or special support details may be needed.

Panel-to-panel transfer, shear and compression perpendicular to the grain at supports, deflection and vibration all need specific checks. Feasibility depends on the whole system, not only on erection accuracy.

• Where it appears. Offices, student housing, hotels.
• The upside. A cleaner soffit and potentially fewer clashes with services, where the design allows.
• The cost. Checking point supports, possible stress concentrations, two-way stiffness, joints, vibration and deflection limits.

Vibration is one of those issues that never shows up in the model and shows up when someone walks across the floor above.

3Load-bearing walls: efficiency through repetition

Loads travel down structural walls instead of columns. Walls can be CLT or light timber frame, depending on the scale of the project.

Walls and slabs can form a box system when diaphragms, collectors, anchors and panel joints are designed for it. It performs best when the plan repeats.

• Where it appears. Multi-family housing, hotels and multi-storey residences, depending on the system and applicable codes.
• The upside. Fast erection, good integration with prefabrication and modular construction, structural efficiency where there is repetition.
• The cost. Limited flexibility to relocate bearing walls, and the need to check openings, lintels, panel edges and continuity of load paths.

This system makes a strong implicit decision: it assumes the building will not change its plan for thirty years. Sometimes that is true. Sometimes it is a bet.

4Hybrids: each material where it performs best

They combine timber with steel, concrete or other materials, giving each a different structural role. Even when they do not form a composite section, all of them must work compatibly within the global structural system.

Typical cases: steel frames with CLT slabs, concrete cores with a timber perimeter structure, steel beams and columns supporting CLT panels.

• Where it appears. Buildings of various heights and projects that need to combine spans, lateral stiffness, fire resistance or particular construction solutions.
• The upside. Each material is used where it is best. It allows longer spans, more slenderness and more architectural freedom.
• The cost. Coordination between materials and trades, precise detailing of timber to steel and timber to concrete connections, and different site rhythms and tolerances.

Tolerances do not depend on the material alone: they must be agreed according to fabrication, execution, setting out and erection. Joints are detailed to transfer design forces, absorb planned adjustments and control differential movement. It is construction coordination and structural verification at the same time.

The choice is justified once the gravity and lateral load paths, deformation compatibility and the feasibility of the erection sequence have been verified.

+1Composites: two materials, one element

Here the scale of the problem changes. In a composite system, two materials work together within a single element through force-transfer mechanisms. Interaction may be partial, depending on connection stiffness.

One example is the timber–concrete composite slab (TCC): a CLT panel mechanically connected to a concrete topping. Under positive bending, the concrete contributes mainly in compression and the timber can contribute in tension. The degree of composite action depends on connector stiffness and strength, slip and long-term effects.

It is an elegant solution because it asks each material for exactly what it does well.

Other composite systems exist, but a truss with members in different materials is not, by itself, a composite section: the specific structural interaction has to be demonstrated.

• Where it appears. Offices, multi-family housing, and generally wherever high stiffness and good acoustic performance are needed.
• The upside. It can increase stiffness and capacity, reduce deflection and improve acoustic and vibration performance, depending on the configuration.
• The cost. More coordination of pours, moisture, site time, and control of the mechanical connection and of slip between layers.

The composite is the system with the least room for improvisation of the five. If the connector does not deliver the expected stiffness or strength, the degree of interaction, the internal forces and the calculated deflection all change.

Designing for efficiency from the structure

Post & Beam, Post & Plate, load-bearing walls and hybrids are ways of organising the building. Composite action (+1) is a strategy that can be built into several of them.

None of them is the best. Each solves one type of problem well and others badly, and most costly mistakes seen in mass timber come from choosing the system on aesthetic preference and discovering the consequences in the shop drawings.

It is a decision that takes a few hours and shapes years.

It is worth making with the engineer and the manufacturer at the same table, before the project takes shape. After that it is no longer a decision. It is a redesign.

Pablo Baldomá Jones · Cambium.Build · October 2026