- Quality and reliability is better – few defects and call-backs.
- Waste on site is cut by up to 70%.
- Build time is faster – by perhaps 40-60%.
- Structure is lighter in weight, by up to 30% .
- Economies of scale for volume production can cut capital costs by 10%.
- It is proven to be a safer means of construction.
- Less site labour is needed and better productivity can be achieved.
- It offers better predictability of delivery.
- It meets Egan targets.
If it's that good, why isn't everyone using it now?
- Public perception is a barrier to its acceptance as saleable housing.
- Financial institutions' perceptions of the "risk" of financing prefabricated building systems and the acceptability of the form of construction to mortgage lenders in the case of private sector purchasers.
- It is criticised for the environmental impact of the large number of journeys necessary in the transportation of the component parts of prefabricated buildings.
What are the main build options?
Modular or panelised, in a variety of materials.
What is modular?
It most commonly comprises pre-finished lightweight steel-framed module systems, which are linked together to form homes and stacked to create complete buildings. They can be clad externally in a variety of ways.
What are its benefits?
- It is lightweight (reducing substructure and therefore design costs).
- It is suitable for small compact inner city sites where space is at a premium, as long as there is sufficient vehicular access.
- Less scaffolding and temporary works are needed as modules are self-supporting
- Modules can be stacked to make multistorey buildings without the need for a supporting structural frame.
- Off-site trials can be carried out to "prove" the system before installation.
- Thermal and differential movement defects in internal finishes are greatly reduced due to the rigidity of construction of the modules
- It is an environment-friendly form of construction offering energy savings in manufacture and use (lower running costs for occupiers), ease of dismantling for demolition and scope for recycling. Use of lightweight modules allows designers to offer innovative solutions to problems such as building over and on top of existing buildings or in open spaces where traditional construction would be impractical.
- Lightweight modules lend themselves to refurbishment or conversion projects
What are the disadvantages?
- Cost is marginally higher than for traditional construction at present but this depends on the size of the proposed development and the extent of repetition of the design.
- The size of modules is restricted by transportation constraints.
What about panelised systems?
- These are made from lightweight steel, timber or concrete, but do not give the same level of prefabrication as modular. Timber and metal systems generally comprise insulated framed wall and floor panels sheathed both sides with a prefinished boarding material and trussed rafter roofing which, when assembled, form the structural frame. Internal finishes are generally traditionally applied and the external finish is often masonry cladding and conventional roof tiling.
Concrete systems comprise precast internal and insulated external wall panels with window and door openings. Floors comprise precast panels with stairwell openings and are supported on the internal wall panels. Staircases are precast concrete and are also supported from internal wall panels. Service ducts and outlet points are cast into internal wall panels. Roofs can be fabricated from trussed rafters with traditional roof finishes and external wall panels can be either prefinished or clad to resemble traditional construction.
- There has been innovation to reduce the extent of on-site finishing, such as prefabricated bathrooms and kitchens in pod form and internal finishes that require little on-site finishing
Sounds interesting, so what are the benefits?
- Similar to those of modular systems.
- There has been a re-emergence of timber frame for buildings up to six storeys high. This gives fast construction time, is lightweight and reduces foundation loading, and therefore the cost of substructures. It reduces on-site labour, uses sustainable materials and recent improvements in dimensional accuracy have enhanced the accuracy of systems.
- Metal-framed panelised systems can offer similar benefits.
- Concrete "tunnel form" systems can offer a degree of versatility to accommodate varying designs up to six storeys high, offer excellent sound insulation and are unaffected by concerns in relation to progressive means of collapse.
But there must be drawbacks?
- It cannot generally be used for multi-storey developments above six storeys.
- Systems generally require traditional masonry cladding and roofing.
- Consumer confidence – due to past poor performance particularly in the case of timber frame systems.
- In the case of concrete systems, the preplanning of service runs and points requires careful co-ordination in the preliminary design phase of a project as there is no margin for error.
- Less design flexibility in the case of concrete panel systems.
- Concrete systems are heavier than comparable timber or lightweight steel systems and generally need to be supported on a piled foundation.
How much will it all cost me?
Modular works out at £1100 to £1300 per m2 of gross internal floor area, including contractors' fees. Timber and concrete panelised systems are £1200 to £1400 per m2. We estimate metal panelised systems to have similar costs, although no cost information has been provided by makers.