Cross-laminated timber (CLT) erection
Panels arrive numbered, cut and drilled - the gang screws the building together in the dry, and the only real enemy is water.
Last updated 2026-09-05
What is Cross-laminated timber (CLT) erection?
Cross-laminated timber is board stock glued up in layers laid at right angles to each other, always an odd number of layers so the two outer ones run the same way. Panel thicknesses commonly run from around 60 mm for a wall up to 300 mm or more for a floor, and panels come out of the press up to roughly 3-3.5 m wide and 12-16 m long, with the lorry usually the binding limit rather than the press. Everything is machined on a CNC before it leaves the factory: window and door openings, service holes, rebates, recesses for connections, and a mark sprayed on each panel that ties it to one position on the erection drawing.
Weight is the headline. CLT runs around 450-500 kg per cubic metre against roughly 2,400 for concrete, so a floor plate weighs a fraction of the equivalent slab. That means a smaller crane, smaller foundations, and often the ability to add storeys to an existing building that could never take concrete. Erection is dry and quick - a gang of four to six with one crane will commonly put up a few hundred square metres a day, and a floor a week on a residential block is a normal rate. There is no curing, no drying-out period and no wet trade following behind, so finishes can start far earlier than the calendar would suggest.
Two disciplines make or break it: moisture, and the freeze date. CLT leaves the factory dry - somewhere near 12% moisture content is typical - and it has to stay that way. Rain ponding on a flat floor panel with nowhere to drain is the classic problem; so is standing water sitting in a service rebate, and exposed end grain drinking through a wet weekend. That means a protection strategy agreed before the first lorry books in, not improvised in October. And because every hole is machined in the factory, the design is fixed early. A duct that moves after the panels are cut is a chainsaw, an engineer's approval and a bad week.
How does Cross-laminated timber (CLT) erection work, step by step?
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Step 1: Freeze the fabrication model before anything is cut
The model that goes to the factory is the building - it is not a drawing of it. Every opening, every service penetration, every connection recess and every fixing has to be coordinated into it first, which means the mechanical and electrical design has to be far further advanced than a masonry job would need at the same stage. The structural engineer and the supplier's engineer agree the connection scheme at the same time. This is the step that buys the speed later, and the step people try to skip.
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Step 2: Prepare the substructure to timber tolerances, not concrete ones
CLT is cut to millimetres and it does not stretch. Survey the slab or foundation level, the perimeter line and every holding-down position against the fabrication model before the first delivery, and correct anything outside tolerance while it is still cheap. Sole plates, separation layers and damp-proofing go in first so the bottom of the panel never sits on a wet surface, and packers are planned rather than found. An error here is not absorbed anywhere - it shows up at roof level.
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Step 3: Deliver just in time, in lift sequence
Loads are ordered so the panel needed first is the panel reached first, and they arrive on the day they are erected. There is no laydown phase; a stack of unwrapped panels standing in a compound for a fortnight is a moisture problem waiting to be discovered. Panels are checked on arrival for transit damage, wetting and the right mark numbers before anything is unstrapped. If the crane cannot take the load that day, the sensible answer is to turn it round, not to lie it down.
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Step 4: Erect wall by wall, floor by floor
Panels are lifted on a vacuum lifter or on straps and screw-in lifting anchors, landed, plumbed and held on temporary props while the connections go in. Walls go up first on a level, then the floor deck lands on them and becomes the working platform and the stability diaphragm for the level above. Connections are made with the plates, brackets and structural screws in the connection schedule, and the erection engineer sets what may be propped, loaded or released and when.
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Step 5: Manage water every single day the timber is open
Drainage holes are drilled through floor panels at the low points so water cannot pond, standing water is swept or vacuumed off rather than left to dry, and exposed faces and end grain get the protective coating or sheeting the design calls for. On a long programme the frame may be tented. Moisture readings are taken and written down as the build goes up, because the argument later is always about what the moisture content was on the day something was covered over.
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Step 6: Close up, encapsulate and hand to fit-out
Nothing gets permanently covered until the moisture readings say it is dry enough - closing damp timber inside an airtight build-up is the one mistake that cannot be undone. Then the airtightness layer, the acoustic build-ups, the service zones and any fire encapsulation go on in the order the design sets, with penetrations sealed as they are made. Where timber is left exposed as the finish, it needs protecting from site damage right through to handover, because you cannot make good a bruised CLT soffit.
What are the benefits of Cross-laminated timber (CLT) erection?
- Very fast, dry erection - no curing and no drying-out period, so finishes can follow within days rather than weeks
- A fraction of the weight of concrete, which cuts crane size and foundation size and makes rooftop and over-site additions viable
- Factory precision - panels arrive cut, drilled and numbered, so site work is assembly rather than fabrication
- Small, quiet crews with little waste and few deliveries, which suits constrained urban sites and occupied neighbours
- The panel is structure and finished surface at once wherever exposed timber is part of the architecture
- Lower embodied carbon than an equivalent concrete or steel frame, which increasingly carries weight with planners and funders
What are the limitations of Cross-laminated timber (CLT) erection?
- The design has to be frozen early - change after the panels are machined is slow, expensive and structurally awkward
- Water is the enemy from the day the first panel lands until the building is weathertight, and a wet frame is a programme problem, not a snag
- Panel size is capped by what will travel on a road, and factory lead times can be long - the erection programme depends on loads landing in exact sequence
- Fire and acoustic performance comes from build-ups and detailing designed in from the start - it cannot be bolted on later
- Insurance, warranty and lender attitudes to timber structures still vary, and need settling at the outset rather than at practical completion
- Light panels are wind-sensitive, so crane work stops at lower wind speeds than it would on a concrete frame
What is Cross-laminated timber (CLT) erection best suited for?
What plant does Cross-laminated timber (CLT) erection need?
- Crane sized for the heaviest panel at radius - usually far smaller than a concrete job on the same footprint would need
- Vacuum lifter, or lifting straps with screw-in lifting anchors
- Cordless drivers and long structural screws, with the plates and brackets the connection schedule calls for
- Temporary props and bracing to the erection engineer's scheme
- MEWPs and podium steps for making connections at height
- Moisture meter, temporary sheeting or tenting, and a wet vacuum for clearing standing water
How is Cross-laminated timber (CLT) erection quality-checked?
- Panels inspected on delivery for damage, wetting and correct mark numbers before anything is lifted
- Substructure level, perimeter line and holding-down positions surveyed against the fabrication model before the first panel lands
- Plumb, level and squareness checked at every level - CLT gives you no way of taking up an error later
- Connections checked against the connection schedule while they are still visible, not after the linings go on
- Moisture readings taken and recorded before any panel is encapsulated or covered over
- Protection regime inspected daily during erection, including drainage of standing water off every floor panel