When the Second World War ended, the UK faced a severe housing shortage: houses were needed immediately, but there wasn't time to build them using typical construction methods.
In response, the then Ministry of Works built thousands of prefabricated, or prefab, homes. One type could be built in aircraft factories, and was known as the AIROH house for the Aircraft Industries Research Organisation on Housing. The 156,623 AIROH prefab homes constructed proved successful, with some such as the Excalibur Estate in Catford, south London, remaining in use until very recently.
Sadly, other post-war experiments in prefabrication and alternative methods of construction left a legacy of unmarketable and unmortgageable properties, which has cast a shadow over subsequent attempts to devise innovative building methods.
Today, some parts of the UK face a housing shortage on a massive scale. If the government is to achieve its stated goal of building 300,000 homes a year and satisfy not just time pressures but the need for quality, economies of scale and, more pressingly, zero carbon, then new, efficient and sustainable methods of construction will be required. Of course, modern methods of construction (MMC) are not simply about building homes; increasingly, commercial developments are benefiting from the drive for innovation.
The benefits of MMC have yet to be fully realised, however. There are significant barriers to adoption, and until these are addressed the supply of suitable homes in particular is likely to be particularly challenging.
The Commons Communities and Local Government Select Committee report Capacity in the homebuilding industry of 2017 defined MMC as a 'a collective term for a wide range of non-traditional building systems. These include modular construction where units are fully fitted out off site, panelised systems (such as timber or light-steel frames), site-based MMC such as thin-joint blockwork and sub-assemblies and components (such as prefabricated chimneys, porches etc.)'.
The Construction Industry Council's Offsite Housing Review of 2013 also described a number of potential advantages to MMC.
A 2005 report from the National Audit Office on MMC had already suggested that it should be possible to build up to four times as many homes with the same on-site labour using MMC, and that site construction time could be reduced by more than half.
In a 2019 report, the Commons Housing, Communities and Local Government Select Committee estimated that construction programme time could be reduced by 20–60% by using MMC, and went on to predict a 20–40% reduction in costs as well as the potential for lower whole-life cost.
Innovation entails a departure from the tried and tested, but the construction industry can be reluctant to change or modernise. It tends to revert to what it has always done.
In an attempt to counter possible negative impressions of MMC, the report also recommended that better warranty and assurance processes and products are needed. Although the Buildoffsite Property Assurance Scheme was set up in 2013, to demonstrate the quality and durability of property built using MMC by conducting risk-based evaluations, this is not a warranty scheme and does not provide funding for remedial works should these be necessary. The adoption of an MMC warranty scheme by developers – or what the National House Building Council terms a warranty assessment protocol – would therefore help to remove some of the obstacles to wider use of such methods.
The Hackitt review was critical of the Approved Documents and the difficulty of integrating the requirements into a single compliant specification – something that is relevant when considering the adoption of MMC.
The review's concerns were echoed in comments from firefighters following delays in the completion of a twin-tower, 38- and 44-storey modular residential development in East Croydon recently. A London Fire Brigade spokesperson was quoted as saying: 'As we increasingly see boundaries pushed in terms of developments built using MMC, we have found that there is a need across the industry for more research on how this affects building and fire safety.
'We are concerned that buildings are being built using more unusual methods without a complete understanding of their performance in fire, so we would encourage developers proposing new construction approaches to engage fire and rescue services at an early stage in the design process.'
The use of MMC does not excuse the designer for taking liberties with building physics or good practice. Construction principles that have been established over many years are ignored at the designer's peril. Indeed, the more that buildings are engineered the more dangerous they will be if not constructed properly. The following case study illustrates the point.
The development involved prefabricated houses shipped from a supplier in eastern Europe, highly engineered products that all had European Technical Approvals. The roof cassettes were insulated with around 400mm of mineral wool between the joists and around 300mm of polyurethane foam insulation on top of the oriented-strand board (OSB) decking.
The form of construction was therefore neither a true cold roof nor a warm roof; it was a hybrid and so did not follow conventional UK practice, which generally cautions against the use of unventilated cold flat roofs because of the condensation risk they present. Opening up indeed revealed that there was a serious moisture problem from entrapped water and condensation and that the OSB had decayed, with the only feasible repair option to reconstruct the roofs down to joist level – all this in a property less than a year old. The message is simple: avoid unventilated cold roofs, and ensure that any early leaks are dried out before the building is enclosed.
Another example involved a series of modular buildings constructed on simple concrete foundations. Airbricks that were required to provide ventilation to the ground floor void – and so avoid the risk of condensation – were located in the external plinth wall, but many of these were obstructed by terraces and thus ineffective. The gravel oversite contained no waterproofing measures. Condensation in the floor voids led to timber decay and the partial collapse of some floors.
The provision of suitable ventilation and damp-proofing has been part of mainstream building for many years, so why ignore it when installing a prefab?
'The use of MMC does not excuse the designer for taking liberties with building physics or good practice'
The above examples illustrate some of the challenges to widespread adoption of MMC; prejudice can be fuelled by performance failures and defects that are costly to repair. One can innovate, but must not lose sight of the way materials perform. However, with care, MMC can play an important role in addressing the housing crisis.
The key thing to remember is that although some MMC may be unfamiliar, the physical forces that act on them are all well known. As building surveyors, we therefore have to apply our knowledge of materials and their performance characteristics to help work out what we don't know.