Getting to grips with prefab
In a new series, we look at the different types of prefab – from components, panels, modules and hybrid, to complete building – and how to apply them. First up are component-based systems.
PREFAB – short for prefabrication – refers to any part of a building made away from the building site. It is also known as off-site construction.
It is a complementary system to traditional construction and can range from small components such as the widely accepted prenailed wall frames to panel systems and even complete transportable buildings. They can be made of many materials, such as timber, concrete, metals or plastics or any combination.
Prefab buildings are not limited to residential or commercial, can be temporary or permanent and, like other buildings, are designed according to budget and space requirements in a range of aesthetics.
They can be fully compliant with New Zealand building standards and consent processes.
Component-based systems
Component-based construction can also be known as kitset construction. The components may include:
- stick prefabrication – individual lengths of timber or steel that are precut, presized or preshaped and brought to site unassembled
- subassembly prefabrication – preassembled windows and doors, fixtures and fittings, and structural members such as prenailed roof trusses and wall frames, and elements such as composite flooring and framing.
Component-based systems include solid wood, concrete composites and steel framing, each offering alternative solutions to traditional methods of construction and delivering benefits in build time, overall efficiency and quality control.
Solid wood systems
Solid wood systems consist of precut, preformed and premade timber components made off site in controlled conditions, enabling better quality control and faster build time on site. They are suitable for residential and light commercial buildings.
Solid wood technology dates back over 70 years in New Zealand and was imported from Northern Europe.
Design detail
The solid wood kitset home system is engineered with a system of interlocking tongue-and-groove timber boards that are joined using connectors or profiles.
Solid wood systems are generally made from radiata pine prestressed, treated, kiln-dried timber – it provides a solidity and ductility that can withstand extreme weather and earthquakes.
Traditionally, these systems were noisy, with the timber moving as temperatures changed. This has now been largely alleviated with a central cavity between the interior and exterior panels. Some manufacturers also use this cavity for insulation material, increasing the energy efficiency of the building.
Exterior finishes on solid wood systems can vary from the traditional stained wood, with options including aluminium, weatherboard, brick and cedar.
The design process is key as the precise precut dimensions of the factory-controlled system means customisation can only occur before the build begins.
Installing and assembling
Typically, the builder assembles the components in a kitset home pack on site and manages overall construction and sub-trades. Assembly takes about half the time of traditionally built houses.
With solid wood system installation, stability and lateral support are critical. In some systems, the walls are joined to the foundations and roof using vertical steel tie rods, providing structural support to resist uplift loads from wind. Stiffener posts are slotted between the interior and exterior walls for additional strength. Walls acting as sheer panels also provide lateral stability.
Uses renewable materials
Overall, the insulated solid wood system provides a relatively energy-efficient option utilising a renewable resource. The design process enables all aspects of the building to be considered before construction.
In some cases, the manufacturers’ door and window joinery must be used to ensure the overall effectiveness of the building design.
Concrete components
Concrete can be used in walls, floors, beams and decorative panels or as prefabricated structural panels and masonry to form the entire building. These mainly panel-based systems will be covered in the next Build.
The use of precast concrete for stair systems manufactured off-site and then delivered and installed is not new. Traditional concrete stair systems are reinforced with steel and can be moulded to suit any design.
The Flexus™ flexible stair system is a lightweight concrete option providing the structural integrity of reinforced concrete but without the weight and thickness. The stair system can be installed on site by two people.
Steel framing
As an alternative to timber wall frames and roof trusses, steel framing systems are about one-third the weight, providing a system that is non-absorbent, recyclable, straight and will not burn. Light-gauge steel may be the choice for panel manufacturers where measurement tolerances are critical.
Steel frames are designed using integrated engineering software, with the information transferred to a roll-forming machine for manufacture and assembly to exact dimensions, including prepunched services holes.
This delivers simpler installation of services and a better fit for doors and windows, but the design and layout of services is critical.
Design detail
A steel framing system utilises cold roll formed 90 × 40 mm galvanised, lipped C section members in either 0.55, 0.75 or 0.95 mm gauge high-tensile G550 steel.
The framing comes with bracing, battens, nogs, window and door lintels, structural members, openings and prepunched holes for services.
Generally, frames are delivered to site preassembled. However, the C profile allows for flat-packing for export.
Getting the frame up
Steel framing can be fixed to concrete floor slabs, steel floor joists or traditional timber floor framing. With reduced need for temporary bracing, there is less rework required resulting in faster completion.
Frames are transported to site pre-assembled for installation. Depending on the specifications and what the steel framing is being connected to, screw fixing types, such as Pan-head, tek and wing-tek screws are included. A range of proprietary fixing systems simplify specific requirements, such as truss fixing brackets.
Things to think about
Designing a steel frame building is similar to designing a traditional timber-framed building. While it is easy to convert a pre-consented timber frame design to steel frame, consultation with the designer and the fabricator early on is valuable to ensure that the structural performance benefits of steel framing are maximised and structural steel requirements are minimised.
Care must be taken when the steel adjoins CCA treated timber – not LOSP – and dissimilar or bare metal.
Depending on the system, with the correct tools alterations may be made to steel framing on site.
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Articles are correct at the time of publication but may have since become outdated.
