construction’? ‘How do we decide when one is more appropriate that another?’ ‘What pushes the decision in one way or another?’ The simple answer is usually: ‘Money’, apart from when it isn’t, and then the answer is: ‘Money, but in a complicated way’. There are six main materials on offer in construction prefabrication when it comes to structures. Timber frame and mass timber, light gauge steel and hot rolled steel, precast concrete, and in-situ concrete. (There is another batch of materials based on waste processes which I’m not going to cover here.) Each of the ‘big six’ has its benefits and problems, and each one has its fervent supporters (usually suppliers), who think their material is the best and that the others are rubbish, and won’t hesitate to tell you this at great length. But architects are supposed to be independent, and give the client a disinterested view. Some architects openly adopt one material as their preferred option, and while there are good reasons for it, I don’t think it’s an ideal approach. Inevitably, as design practice exposes architects to experi- ence in a particular material, where it’s a successful building, it often leads to another similar building, and the practice gains a reputation for having skills and expertise in a particular material or construction methodol- ogy. But in the long term, the profession needs to keep a professional distance. It helps to look at each material with an open mind and compare them to each other in terms of a number of attributes that demonstrate the differences between them. It is difficult to be generic, for every generalisation I can make, there will be a specific manufacturer or supplier who can demonstrate how their product or widget makes that benefit irrelevant or this problem relating to that material go away.



lients often ask us, ‘why should we use one material versus another, when we are considering prefabrication in

Architect Rory Bergin previews a talk by his practice at Materials for Architecture later this month, weighing up the merits of the ‘big six’ offsite prefabrication materials

Acton Ostry Architects and University of British Columbia

But generalisations are a good starting point, even if they don’t get us to a final decision.

Dimensional accuracy

Steel and precast concrete win the argument here, materials that won’t shrink, swell, warp or twist and which can be machined with millimetre precision. The CLT (cross laminated timber) advocates will say that their favourite material can do the same, which is true, but it can change shape if not looked after properly.

Environmental impact Timber products win this argument, being renewable; even if there is some energy consumed in manufacture and transport it is still much less than for most other materials, there is no real contest here. Concrete is easily the worst, due to the environmental ravages imposed by material extraction and

the CO2 produced in the curing process. 6 per cent of global CO2 output comes from concrete processing.

Structural strength to weight ratio Steel wins this argument, being the lightest material relative to the weight it can carry. You need very little steel to build a substantial structure. Comparing the environmental footprint of a kilogram of steel to a kilogram of concrete makes no sense, comparing the amount of steel you need to build a house with to the amount of concrete does make sense.

Fire performance Steel needs to be protected, concrete doesn’t, and while timber products are obviously flammable, in practice they perform well due to charring. Concrete wins this one, being noncombustible, but most of the problems we have in construction have to do with human behaviour in buildings, and not the behaviour of the material. Any of the materials can be deployed successfully provided that they are detailed well for performance during construction and during the operational life of the building.


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