Auditing off site manufacturing

The onus is on building control inspectors to find ways of auditing housing that is manufactured off site


  • John Miles

07 September 2019


In July 2013, the coalition government published the policy paper Construction 2025. This set out a vision of the industry's future according to three strategic priorities: smart construction and digital design; sustainable construction; and improved trade performance.

For this vision to be realised, I believe smarter and more effective methods of construction such as off-site manufacturing must be adopted across the industry. This view has been supported by a number of government and industry publications, including the 2018 House of Lords Science and Technology Select Committee report Off-site manufacture for construction: Building for change; last year's Proposal for a New Approach to Building consultation from the Infrastructure and Projects Authority; and the 2016 KPMG report Smart construction: How offsite manufacturing can transform our industry.

This April, a specialist subgroup of the Ministry of Housing, Communities & Local Government cross-industry working group on modern methods of construction (MMC) published a definition framework,  a project that was led by the chair of the wider MMC working group Mark Farmer of Cast Consultancy, and supported by representatives of Buildoffsite, Homes England, NHBC and RICS.

The intention is that this framework will regularise and refine the term MMC by defining the spectrum of innovative construction techniques being applied in the residential market, both now and in the future. 

RICS plans in due course to integrate this definition into supporting guidance for its new home survey standard, new-build valuation guidance and the second edition of the International Construction Measurement Standards.

Modern methods are less disruptive and offer more control, as homes are built in a matter of days and assembled on site instead of being constructed over a period of months. Productivity is also improved because there are no delays due to bad weather, materials shortages or worker error; buildings can be manufactured in a factory, like aircraft or cars, and transported to their sites.

But while this represents a step forward in housebuilding, it also presents a challenge when it comes to regulatory compliance in some key areas. Every building is assessed against the regulatory requirements and against the guidance used to demonstrate compliance at design stage and during construction, and this typically covers the following eight inspection stages:
  • foundations
  • drainage
  • damp-proof course (DPC) or damp-proof membrane
  • first floor
  • roof
  • first fix
  • second fix
  • final.

Off-site construction does not follow this traditional approach, and systems are needed to ensure the regulator is satisfied that the functional requirements have been met both in the factory and on site. These systems are critical, because the regulator on site may not have been able to inspect the structure or key components of a building that is substantially complete when it arrives on site.

Modular or off-site construction relies on a production line, and building control will normally undertake a factory audit to inspect the manufacturing process while it is under way. This allows the inspector to assess units at various stages during construction, checking their structure, fire resistance, cavity barriers and insulation.

It also enables them to review the factory quality-assurance process and the records kept to ensure each unit is checked at critical points during manufacture, confirming that Building Regulation 7 on the standard of work is being met and that appropriate materials are being used.

"Modern methods of construction are less disruptive and offer more control as homes are built in a matter of days and assembled on site instead of being constructed over a period of months"

Traditional methods of construction are a lot more forgiving in some respects, as they enable design and material changes throughout the construction process. Off-site construction and manufacturing does not allow for this, and any changes required to ensure compliance cannot be easily implemented on site during the brief construction time. This makes the factory audit process all the more important.

Building control bodies can take all this information and issue a type or system approval for anything from a foundation to an entire off-site manufacturing process, similar to the kind that major national housebuilders would receive for standard house types that can be adopted on numerous sites without having to start afresh each time. This audit and approvals process should provide a framework where the building control body is able to sign off schemes without having inspected every unit being manufactured before its assembly on site.

The eight inspection stages identified above can typically be reduced to four, namely foundations, drainage, construction and completion. However, regulators need to be satisfied before sign-off that there is sufficient evidence from the manufacturing and subsequent site inspections to indicate that the regulatory requirements have been met. Fewer stages do not equate to less inspection time needed on site.

High-profile cases

The reputation of modular and off-site construction has been dented by several high-profile cases where things have gone wrong. In 1983, ITV's World in Action series aired a damning documentary on timber-frame housebuilding, highlighting failures in new homes due to a lack of timber treatment, poor membranes, ventilation and subsequent rot.

Then there was the CASPAR project – City-centre Apartments for Single People at Affordable Rents – completed in Leeds in 2000 and hailed as a glimpse into the future of modular construction. By 2005, however, it had been condemned and the occupants rehoused.

These two projects used different off-site construction techniques, but were both affected by poor sequencing, manufacturing and site supervision. The timber-frame homes for instance were built to an unreasonable timescale and without a full understanding of the detailing around vapour control, DPCs and interstitial condensation in timber frames, all of which led to the issues encountered on site.

Meanwhile, the CASPAR project was constructed from modules stacked on top of each other to form the apartment building, but during construction the build sequence was disrupted and what appeared to be identical units were assembled incorrectly. Ultimately, this fault resulted in structural elements that were meant for the top floor ending up in the wrong place, with the building structure failing and being put at risk of collapse.

Such failures show that building inspectors need to understand the principles and sequencing of the modular units they are inspecting. Checks should be made on all critical connections, and inspections sequenced so that vapour barriers and joints in the construction can be examined, which is key to ensuring that there will be no water ingress or cold bridging once the units are completed. Further checks should be made on items such as service connections, drainage connections, and the positioning of any cavity arriers and fire-stopping installed after construction.

Efforts to develop construction into more of a manufacturing-based industry are supported by the Hackitt report, Building a Safer Future: Independent Review of Building Regulations and Fire Safety . One anticipated outcome of this report is that the changes it sets out will lead to greater systems being constructed off site, and less elemental construction on site.

There has, however, been some concern expressed by insurers about the dangers of certain materials used in off-site construction being less resilient to fire, water and physical damage. However, the government is of the view that robust regulations and careful consideration at design stage should be able to mitigate any risks posed.

Competency evidence

This process and the assessment of the design, risks and site inspections involving modular construction count as evidence towards the following competencies on the Building Control pathway of the APC.

Building control inspections: achieving this competency involves carrying out site inspections to ensure building work meets relevant performance requirements, and having the ability to observe, assess and take action against contraventions on site. It also entails specialist knowledge of modular construction, understanding the risks associated with it, and sharing this knowledge with the design and construction teams during the manufacturing and building phases.

Building pathology: this requires understanding of defects analysis and being able to explain building fabric failure. An ability to undertake preventative maintenance inspections is also needed to reduce the likelihood of defects occurring.

Construction technology and environmental services: this entails an understanding of design and construction, and being aware of construction solutions to problems.

Works progress and quality management: you should know about construction technology techniques and their relevance on site, good-quality work being vital in ensuring long-term functionality.

While carrying out inspections, the surveyor should be able to identify the types of modular construction being adopted, and provide suitable notes, measurements and photographs in site reports. The detail should be reviewed appropriately using relevant guidance such as Approved Document A (Structure), Approved Document B (Fire Safety) and other guidance including the NHBC technical standards, to determine whether the structure is stable.

From there, the surveyor would be able to take suitable action, for example advising on remedial measures or the need for further review by a structural engineer. Where a candidate is identifying and resolving such site defects, it should be possible to record their experience under at least Level 2, and potentially Level 3 where advice has been given for the competency of Building control inspections, as modular construction can be considered a speciality.

John Miles MRICS is a technical and business development manager at Assent Building Control johnmiles@assentbc.co.uk

Related competencies include: Building control inspections, Construction technology and environmental services, Works progress and quality management

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