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For as long as I can remember, there has been the promise of a construction revolution where prefabrication will make building cheaper, quicker and better quality. Whilst offsite construction is gaining headway, it still isn’t dominating how we thought it would.
Modern methods of construction: Prefabricated construction systems fall under modern methods of construction or MMC. MMC is a broad term covering more efficient methods of building buildings. A large number of products and systems fall under the MMC banner. All prefab can be regarded as MMC, but not all MMC is prefab. We can further divide modern methods of construction into systems designed for construction or manufacture.
Design for construction: These typically use site-built systems but undertake the site construction activities in a factory and then bring the product to the site. A large proportion of prefabrication in the UK occurs in a dry environment, allowing for a flexible workforce that uses readily-available construction skills and a factory base. Utilising site trades and tools that are generally readily available avoids the risk of specialisation and bespoke tooling.
Design for manufacture: This approach develops fabrication in the factory and optimises this production for the environment, tooling and opportunities of factory manufacture. This has the potential to be substantially more efficient in material and labour use than offsite construction. However, it requires a different level of training, tooling and investment. It also needs a throughput of products to support the infrastructure, which is hard to maintain and we’ve seen the loss of a number of businesses in this space over the last 24 months.
Different types of offsite prefabrication
You can broadly split the outputs from prefabrication for construction into three product scales: Component, panel (open or closed) and volumetric (whole building, internal pods, etc.).
Components can be non bespoke but can work together to make a bespoke product, panel approaches are generally likely to lead to a bespoke product and volumetric outputs are typically either bespoke at a large scale (lots of homes/pods) for a single client or non bespoke from a catalogue for small orders.
Waste
This is one of the biggest issues in construction. MMC can be lower carbon, even with the same construction type, as the process can better reduce waste (through design and fabrication methods) in the factory environment.
A factory environment also has greater potential to reuse or make more effective use of its waste. A busy, dirty site is much more likely to skip any waste rather than store it and feed it back into the construction at an appropriate time.
Quality
This doesn’t necessarily improve when using prefabricated construction systems, but all other things being equal, the conditions in the factory make higher quality easier to achieve. Operatives are likely to repeat tasks in warm and dry spaces, so materials stay dry and are more likely to be dimensionally stable and have clean joints. Tighter tolerances are more accessible to achieve.
Speed
It’s generally agreed that bespoke prefabrication is not much faster overall. It is faster once on site but requires a longer lead time. There is much more design, coordination and manufacture that happens before the building elements reach site. However, this shift of time – making site time shorter – can reduce site costs.
Speed can be improved if you use non-bespoke products, as they can be manufactured quicker or may already have been manufactured. A classic example of this is rooflights versus most windows. Rooflights are manufactured in a range of standard sizes and readily available to buy off the shelf. Most window systems are fabricated to bespoke sizes and need anywhere from two to 12 weeks’ notice.
Cost
This isn’t very clear cut. If done well, where the system arrives on site with minimal waiting time, there are definite savings in site costs. These could be anything from site management and labour to insurance, security and finance.
However, prefabricated building systems can be more expensive as you have ongoing factory overheads and staffing to cover. You also have to transport the finished products to the site. The return benefit is through higher-quality control and shorter site time. At best, though, we usually see these come out as cost parity with more traditional systems for bespoke self-build projects.
Cost savings can be made with non-bespoke products or on large sites with many repeating elements.
Transport is a significant issue in cost terms that is often overlooked. Most onsite construction uses materials that are easily sourced locally. Offsite prefabrication may involve manufacturing at quite some distance from the site. Volumetric systems, in particular – where you’re transporting air inside the volumes – can become expensive because of this. ‘Flying factories’ have been created for larger developments where the factory travels to the site to avoid this transport cost issue.
In the UK, though, the preference amongst most builders for masonry cavity wall construction still makes this the cheapest method in most instances.
Health and safety
This is one of the most significant benefits of factory or prefabricated construction. While construction has improved considerably, it is still quite a dangerous work sector. In a factory environment, conditions can be much easier to make safe. Working at height can be reduced, proper dust extraction can be installed, fixed workshop tools rather than hand tools and even better staff welfare facilities make for much better working conditions. For precisely these reasons, prefabrication also suits our increasingly ageing construction workforce.
Conclusion
The prefabrication, or offsite construction, industry still plays a relatively small part in the construction of our homes, especially in the self-build sector. A big reason for this is how embedded traditional masonry construction is with smaller builders. Whilst not necessarily cheaper, prefabrication gives the opportunity to create better quality and lower-carbon buildings.
