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PRC.03 · Construction Process

CLT (X-LAM) Timber Structure

A CLT structure is not built: it is assembled. The real work already happened at the factory, at cutting; on site remain the precision of the first wall, the right screws in the right places, and one true rule — timber must never touch concrete nor stay wet while standing still.

System
prefab, dry
Weight vs RC
~ 1/5
Carbon
stored
Assembly
days

Overview

TAV. 00

A CLT (cross-laminated timber) structure uses large solid-timber panels as load-bearing walls and floors. It is a prefabricated, dry system: panels arrive cut to the millimetre and are assembled in days, cutting time, noise and waste. For the same performance it weighs a fraction of concrete, stores carbon instead of emitting it, and offers timber comfort and aesthetics that are hard to match.

The site score

TAV. H
The beats — with site daysbeat 1 of 5

click a beat · arrow keys ← → walk the site · dashed pauses are the chemistry at work

NOT TO SCALE0 of 4 layers in place

The detail of the craftfrom model to schedule

P-01each panel has a name, a way upholes and joints: pre-cutno cutting on sitePANEL TO DRAWING · CNC CUT
01Weeks before · at the factory

Design and prefabrication

BIM modelling and CNC cutting of the panels in the factory, with pre-arranged holes and joints.

Why it is done this wayIn CLT the drawing IS the construction: every hole, joint and services route is born in the model and CNC-cut. On site you don't cut: you only fix what the survey didn't see.
CNC · millimetric cuttingun modello · shared BIMrilievo · of the real pour
In handBIM modelassembly scheduleslifting plan
The mistake that costs dearlyReleasing to production without surveying the real pour: a kerb two centimetres off is paid for by every wall that lands on it.
Site supervision checkConsistency between model, schedules and drawings; assembly and lifting plan approved (weights, centres of gravity, lifting points).
"In CLT, mistakes are made at the desk and seen at the crane."site notebook — editorial synthesis

Time that doesn't wait

The dry site

TAV. G1

No water in the processes means no chemical waits: the CLT score has no dashed pauses. The risk moves entirely to logistics — crane, deliveries, weather — and to the desk, where mistakes are born.

TRADITIONAL STRUCTURE, WET WORKScuringcuring→ weeksX-LAM STRUCTURE · DRY ASSEMBLY→ days, no breaksthe time saved is not haste: it is no chemistry left to cure

The flip side: such a fast site does not forgive improvised logistics. An idle crane is CLT's "dead time" — prevented at beat 01, not on site.

The rule that rules

Water is the only enemy

TAV. G2

Timber tolerates rain that runs off and dries; it does not tolerate rain that stands. Three points decide everything: the base separated from concrete, horizontal joints that drain, panel ends covered at day's end.

1 · foot membrane: always2 · vertical joint: no standing water3 · heads covered at day's endrot is always a missed detail, never a destinyTHE THREE THINGS TO CHECK NIGHTLY

It applies to storage too: panels raised off the ground, spaced, covered yet ventilated — a clinging tarp that condenses is standing water as well.

How it ages (and what betrays it)

TAV. P
PIEDE MARCIOdecay at the wall base → timber in contact with concrete or missing membrane (beat 02)
VITI SBAGLIATEseismic behaviour off-design → undeclared "equivalent" substitutions (beat 03)
PASSI DI SOPRAaudible footsteps between floors → screed not decoupled or missing strips (beat 04)
SPIFFERI INVISIBILIworse consumption and comfort than designed → incomplete tapes closed too early (beat 05)
PANNELLI IMBARCATIgaps and warping appearing on site → uncovered storage and repeated wettings (beat 02)

Questions from the site

TAV. Q
Doesn't timber burn?

It burns predictably: solid timber chars at a known rate and the charred layer shields the section's core, which is designed exactly that way. That is why a well-sized CLT wall keeps bearing for a long time even in fire — details and protections matter, as always.

What if it rains during assembly?

A brief wetting that can dry out is no drama: the enemy is water standing in horizontal joints and panel ends. Hence: fast sequences, sheets at day's end, and an envelope closed early.

How long does assembly really take?

For a two-storey house the panels go up in days, not months — the most photogenic phase and the shortest. The real site time stays in foundations before and finishes after.

Do you still need external insulation?

Almost always yes: the panel alone insulates little. Wood fibre is the natural companion — summer phase shift and vapour permeability in order — and the envelope can close as rendered ETICS or a ventilated facade.

Materials involved

TAV. M
MAT.17/40X
Engineered panel
X-LAM (CLT)
MAT.18/41G
Linear element (beams)
Glued Laminated Timber (GLT)
MAT.21/44L
Laminated veneer lumber
LVL (Laminated Veneer Lumber)
MAT.38/61W
Wood Fibre (WF)
Wood Fibre
MAT.74/97S
Structural Solid Wood
Spruce / Larch (Carpentry Timber)

Reference regulations

5 norms
  • D.M. 16/02/2007Fire-resistance classification of construction products and elementsNazionaleIn force
  • D.M. 17/01/2018Technical Standards for Construction (NTC 2018)NazionaleIn force
  • D.M. 58/2017D.M. 58/2017: the Sismabonus and the seismic-risk classesNazionaleIn force
  • Eurocodici (EN 1990–1999)The Eurocodes: the European language of structural designProtectedUEIn force
  • OPCM 3274/2003Seismic classification of the territory and technical rules for construction in seismic zonesNazionaleIn force

Informational links to the regulatory framework. Always verify the current text on the official source.

Stratigraphy

TAV. S
↑ OuterInner ↓
Cladding
Ventilated facade or rendered ETICS
Cavity
Ventilated battens
Insulation
Wood fibre or mineral wool
CLT panel
Load-bearing, cross-laminated; interior finish downstream

Advantages

TAV. V
  • Fast, clean, dry site
  • Lightweight: ideal for vertical extensions
  • Stores carbon, renewable material
  • Excellent hygrothermal comfort
  • Good seismic behaviour thanks to low weight