What lies behind it
Two things that belong together. Forestry logistics means the whole chain from the standing tree to the site: harvesting, transport to the mill, sawing, drying, intermediate storage and delivery. Every step has its own dates, and if one slips, the carpenter is left waiting.
Prefabrication is the machine cutting of the timber to the shop drawing — today mostly on CNC lines. The machine cuts to length, forms tenons, birdsmouths, dovetails and lap joints, drills and labels every piece. What leaves the works is a numbered kit.
Everything that makes up the structure passes through: structural timber for rafters and studs, glulam for long-span beams, cross-laminated timber as planar elements. Panel products for sheathing and bracing run through the same chain.
Roof structures used to be marked out and cut by hand on the framing floor. The machine has turned that into a data set: what is in the shop drawing gets cut — no more and no less.
Advantages
- Short erection time. A prepared roof structure goes up in hours rather than days.
- Dimensional accuracy from the machine. Joints fit because they come from the same data set as the design.
- Less waste. Optimisation calculates cutting across the whole order — unachievable by hand.
- Less disposal on site, because offcuts never get there.
- Barely any storage area needed. Delivery in erection sequence instead of stacks on the plot.
- Predictable dates. With the chain to site in one hand there are fewer interfaces where things get lost.
- Less noise and dust on site, a real argument for refurbishment in occupied buildings.
- Traceable documentation of origin and delivery.
Limits and drawbacks
- The design must be complete. Prefabrication starts only once shop drawings and structural design are released. Without that, nothing runs.
- Changes after cutting are expensive. A wrong dimension in the drawing gets cut into timber twenty times over.
- Lead time. Weeks pass between release and delivery, depending on capacity.
- Access and crane must suit the delivery date — tight urban sites need their own planning.
- Weather protection on site. A finished kit lying in the rain for a week was not worth the accuracy.
- Fixed sequence. Deviate from the plan during erection and the advantage is lost immediately.
- Survey errors in existing buildings have full effect. The machine does not check whether the old wall is really plumb.
Applications
- Roof structures from simple gables to complex valley constructions
- Timber frame construction and element production
- Hall structures and skeleton frames
- Vertical extensions and additions
- Series and standard-type buildings with recurring details
- Carports, dormers and canopies as prepared kits
- Exposed structures where clean joints matter
When prefabrication does not pay off
- Small repairs and replacements. A single rafter is quicker made on site than designed and delivered.
- Existing buildings without an accurate survey. Where the real dimensions only appear once things are opened up, cutting on site is more honest.
- Very short-notice requirements. No optimisation beats the lead time.
- Very small quantities. Set-up and programming bear no relation to a handful of members.
- Structures that will need adjusting anyway, for instance when tying into grown historic geometry.
- Sites without access for lorry and crane.