When should construction go off-site and when should it not?

For pretty much forever, on-site or in-situ construction has been the default way of building. However with accelerating interest in Design for Manufacture and Assembly (DfMA) and Modern Methods of Construction (MMC), it sometimes seems that on-site construction is falling out of favour.

In 2019, the UK’s Ministry of Housing, Communities and Local Government (MHCLG) – as it was then called – published an MMC Definition Framework (see my earlier article, What does DfMA stand for?). This framework proposes Pre-Manufactured Value (PMV) as a metric for assessing the scale of MMC adoption on a project. PMV is calculated as the percentage of cost that is derived before any site works and strongly encourages maximising the proportion of off-site works.

As a result, many parts of the construction industry are focusing more and more on off-site prefabrication in pursuit of benefits including lower cost, reduced delivery timescales, higher quality, improved health and safety, less waste and more productive site labour.

But is off-site always the best way to achieve these benefits? Construction sites have been around since the dawn of civilisation. Do we really want to walk away from all that learning, and start moving as much of the construction process as possible into distant sheds?

Is on-site construction really all that bad?

Construction sites could be defined as locations where finished assets are built. They can be messy, congested, dirty and chaotic places. But they don’t have to be. If the construction process can be transformed into the streamlined and predictable assembly of pre-manufactured parts, combined with on-site construction processes that are carefully managed to add the maximum value, much greater productivity can result.

To give a slightly left-field example, consider a circus tent. Typically, these large structures are put up overnight by a small team of trained operatives. In a budget-conscious industry, every hour counts, so assembly is planned to be as quick as possible, and disassembly is just as quick. It would be even quicker if the tent was pre-erected, of course, but this doesn’t make sense from a transportation and logistics perspective. Instead, they use a component kit-of-parts that is easily handled and takes up very little space during transportation. A permanent building may have different drivers, but lessons can still be learned from highly efficient on-site works such as the circus tent.

One aim of a fully considered DfMA strategy is to enable smooth running of the construction site. To create a well-orchestrated assembly line with productive workers carrying out pre-determined, standardised and well-understood tasks in predictable timeframes.

In short, we want construction sites to be more like factories.

Off-site and modular construction: are factories really all that great?

Prefabrication in factories (off-site construction) is often thought of as a panacea, a sure-fire way for construction sites to achieve greater productivity. However, factories can also be run inefficiently, and if traditional construction methods are simply shifted into a factory setting, the benefits of MMC can be diluted or lost.

In some cases, building off-site in a factory may even be less efficient than on-site construction. For example, prefabricated 3D modules (used in modular construction) involve many additional costs compared to conventional build. These include transportation (a pre-fabricated room is mostly air, after all) and heavy plant for lifting modules into place. Unless modules are fabricated on a just-in time basis they also have to be stored which costs money, especially if that storage needs to be sheltered from the elements. 3D modules also occupy large amounts of factory floor space and therefore absorb a high proportion of factory overheads.

If these additional modular construction costs can be offset by large improvements in construction site efficiency, for example by relocating wet trades or complex specialist trades away from the site, a 3D module might make sense. But, with modular construction, it’s often the case that prefabricating comparatively simple parts of a building as 3D modules adds cost and complexity, especially if the required trades need to be present on-site anyway.

The problems found in modular construction are only compounded by inefficient factory working. The cost of any prefabricated component (indeed, any component of any building) can be divided into materials and labour. If we ignore the cost of the labour that has gone into making the component, we only have material costs left, resulting in limited opportunities to add value. Manufacturers have understood this for decades and spent a great deal of effort developing highly productive assembly routines that enabled the mass production, automation and commoditisation that fuelled the consumer age.

Too often factories are treated as ‘construction sites in a shed' producing bespoke, custom components with overlapping trades and poor works sequencing, causing reduced value and the same inefficiencies that are often found on construction sites. We want the factories that produce components for the construction industry to be more like the best factories making consumer goods; highly efficient, controlled and focused on achieving the highest throughput for the lowest cost, without compromising on quality.

In short, we want factories to be less like construction sites.

Construction Platforms: our MMC approach to achieving the best mix of on-site and off-site construction

At Bryden Wood, we have over 25 years’ experience applying all types of MMC solutions to a huge range of projects. Our industry-leading approach to MMC has developed over the years to consider ways to maximise the productivity of both off-site and on-site construction works to achieve better project outcomes for our clients.

Much of our thinking has been published on our website, particularly in our series of books about Construction Platforms, including the recently updated Delivery Platforms for Government Assets. The concept of a platform approach to DfMA (P-DfMA) has been adopted by the UK Government, particularly the Infrastructure and Projects Authority (IPA), who have said they will mandate construction platforms by 2023 for all social infrastructure.

The pre-manufactured components of platform construction (P-DfMA) systems are specifically designed to be standardised and mass-produced by a wide supply chain, like the mechanical components of a car, to deliver the greatest possible factory efficiency and economies of scale.

The components can be easily transported and moved, and they fit together quickly and simply on site with the least amount of handling. Platform construction systems are designed to be flexible and customisable to avoid limiting design freedom and allow massing, form and aesthetic treatments to be considered on a plot-specific basis.

Driving value with MMC and P-DfMA

Construction platforms are an example of an approach to DfMA that relies on both off-site and on-site construction works, bringing the best aspects of both. The division of fabrication and assembly tasks has been carefully considered to maximise the efficiency of both factory prefabrication and on-site assembly. This is a key aspect of Bryden Wood’s Design to Value approach: considering how the benefits promised by MMC can be maximised to deliver better outcomes for clients, contractors, end users and society as a whole.


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