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Modern methods of construction (MMC) - A long and ongoing journey!

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In this article, Stuart Matthews (FIStructE) gives his views on the history and development of MMC, types of construction, their advantages and disadvantages and what the future may hold.

1. Introduction

References to modern methods of construction (MMC) appeared following the publication of the Rethinking Construction / the Egan Report (Egan, 1998) which put forward recommendations on how to modernise the then UK construction industry by focussing on (amongst others matters) standardisation, prefabrication, preassembly and off-site manufacture (OSM). Joint UK Government and construction industry initiatives to implement the recommendations of the Egan Report included the formation of the Movement for Innovation (M4I) and the Construction Best Practice Programme (CBBP). However, prior to that time, UK government initiatives targeting the building industry had used other terminology, with the focus generally being upon what were then termed ‘non-traditional’ methods of construction and were principally focussed on housing. As much of the recent debate upon MMC has been in the context of housing, this viewpoint article examines the issues mainly from a housing perspective.

Whilst M4I and CBBP and other bodies did sterling work, the process of getting to where we are today with MMC has actually been a long and uneven journey, which started much earlier than many may realise. It is over a century since the first government initiatives in the 1920s promoting the then new ‘non-traditional’ methods of house construction, as a response to the national housing crisis following World War I. The development journey of ‘non-traditional’ housing systems in the UK has included changes in terminology as they became part of various government initiatives, such as the ‘Industrialised Building Drive’ of the 1964-1970 Labour government. Numerous political and technical challenges have been encountered on the journey.

1954 was a high-point in overall UK housing production, with just under 350,000 dwellings (of all types) completed. After that output dropped steadily, before stabilising at around 300,000 in 1960. However, the need for new housing remained as pressing as ever and the vision was that ‘industrialised building’ could fill the gap between the desired political target of 500,000 homes a year, and the then output. 1970 saw the peak of non-traditional construction, with 55,701 dwellings reportedly completed. After that, as the then national economic situation worsened, non-traditional production tailed off to less than half that number in 1975. Without considering the later social consequences which arose from the then focus upon high- and medium-rise flats, it is clear that the UK government’s industrialised building drive did make a significant contribution to both the size and the physical improvement of the nation’s post-war housing stock. However, the volume of output could not be grown enough to achieve the targeted 500,000 homes a year.

The above figures for home construction are appreciably higher than the achievements of more recent years. A total of 209,460 homes were built in 1979. However, completions dropped to about 106,000 in 2010, the lowest total between 1979 and 2019, but increased to 178,800 homes in 2019. Again, these levels of construction did not meet the demand for new homes. A major influence upon this situation was the UK and local government’s decision to greatly reduce its direct role in the building of social housing. It is understood that the private sector was expected to expand its output, but it has not been able to achieve that goal. The outcome is the ongoing shortfall in the number of new homes created. It is argued (De’Ath & Farmer, 2020) that, in respect of the development of MMC in the UK, that some form of government stimulus or intervention could be necessary to create an ongoing social new-build housing market to facilitate the commercial response necessary to address the established UK social housing need.

MMC take-up is also occurring in the infrastructure arena (eg prefabrication / OSM for bridge and non-domestic building construction), with recent press reports highlighting a number of these applications, such as forms of segmental construction. Generally, although the term MMC is not employed in civil engineering, reference may be made to the use of offsite techniques. However, many of these techniques are reportedly now well embedded in contemporary civil engineering design and construction practice. The uptake of forms of MMC resulting in the evolution of ‘traditional’ forms of house / dwelling construction is briefly discussed in Section 6, which provides examples of some of the positive spin-offs from MMC.

But where are we currently with MMC and where are we seeking to go in the future? Let us first ‘look back’ and ‘look now’, before we attempt to ‘look forward’.
 

2. Types of MMC recognised at different times – An evolving situation

The types of construction and activities considered to be MMC has broadened over the years. Early UK government housing initiatives (1920s to 1970s) initially focused upon off-site / factory and related manufacturing, with innovative site-based methods being included later (from circa 2003, refer Table 1).

Table 1: The Housing Corporation construction classification system for dwellings, 2003

1. Off-site manufactured – Volumetric construction
- Volumetric construction units
- Bathroom and kitchen pods

2. Off-site manufactured – Panellised construction systems: Typically - steel, timber and concrete
- Open panels
- Closed panels
- Concrete panels
- Composite panels
- Structural insulated panels (SIPs)
- Infill panels
- Curtain walling

3. Off-site manufactured – Hybrid construction: Volumetric units integrated with panellised systems
4. Off-site manufactured – Sub-assemblies and components
- Pre-fabricated foundations
- Floor cassettes
- Roof cassettes
- Pre-assembled roof structure
- Pre-fabricated dormers
- Wiring looms
- Pre-fabricated plumbing
- Timber I-beams
- Metal web joists

5. Site-based modern methods of construction
- Tunnelform
- Insulating concrete formwork
- Aircrete (aerated concrete) products, including aircrete panels/planks for walls, roof and floors


In March 2019 the Ministry of Housing, Communities & Local Government (MHCLG) Joint Industry Working Group on MMC developed a comprehensive “Definition Framework for MMC Categories”, which identified the seven MMC categories listed in Table 2 below. These include MMC systems covering a range of approaches to construction including off-site, near-site and on-site pre-manufacturing, as well as site-process improvement gains and technology led applications. This updated definition of MMC encompasses a broader range of approaches, extending beyond OSM and those listed in Table 1 above, and provides a home for other technologies promoting change in the processes used in construction.

Table 2: MHCLG Joint Industry Working Group on MMC - Definition Framework for MMC Categories 2019

Category 1 – Pre-Manufacturing - 3D primary structural systems
Category 2 – Pre-Manufacturing - 2D primary structural systems
Category 3 – Pre-Manufacturing - Non systemised structural components
Category 4 – Pre-Manufacturing - Additive Manufacturing
Category 5 – Pre-Manufacturing – Non-structural assemblies and sub-assemblies

Off-site and near site pre-manufacturing led approaches

 
Category 6 – Traditional building product led site labour reduction / productivity gains
Category 7 – Site process led labour reduction / productivity & assurance improvements
Site-based process improvement led approaches

Note: There are an extensive series of sub-categories for the above classes which are described in the document MHCLG Joint Industry Working Group on MMC - Definition Framework, March 2019
 

3. Some advantages and limitations of MMC / OSM

When properly integrated early in the design process, OSM components and services assemblies can have a major positive impact upon the construction process. This can lead to improved performance by way of:

  • Reduced time for on-site build and enhanced quality
  • Precision of manufacture
  • Standard of finish
  • Reduced waste
  • Less impact upon the neighbourhood during site works
  • etc

Critically these techniques improve predictability of key aspects of project delivery (eg timing, quality, cost, etc). OSM increases the proportion of the construction value which can be delivered under better control in a factory environment.

While prefabricated assemblies may initially cost more, reduced on-site assembly times and increased control over on-site processes should reduce risks and, in many cases, the overall project costs should be no more than those of conventionally constructed projects. Where there is a repeat aspect to the project or where there is a large volume of prefabricated components, overall cost may be appreciably reduced. OSM usually requires a minimum number of units to be put through the factory in one batch (40 dwelling units has been quoted by some as the minimum viable number).

An oft-quoted disadvantage of OSM is that the design needs to be finalised in much more detail well in advance of the site-build, so that material and components can be ordered and stocked at the factory ready for use. However, is this an illusionary problem? If the total project period for design and build is taken into account, OSM requires a much shorter period for the site-build phase than conventional construction. Would this effectively largely offset the constraint of the earlier project ‘design freeze’ date?

Another significant issue is reliance upon one supplier and concern about the risk that they could go bust, which typically causes considerable problems, delay and additional expense upon affected projects.

Future maintenance and functional adaptability are also potential concerns for OSM buildings, presumably scenarios for both circumstances could be addressed in design. Many buildings have great longevity and some undergo significant adaptation during their lifetime. As it is recognised that holistically designed and manufactured MMC components can provide enhanced air tightness, thermal efficiency and build quality, etc, inevitably some specialised components and materials may be used in the manufacturing process. While it is accepted that design involves making compromises to achieve a satisfactory balance between conflicting performance and other requirements, will such materials and replacement / alternative units be available decades after building manufacture? However, such problems could also arise with unique / specialised elements employed in evolved ‘traditional’ and other forms of construction (refer Section 6), so making the issue more generic. Repair / replacement of such elements could be considered during design. Perhaps future advances in 3D printing techniques could allow suitable replacement units to be created.

Another issue which could become increasingly relevant is that many of the composite materials used in MMC projects are likely to be difficult to reuse / recycle at the end-of-the-life of the building. This would make a negative contribution to a life-cycle analysis (LCA) of the environmental impacts and benefits of a project. Conversely, materials used in conventional construction will usually be more easily recycled. However, the issue of reuse / recycling at the end-of-the-life of the building could be addressed if OSM components used in MMC projects are designed for deconstruction and re-use as components.

OSM processes alone do not guarantee quality. Good conceptual and detailed design, appropriate choice of system, careful construction detailing etc in the early stages of a project can mitigate many of the risks.

One fundamental problem of mass production always applies – if a mistake is made, it will generally be replicated many times. The error is then both ubiquitous and expensive to remedy. Quality assurance checks are therefore key, especially those by a third party. In-depth reviews and checks must occur in design, before products leave the factory, once they are installed on site and during the remaining site operations. Oversight of on-site operations is essential to deliver high-performing, long-lasting structures.
 

4 Contemporary context and developments - Recent MMC projects and advances

Barriers to the wider uptake of manufactured housing have historically been associated with risks (real and hypothetical) perceived by the different players involved in the delivery of housing (eg financial institutions, mortgage providers, warranty providers, insurance companies, registered social landlords, designers, surveyors, house builders, building control, manufacturers of dwellings, etc).

However, De’Ath and Farmer (2020) – who are acknowledged to be passionate advocates for MMC - observe that following the 2008-2009 recession, which caused consolidation and change in the UK housing sector, new companies with different business models had entered the sector and grown quickly. These included manufacturers such as Berkeley Modular, Ilke, Legal & General Modular Homes (L&GMH), Swan Nu Living, TopHat and Urban Splash who had invested in factories, products and system development with the ambition to deliver thousands of homes a year. These companies focused on low rise modular homes, predominately houses, with potential to deliver apartments in medium rise blocks. De’Ath and Farmer also noted that medium to high rise volumetric modular delivery had been honed by specialist manufacturers, such as Caledonian, Elements and Vision Modular Systems.

These developments were coupled with the steady growth of existing housing associations and resurgent local authorities who had begun building once again, investor backed ‘build to rent’ developers who had quickly established themselves, delivering high quality homes at scale. For example, Greystar who set up its development team in 2015 had, by late summer 2020, delivered over 1,250 homes and established a pipeline of over 2,000 more. Private housing associations also emerged over the same period. Institutional investors also entered the sector, with Legal and General announcing in 2019 its intention to buy and build 3,000 homes a year within four years.

Thus, in that period various new players, which are not linked to existing homebuilders or construction companies, had entered the housing market with the objective of taking forward modular MMC. An upbeat view was put forward that the existing homebuilders and construction companies could be left to build traditionally or combine it with MMC. However, several of the large companies operating in the housing market, including Urban Splash and Legal and General (L&G), subsequently went into administration / ceased trading demonstrating the considerable difficulties experienced operating in the volatile UK housing sector.

Several examples of contemporary volumetric / panellised / hybrid MMC are given in the Build homes - Build jobs - Build innovation Report by De’Ath and Farmer, September 2020. In their discussion De’Ath and Farmer focus upon the Category 1 - Volumetric segment of the market place as they indicate that this is the single MMC approach most aligned to creating true additional capacity in the market. Category 1 MMC modular means the construction of fully finished modules, or structural boxes, that include fitted kitchens and bathrooms, etc. These are brought to site and assembled either with or without external fabric materials. As the Farmer Review (2016) notes, this form of MMC enables over 70% of the home’s construction value to be manufactured. This is termed its pre-manufactured value [PMV] and compares to traditional build’s PMV of circa 40%.

A recent example of the above-mentioned developments is Greystar’s George Street build-to-rent development in Croydon, south London (Figure 1). This comprises 546 homes situated in 44 and 38-storey blocks, reportedly achieving a very high-quality build which demonstrates the technical excellence achievable in modular delivery. At the time of writing this is the tallest modular residential scheme in the world. Its 1500 modules were assembled and installed in just 31 months by Tide Construction in conjunction with its manufacturing company Vision Modular Systems.

Not all commentators share the same positive view of MMC. Stuart Green (2019) in his article Modern methods of construction: Unintended consequences investigates whether MMC can resolve the UK construction industry's challenges of a diminishing workforce and low productivity, whilst also delivering a more energy efficient built environment. He explores some negative consequences and considers how an alternative conceptualisation might help realise better outcomes.

Figure 1: George Street, Croydon, UK. Credit: Tide Constuction
 

5 Looking to the future – Some contemporary examples / recent case histories

Two examples are given below of future homes employing contemporary and future looking / experimental MMC and associated technologies for sustainable living.

Figure 2: Midland Heart’s ‘Eco Drive’ Development, Handsworth, Birmingham. Credit: Midland Heart Housing Association

 

  • The Barratt ZED-House - Exceeding the Future Homes Standard (October 2021): This is a zero-carbon demonstration concept home, built on the University of Salford’s main campus, which is intended to showcase the future of the sustainable living in the UK and whose performance is intended to go substantially beyond the requirements of the Future Homes Standard. (See Figure 5)

Figure 3: The Barratt ZED-House. Credit: Barratt Developments

While the MMC and related construction and services technologies are extremely important to meet future performance requirements and aspirations, perhaps the most critical aspects to future achievements are the contractual and organisational relationships and collaborations. For example, the Zed House, built using MMC, was part-funded by the UK government and was developed in partnership with over 40 leading organisations from across the housebuilding, sustainability and technology sectors. These collaborations contributed to broadening knowledge and enabled the lessons learnt to be shared across the industry. Potentially lessons might also be learned from the design and construction practices adopted in other countries / parts of the world.
 

6 The evolution of ‘traditional’ forms of construction - Positive spin-offs from MMC

While the appearance of UK homes has remained largely unchanged - attributed by some commentators to be the result of restrictive planning policies - there are many examples of OSM / ‘non-traditional’ / MMC methods which have over time been incorporated into the mainstream ‘traditional’ construction process (eg timber roof trusses, stairs, doors and window sets, precast beam and block flooring, composite cladding panels, prefabricated foundation systems and insulated walling panels, to list just a few), as well as building services pods (such as air-conditioning or refrigeration packs). Thus, contemporary ‘traditional’ construction now incorporates many forms of construction which were once seen as being MMC. These have now become well established as elements of ‘standard’ systems used in contemporary construction.

NHBC Guide NF85 describes some of the many technical advances incorporated into traditional methods of house construction over the last century or so. This process has resulted in a significant improvement in the performances achieved, especially over the last two decades. The NF85 report makes clear that the contemporary ‘traditional’ construction of new homes is quite different from what formed ‘traditional’ construction in earlier years. It is expected that this gradual evolution of ‘traditional’ (conventional) construction, with associated improvements in performance, will continue in the future.
 

7 Concluding remarks

While MMC methods have not yet become a significant direct challenge to mainstream ‘traditional’ forms of home construction, the recent developments noted above suggest that MMC / OSM had gained more momentum over recent years. This led to the impression that the UK was approaching the time when factory-made homes could realise their envisaged potential to help resolve the UK’s ongoing housing crisis and contribute to tackling the climate emergency. However, the strength of this perception has been diminished recently after several large commercial companies operating in the housing market, including major players Urban Splash and Legal and General, have gone into administration, demonstrating the considerable difficulties of operating in the volatile UK housing sector.

In recognition of the failure to meet the demand for new homes over recent years - the UK government has a recently stated objective to deliver 300,000 new homes a year by 2025 - De’Ath and Farmer (2020) in their “Build homes - Build jobs - Build innovation” Report have proposed a programme using MMC to build an additional 75,000 high-quality manufactured homes a year by 2030.

Has MMC / OSM finally reached a tipping point after all these years?

Will MMC / OSM supplant mainstream ‘traditional’ forms of home construction? Pros and Cons?

How might MMC / OSM impact the wider construction industry - is this just another evolutionary step?

The UK Government has indicated that it will use its weight as a major construction client to transform and modernise the construction industry and that it will facilitate the adoption of MMC / OSM by standardising components, designs and interfaces. How can the prospects for success be maximised?

Nevertheless, there are always ‘howevers’ and ‘buts’. Are there factors which might impede such progress?

In their “Build homes - Build jobs - Build innovation” Report, De’Ath and Farmer seek to identify and discuss the need to overcome previous market failure.  They identify ten key factors they consider to be responsible for ‘market failure’ and which “go to the root of why modular housing delivery has not matured in the UK and indeed in many other countries”. Many good points are made (see Figure 4).

Figure 4: Ten critical points of market failure. Credit: Cast Consultancy

Could there be issues which would adversely impact upon the take-up of MMC in the owner-occupier homes market? Financing and the ability to get a mortgage are critical factors.  Owner-occupier’s may need reassurance that their home / their largest financial asset will be durable. Also, could the ‘shadow’ of the Grenfell Tower fire disaster potentially raise concerns about the safety of modern construction technologies more generally. Could these have adverse connotations for the wider take-up of MMC?
 

8 References and further reading

 
 

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