Cost model: Modular construction

Debates about modular construction and off-site manufacture are nothing new. But with more suppliers, better quality and greater awareness within the industry, could it finally achieve critical mass? Rob Mills of Aecom looks at the figures

01 / Background

It may have had different names over the years – prefabrication, modular building, design for manufacture & assembly, or off-site construction – but the idea of constructing buildings away from the site where they will eventually stand has had a long history, dating as far back as the 16th century.

Perhaps the most notable use of modular construction was during the UK’s post-war transitional period, which drove the need for homes and accelerated the search for a remedy to meet the country’s housing supply issues. The legacy of that building programme survives today, with many of those postwar homes still standing.

In that period, redundant arms factories were adapted to allow ex-servicemen and women to produce prefabricated housing in controlled factory conditions. This addressed the pre-existing housing supply problems and targeted employment issues – an idea that would surely resonate today.

Yet this was an opportunity missed. A relatively small number of houses were delivered in that post-war push. Technological innovations were not adopted, and so there was little change to the development of the industry.

Throughout the 1950s and early 1960s, demand for housing was a result of major slum clearances in inner cities. This set a precedent for government housing targets and saw the introduction of a large-panel residential systems on high-rise blocks, based on their successful application in Scandinavia.

The first use of such systems was a nine-block residential scheme commissioned by the London Borough of Newham using the Larsen-Nielsen system.

The subsequent progressive collapse of the structure in the Ronan Point block led to concerns around durability and structural performance. This only added to existing negative views of methods of prefabricated construction.

02 / Modular construction today

Fast-forward to present day, and things have changed. Performance is now more than credible. Quality is very good and viability assessments are encouraging when compared with a traditional approach.

Recent cost estimates suggest that, for medium-size schemes, modular construction could be 10-20% more cost effective than traditional methods. In the past, the modular approach has been seen to cost more and this can be attributed to designers trying to fashion a design for a traditional build into something that fits with the modular approach. This doesn’t always work and economical projects have been designed as modular from the outset.

The modular supply chain is still fairly immature. However, modular suppliers themselves have very strong supply chains and so can compete on price versus their traditional counterparts.

It is no surprise that interest in this approach is gaining traction. Contractors like the enhanced health and safety benefits gained from having less heavy and time-consuming construction work occurring on site and the logistical challenges eased, especially when faced with tight inner-city sites where storage or lay-down areas are sometimes non-existent. There is also less waste.

Project managers like the lean, on-site programmes that result from off-site manufacture. Developers often approve of the reduced finance charges and the revenue streams flowing from the earlier marketing and viewing of completed homes that result.

It all sounds too good to be true. But the usual hurdles around quality perception have been resolved and investors are placing cash and expectation into expanding factories.

Earlier drawdown of social housing grants by social registered landlords and earlier completion also make this approach more attractive, where applicable. With an earlier and faster completion, the additional costs associated with inflation and cost escalation is minimised by reduced contract duration.

The government is also interested in modular construction, notably in the potential for upping the bandwidth of housing delivery and closing the gap between supply and demand, which is at the forefront of the current industry debate. Add to that the prospect of mitigating the skills shortage across the industry as a whole – not to mention an estimated 20-30% decline in skilled labour over next decade due to migration and retirement – and it’s no wonder that modular construction’s time is coming.

It has taken a while to reach fruition. “Modern methods of construction” has been industry jargon for years, but until recently the percentage of schemes completed using these methods was stuck in the single figures.

Small-scale prefabrication and off-site construction – bathroom pods, MEP elements, facades and the like – have helped larger, traditional schemes make time and cost savings. But wholesale adoption has been slow, held back in part by negative perceptions around quality and product lifespan.

However, economies of scale seen as standard in manufacturing industries suddenly seem achievable. “Pure modular” projects, usually based around timber-framed structures, are now a reality, with their standardised components and on-site installation making for a safe, efficient and sustainable delivery.

The recently published government white paper (Fixing a broken housing market, February 2017), talks about how the UK needs 225,000-275,000 or even more homes to be built per year in order to keep up with population growth. This gives an indication as to what the modular market size could be.

Within the same report, the government announced that there will be £7.1bn made available for housing associations and non-profit making developers to assist with funding such projects, along with a further £2.3bn for private developers from the housing infrastructure fund. A broad range of developers are exploring this methodology, especially for multi-building sites where standardised, repetitive floor plates work well.

Planning conditions often dictate that affordable accommodation is built first when delivering a multi-block and mixed tenure site. Typically, with margins on lower-value sites being tight, the efficiencies that modular construction brings can improve viability assessments.

This is a developing sector within the industry, which does have challenges. There is still a limited supply chain, with around six sufficiently mature manufacturers able to support projects whose order books are full and a number of off-site manufacturers are keen to enter the arena.

The modular approach, however, is scalable. Factories are generally located in the Midlands and require very limited equipment and low utility capacity to set up. They assemble a kit of parts, rather than manufacturing anything themselves. A mixture of skilled and semi-skilled labour is required and quality is tightly controlled under factory conditions by supervisors.

At the moment very few large-scale projects have been executed, and these are typically low-rise, affordable apartments.

However, engineers are now exploring the possibilities of building to 20 storeys plus (logistics and access permitting) and producing private tenure blocks with efficient massing. More and higher-profile schemes are sure to come on line in the future.

03 / Design

To many designers, modular buildings always look like modular buildings and the stigma of post-war prefabs may take time to shake off. Architects and their design teams will need to develop better facade details for private-tenure modular schemes to become possible. Clearly, some premium architects have been nurturing this idea for a while now, and foresee the opportunity and growth.

Modules usually consist of a timber or lightweight steel frame held together using heavy-duty brackets. A sub floor and outer ceiling is then inserted using timber joists or a metal equivalent, with finishes applied afterwards. Internal partitions, linings, doors, wall, floor and ceiling finishes, kitchens, bathrooms, joinery and MEP are installed as per a traditional fit-out solution, albeit in the factory.

For MEP, modular construction is flexible. A common MEP strategy (made possible by the super-insulated nature of the modules) is to design the modules to utilise an all-electrical system for heating and hot water. However, where an energy centre is available, along with centralised low temperature hot water being piped to each plot, the overall energy strategy can be more resilient. The capacity that comes with an energy centre is flexible and usually attracts better energy rates from the provider.

External walls usually consist of: a breathable outer; weatherboard; insulation; punched windows; and necessary fire stopping. A solid exterior is then applied as a rainscreen system.

On-site amendments, such as double-height unbroken glazed facades should be avoided, to retain the structural integrity and air-tightness of each unit.

Those preferring tall towers may need to revise their expectations for now – 16 storeys is achievable and most developments to date have not exceeded 12 storeys. But as you go higher, the cost increases – simply because specialised mobile cranes are required for installation at height, for which there are only a few in Europe, as well as the requirements for structural integrity and services requirements, just as would be the case for a traditionally constructed tall building.

A tightly managed design phase is essential for the benefits of a modular project to be achieved. In particular, design will need to be fixed early to allow for lead-in times and mock-up approval. This also reduces risk across the rest of the project. When compared with a traditional build programme, modular methods can reduce the overall programme by around 25%.

04 / Procurement and construction

Moving to modular requires a change of approach, which is not always welcome. Clients may be more aligned to traditional procurement strategies and be unwilling or slow to adapt. Contractors and their professional teams may find it hard not to tinker with detailed designs.

This may be exacerbated by the fact that there are limited incentives for contractors, designers or clients to innovate. Modular construction is rarely specified in tender documents or planning conditions.

Many main contractors lack experience in modular schemes. This has, occasionally, meant that developers engage their supplier as the main contractor. While this makes sense from a product viewpoint, the supplier’s inexperience in tendering work packages or operating in non-local markets has caused problems. A main contractor model is still desirable, to ensure efficient management of the on-site work packages and delivery within programme.

In some instances, to further improve efficiency, suppliers can actually take their factory to site by setting up temporary structures in order to cut out the cost of haulage and risks that come with transporting large loads.

However, because the actual on-site construction is generally less complex, modular schemes may provide an opportunity to use tier two and three main contractors, or even larger trade contractors, to manage procurement and oversee assembly. The simplicity of modular schemes, once detailed, could make single-stage procurement more attractive to main contractors.

The number of highly-trained and experienced construction personnel required to deliver a modular scheme is fewer than on a traditional build, making the construction industry’s skills shortage less of a problem. With further potential labour shortages post-Brexit, this can be both an advantage and an opportunity to invest in a modular-savvy UK workforce.

The supply chain remains an issue. The small number of manufacturers means they need to be engaged early, and design needs to be fixed before starting the production line. Changes during manufacture will be costly and could result in the contractor losing their allocated manufacturing slot.

05 / Cost influencers

Repetition is key to a successful modular construction scheme. Project teams should think of what is required when using bathroom pods in a traditional project and scale that up.

The usual design and cost metrics apply, such as net-to-gross ratios. Square buildings are particularly good for modular construction, and five-plus apartments per core should be targeted.

Haulage is a cost to consider, especially on a high-volume project. An alternative is to build a factory on site, space and logistics permitting.

While not a cost driver, clients should expect to cash flow for an advance payment – typically 10-25% of the total contract value protected by a bond.

There is the potential for big cost savings using modular construction. Pure modular projects can be completed in half the time of traditional schemes once on site. Fewer packages need to be bought (usually ground works / substructure, cores, stairs, apartment modules, shell and core MEP works, builders’ works, lifts and potentially balconies, roof finishes and the like, depending on the project).

As a result, prelims are much lower – usually around 12% for a 160-apartment scheme. This combination of fewer packages and simpler design also means costs are easier to predict.

06 / Sustainability

There are some strong sustainability advantages associated with modular construction, which should be factored in to the design and client expectations. Typically, off-site assembly means reduced waste compared with traditional construction, and the materials used general have a higher recyclable content.

Energy consumption is lower – so much so that each module is almost at Passivhaus standards. Thermal values often outperform traditional schemes and, where there are no central hot water system keeping corridors and risers warm, there are fewer issues with overheating.

07 / Cost model

The cost model is based on the following:


The author would like to thank Barry Nugent, James Barton and Garry Burdett of Aecom for their help in preparing this article

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