System building — also called assembly building or offsite construction — is a method in which the components are made in a factory and on site are only lifted, positioned and connected. The building site turns from a workshop into an assembly yard. It is the counterpart of traditional construction, where brick is laid on brick and concrete is cast in place.
The components can be concrete, steel, timber or a combination: hollowcore floor slabs, lattice-girder floor planks, precast walls, facade panels, dormers, complete bathroom pods and even whole volumetric modules.
Advantages
- Speed on site. The shell of a house can go up in days rather than weeks, so the building is weathertight much sooner
- Consistent quality. Factory conditions mean no rain, no frost, fixed moulds and repeatable dimensions
- Less weather dependency. Frost delays play a much smaller part
- Less waste and fewer defect costs, because everything is made to drawing
- Less disruption to neighbours: shorter build period, fewer deliveries of loose material, less cutting and breaking noise
- Safer work, because many operations happen at bench height in the factory rather than on a scaffold
Drawbacks and pitfalls
- The design has to be finished early. A component in production cannot be changed. Every opening for services, ventilation and electrics must be settled at the engineering stage. This is by far the biggest trap
- Lead time. Precast components have production and scheduling lead times. Decide late and you wait
- Foundation accuracy. The substructure has to be exact; precast components have very little tolerance. A foundation a few centimetres out costs days of remedial work
- Access and crane standing. Large components need a heavy crane and room to set it up. In a narrow city street that is usually the constraint
- The joints determine the quality. Airtightness, weathertightness and thermal bridging all sit in the joints between components. Badly finished joints undo all the factory quality
- Transport limits. Component sizes are partly set by what can legally be moved on the road
System building versus prefabrication
The terms are often used interchangeably, but there is a shade of difference. Prefabrication describes the working method: assembling from components made in advance. System building describes a coherent, standardised building system with a fixed dimensional grid and matched components. System building is therefore almost always prefabricated, but not all prefabrication is system building.
Related terms
- Prefabrication
- Hollowcore slab
- Lattice-girder slab
- In-situ concrete
- Modular coordination
- Volumetric module
- Programme
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