Offsite or Built in Place: What Changes

The precast entry states the misreading directly: precast structures are assumed to be quicker in every respect, when much of the effort moves earlier into design and manufacturing information. The volumetric entry says the same in its own terms, that speed on site is assumed to mean less design when the design has to be finished earlier than in any alternative built in place.

Nothing is removed from the project. What changes is the order in which it has to be done, and therefore who is waiting on whom.

In mass timber, openings, service holes, recesses, rebates and connection preparation are all part of the manufacturing information, so the drawing set closes earlier than for site-built construction and the coordination of services becomes a design activity rather than an installation one.

In precast work, what is cast into the units for services and fixings has to be known before the mould is set, which means service and fit-out coordination precedes production rather than following the frame. In a panelised steel frame the fabrication model records every stud, hole and piece of backing, and where backing for heavy fixings is needed has to be known before the panels are made.

Units arrive finished. A precast unit is cured, faced, and already carries its lifting points, cast-in fixings, service voids and connection preparation, so nothing about it can be adjusted once it is delivered. Mass timber panels arrive in sequence, are craned into position and cannot be trimmed to fit.

A volumetric module is more final again. It leaves the factory as a finished enclosure, already framed, lined and largely fitted out, and a unit landed out of position delivers its load eccentrically into the one below, a correction that cannot be made inside a finished box.

Every precast unit is made to a manufacturing deviation and placed to a construction deviation, and the joint has to absorb both at once. A joint dimensioned for the drawn gap rather than for that accumulation will either refuse to close at one position or open too far at another, and by then the units either side of it have already been made.

In a volumetric stack the accumulation travels. Variation within each unit, the setting-out of the first row and the take-up at every connection add together up the stack, and the connections are the only place that accumulation can be absorbed. Where their capacity runs out, the effect shows at the facade line or along a corridor, well away from the joint that caused it.

Precast units are heavy, arrive in sequence and cannot be trimmed, so crane position, delivery order and access govern the design as much as the plan does, and a unit that arrives out of sequence occupies space the next one needs.

The same constraint reaches mass timber, where access, delivery order and lifting are design constraints and a change agreed on site may have already been manufactured somewhere else. On a tight site this can be the argument that settles the question in either direction.

The common precast arrangement runs load-bearing walls across the building with floor units bearing between them, giving a plan that repeats from bay to bay and from storey to storey. That repetition is what makes the method work and also what makes it inflexible: a plan change is a change to the manufacturing schedule, not a change to a drawing.

It is worth asking early whether the design in hand actually repeats enough for the method to make sense, because a plan that resists repetition removes much of what was being bought.

Offsite does not remove site work, it relocates it. In a volumetric building the units themselves are the least demanding part: the plane they land on, the joints between them, the facade running across those joints and the roof completed above are where the design effort actually concentrates.

Some of that work is also permanently unreachable. Where the wall of one unit stands against the wall of its neighbour, the void between them is closed from both sides at the moment of connection, so whatever is left undone there stays undone.

The most useful thing an offsite route hands over is a record. The manufacturing model holds the position of every hole, recess and connection, and the fabrication model of a steel frame records where every stud, hole and piece of backing sits, which is the only reliable way to find them once the wall is lined.

Both entries suggest asking for that model as part of the information handed over at completion. Without it, altering the building means investigating an element whose internal layout is not visible.