Tolerances, Logistics & the Programme

Tolerance is the allowable deviation from a specified dimension - how far off 'exactly right' a part is permitted to be - and prefabricated timber works to remarkably tight tolerances. Because the components are machined by CNC from a digital model (8.2), they are accurate to a fraction of a millimetre, far tighter than anything achievable by hand on a wet site. This precision is a genuine gift: parts fit together cleanly, connections seat properly, the frame goes up fast and true, and the quality of the finished structure is high. When an entire kit is fabricated to a consistent, tight tolerance, the pieces relate to each other exactly as the model said they would.

But precision is only an advantage when everything it meets is precise too, and in real construction it usually is not - which is the discipline hidden inside the gift. The classic problem is the interface between the millimetre-accurate timber and the centimetre-accurate concrete it sits on. Foundations, ground slabs and concrete cores are cast in situ to much coarser tolerances; the ground itself is uneven. If you simply assume the timber will land perfectly on a concrete base, you will be disappointed, because the base is not perfect and the rigid, exact timber kit has little ability to absorb the difference. The mismatch accumulates and concentrates precisely where the two systems meet.

The resolution is to design the interface to absorb the tolerance difference, and this is a principle every timber designer must internalise. That typically means providing a levelling or packing zone at the base, using adjustable connectors that can take up the deviation, and surveying the concrete accurately before the timber is fabricated or delivered so any correction can be made. The idea is to concentrate the 'give' at a designed joint rather than hoping the mismatch disappears. More broadly, prefab tolerance discipline means being explicit about which tolerances apply where, coordinating the different trades' accuracies, and never assuming that because the timber is exact, the whole building will be. The specific tolerance values, the interface details and the connector allowances are engineered decisions for the structural engineer and the fabricator - the designer's job is to understand that prefab precision is a gift that demands a designed strategy for meeting the coarser world around it.

There is an iron rule of prefabrication that designers sometimes forget: a component that cannot be transported to site and lifted into place cannot be built, no matter how well it is designed. Transport and craneage therefore quietly set hard limits on the size of every element, and understanding these limits early saves a great deal of grief. The truck is the first constraint. A panel or beam has to fit on a lorry and travel legal roads: there are limits on length, width, height and weight for standard transport, and while oversized loads are possible, they require special permits, escorts, careful routing and extra cost, and there is a practical ceiling beyond which transport becomes impractical. So the maximum size of a CLT panel or glulam member is often set not by the factory press but by what can legally and sensibly be driven to the site.

The crane is the second constraint, as we saw in 8.3: every element must be liftable at the radius where it has to be placed, so crane capacity and reach cap the weight and position of parts. Between them, the truck and the crane define the envelope within which the whole kit must fit - a discipline that feeds all the way back into the design grid and panelisation. This is why panelisation - how the building is divided into transportable, liftable components - is a real design decision, and why the route to site, the access, and the space available all matter.

Beyond size, logistics is about flow and sequence. Because parts are numbered and assembled in order (8.3), they must arrive in the right sequence, and often there is little room on a tight urban site to store them - so mass timber frequently relies on just-in-time delivery, with components arriving shortly before they are lifted, sometimes straight from the truck into the crane. This demands tight coordination between factory, transport and site, and it makes the supply chain's reliability critical: a late or out-of-sequence load stalls the whole assembly. Laydown space, site access, delivery scheduling and the transport route are all part of the logistics plan. The specific transport limits, permits, crane capacities and the logistics plan belong to the contractor, the transport specialist and the fabricator - the designer's job is to understand that the truck and the crane cap the size of the kit and that just-in-time flow demands a reliable, well-coordinated supply chain, and to design panelisation that respects those realities from the start.

Prefabrication does not simply make construction faster; it changes the shape of the whole programme, and grasping this new shape is essential to planning, costing and selling a timber project honestly. In a conventional cast-in-place project, design and construction are somewhat sequential and much of the effort happens slowly on site. Mass timber redistributes the effort along the timeline into a distinctive new profile that can be summed up as design longer, build faster.

The front of the programme gets longer and heavier. Because of DfMA and the design freeze (8.1), and because everything must be coordinated into the model before fabrication (8.2), far more design and coordination effort happens early, and it must be complete sooner than a traditional project demands. Then comes a factory lead time - the weeks the fabricator needs to manufacture the kit - which is a new element in the programme, but one that runs in parallel with site preparation and foundations, so two slow processes overlap rather than queue. Finally, the erection is short: the superstructure goes up in a fraction of the time a concrete frame takes (8.3). The net effect, on the right project, is that the total programme is shorter and, crucially, the building reaches weathertight and handover sooner.

This reshaping has consequences well beyond the calendar. Earlier handover can be worth a great deal financially - earlier revenue or occupation, lower financing costs, reduced site overheads and preliminaries over a shorter site period - which is often where mass timber's business case is really made, even when the material costs more (a theme for Module 10). It also changes the risk and cashflow profile: money is committed earlier (design, and paying for fabrication before erection), and the certainty of a frozen, prefabricated design reduces the risk of on-site surprises, but it also reduces flexibility, as we have stressed throughout. Planning a timber project therefore means planning a different programme, with an earlier commitment of design and cost, a factory lead time to build in, and a short, intense erection - not a faster version of the old programme, but a differently shaped one. The actual durations, lead times and the programme itself are the contractor's and the fabricator's to determine; the designer's job is to understand and design for this new shape.

Everything in this lesson meets its hardest test in an emerging market, and an honest, India-aware account has to say so plainly. The tolerance discipline is universal and transfers directly - millimetre timber meeting centimetre concrete behaves the same everywhere. But logistics and the programme are where the Indian reality bites, and pretending otherwise would do a designer no favours. Because domestic mass-timber fabrication is nascent, much of the kit for an Indian project today may be imported, which lengthens the logistics chain dramatically: sea freight, port handling, customs and inland transport add weeks or months of lead time and cost, and the just-in-time flow that works so well when a factory is a few hours away is far harder to orchestrate across an ocean.

Transport within India adds its own constraints. Road conditions, access to tight or congested sites, the availability and cost of suitable cranes, and the smaller pool of contractors experienced in timber erection all affect what is practical, and they can erode the on-site speed advantage if the supply chain is not reliable. The programme benefit - design longer, build faster - still holds in principle, but the 'longer' front end is stretched further by long import lead times, and the 'faster' erection depends on the kit actually arriving complete, in sequence and undamaged after a long journey. None of this makes mass timber unworkable in India; it makes realistic, early logistics and programme planning even more essential than in a mature market, and it favours approaches that reduce the logistics burden - simpler panelisation, hybrid structures, and building local fabrication capacity over time.

The honest position to carry out of this module is therefore twofold. First, the practical disciplines of prefabrication - tolerance strategy at interfaces, respecting the truck-and-crane size limits, planning just-in-time flow and designing for the reshaped programme - are exactly what turn mass timber's promise into a real building, and they must be engaged from the earliest design stages, not left to the contractor at the end. Second, in India today these disciplines are harder to satisfy because the supply chain is immature and often international, so the programme and logistics must be planned with clear eyes and realistic lead times, and the material chosen where its benefits genuinely outweigh these frictions. The specific tolerances, transport limits, crane capacities, lead times and programme are the engineer's, fabricator's and contractor's to determine; your job as a designer is to understand these realities deeply enough to design a kit that can actually be made, moved, lifted and built - on time, in the real world, and in your real market.