CLT (cross-laminated timber) design
CLT is a solid timber panel built from layers glued at right angles, so it works in two directions like a timber version of a concrete slab or wall. We design cross-laminated timber structures to the Eurocodes for clients across the UK.
What makes CLT different from glulam?
Both are engineered timber, but the layers run differently. In glulam every lamination runs the same way, which suits beams and columns. In CLT the layers cross at right angles, which makes flat panels that work in two directions as walls and floors, more like a timber slab than a beam.
What CLT is and how it behaves
Cross-laminated timber is made from layers of graded boards stacked with the grain of each layer at ninety degrees to the one below and bonded into a solid panel, usually in an odd number of layers, three, five or seven. The crosswise arrangement is the key idea, because it gives the panel strength and stiffness in both directions and controls the moisture movement that limits ordinary timber, so a CLT panel behaves as a plate rather than a set of one-way boards. Panels arrive prefabricated, cut to size with openings already formed, and go up quickly as walls, floors and roofs.
Because it works in two directions, CLT is the timber answer to a reinforced concrete slab or crosswall, and it lets timber move into buildings that used to be masonry or concrete. It is light compared with those materials, which eases foundations and cranage, and it stores carbon in the structure. The layered make-up does mean the layers running across the span contribute little to bending, so the effective section is less than the full thickness, which the design has to account for rather than treating the panel as solid through its depth.
Designing CLT to BS EN 1995
CLT is designed within BS EN 1995, Eurocode 5, supported by manufacturer technical data and European assessments for each product, because panel properties vary between suppliers. For floors we check bending and, importantly, rolling shear, a shear failure in the crosswise layers that is specific to CLT and often governs, along with deflection and vibration, which usually control the span of a floor panel just as they do a timber joist. For walls we check in-plane compression and buckling for gravity load and in-plane shear where the wall acts as a racking panel resisting wind.
The connections between panels are what turn a stack of plates into a stable structure, so much of the design effort goes into the junctions. We design the screwed, bracket and hold-down details that transfer floor loads into walls, tie walls together at corners and resist the uplift and sliding that wind and, in a tall CLT building, overturning produce. Serviceability matters too, and we consider long-term deflection under sustained load and the acoustic and fire build-ups that sit around the structural panel, since these often set the final floor and wall thicknesses.
Fire, moisture and where CLT suits
Like other mass timber, CLT chars at a known rate and the char protects the core, so a panel keeps structural capacity in a fire for a calculable period, and we design a sacrificial thickness for the required resistance. That said, fire regulation for taller timber buildings is a live and tightening area, so the exposed timber question, and any encapsulation with plasterboard, is settled early with the fire strategy and the current guidance rather than assumed. Getting this right at concept stage avoids reworking panel thicknesses later.
Moisture is the practical risk on a CLT job, both in service and during construction, since panels must be kept dry before they are closed in or they can swell and their surfaces degrade. Detailing to shed water, protect exposed edges and manage the build sequence is part of the design. CLT suits low and mid-rise residential, schools, offices and any project that values speed, a dry trade site and a low-carbon structure. Its limits are long single spans, where it grows thick or needs downstand support, and its cost against lighter timber framing for simple houses.
What we check.
The points our calculations resolve for a project like this.
- Panel layup and effective section for one-way and two-way action
- Rolling shear in floor panels, which often governs
- Deflection and floor vibration limits for comfort
- In-plane racking and wall buckling under gravity and wind
- Panel-to-panel connections, hold-downs and tie details
- Charring rate, encapsulation and moisture protection during the build
From enquiry to sign-off.
Enquiry
Send drawings or describe the problem. We confirm the scope, the deliverables and a target timescale.
Information
We agree the survey, drawings or data we need and any site access required.
Engineering
Design, calculation or assessment to the relevant Eurocodes and UK National Annex.
Issue
A clear, defensible report or set of calculations, with assumptions and limitations stated.
Common questions
What makes CLT different from glulam?
Both are engineered timber, but the layers run differently. In glulam every lamination runs the same way, which suits beams and columns. In CLT the layers cross at right angles, which makes flat panels that work in two directions as walls and floors, more like a timber slab than a beam.
What is rolling shear and why does it matter?
Rolling shear is a shear failure in the crosswise layers of a CLT panel, where the fibres roll over one another. It is a weak mode specific to cross-laminated timber and often governs floor panel design, so we check it explicitly rather than relying on bending strength alone.
How does CLT perform in a fire?
It chars at a predictable rate and the char layer insulates the timber beneath, so panels keep capacity for a calculable period, and we design a sacrificial thickness. Regulation for taller timber buildings is tightening, so the exposed timber and any plasterboard encapsulation are agreed early with the fire strategy.
Is CLT suitable for a house or only large buildings?
It works for both, but it earns its place best on low and mid-rise residential, schools and offices where speed, a dry site and low carbon matter. For a simple detached house, lighter timber framing is often cheaper. We advise which suits the project and budget across the UK.
Tell us what you are building.
Send the drawings or describe the project. We confirm scope, deliverables and a realistic timescale.
Start a project