Timber - Cross Laminated Timber (CLT) Construction Methods, Properties & Performance
Cross laminated timber, commonly known as CLT or crosslam, is a precision engineered building material that, despite a raft of practical and ecological benefits, is yet to be widely adopted by the UK self-build market, although growth has been strong in the commercial sector.
CLT is made from gluing together layers of solid softwood sawn lumber in a symmetric way at 90 degree angles. By rotating the layers of lumber CLT loses the anisotropic quality of regular timber – its physical properties are the same in every direction that force is applied. This not only makes CLT strong and durable, but allows it to be cut into almost any size and shape.
CLT differs significantly from GLT (glued laminated timber), or 'glulam' as it is more commonly known and LVL 'Laminated Veneer Lumber'. A GLT panel is made when all laminations are oriented the same way.
CLT is formed of kiln-dried spruce or pine boards which are laid on top of each other at 90° (three, five, seven or nine layers depending on structural requirements), coated with a layer of polyurethane adhesive and subjected to immense hydraulic pressure to create large, stiff, dimensionally stable panels.
These timber panels, which can measure up to 3.5m in width and 20m in length, depending on specification, can be used to create the complete superstructure of a building. CLT panels are precision cut by CNC machines (from 3D CAD drawings) and delivered to site to be installed by specialist CLT installers.
Cross Laminated Timber
Window and door openings are created during manufacture to such high degrees of accuracy that glazing can be ordered from the initial drawings. The high tolerances involved greatly improve airtightness and thermal performance and mean that CLT can be used in Passivhaus designs.
CLT buildings are assembled on site quickly, with pre-cut window and door openings designed to exacting tolerances. This allows glazing to be ordered from the drawings and fitted swiftly, getting projects to watertight stage in a matter of days.
As of 2018 there are no CLT manufacturers in the UK, which means all CLT builds here will import directly from Europe, though many of the key CLT producers work, own or partner with UK-based CLT installers who provide services such as consultation, design, supply and installation.
Studies examining the viability of manufacturing CLT from UK resources have found that it could be a viable proposition using sitka spruce (which accounts for around 50% of the UK’s softwood resource).
But there were challenges, including that the UK’s spruce performed differently to the softwoods on the Continent under structural testing and would need to be kiln-dried to a lower moisture content than current UK standards to match European CLT’s material properties.
For the self-builder CLT is currently a seemingly underexposed option for creating highly efficient, design-led and structurally stable homes that can benefit from reduced foundation costs compared to a traditional brick and block build, reduced construction time and a safer construction environment when compared to some other build methods.
CLT properties and performance :-
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High axial load capacity for walls;
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Less susceptible to buckling;
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High stiffness/strength-to-mass ratio;
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High shear strength to resist horizontal loads;
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Less susceptible to effects of soft story failure than other platform-type structural systems;
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Floor span-to-depth ratios 20:30; roof span-to-depth ratios 30:40.
CLT key benefits :-
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Design flexibility;
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Thermal performance and energy efficiency;
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Cost effective;
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Environmental advantages;
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Less waste;
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Fast installation;
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Fire protection;
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Acoustic performance.
PDF Download
Timber - Introduction To Cross Laminated Timber - PDF Download
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