CPD 5 2014: Risks associated with modern methods of construction

Introduction

Modern methods of construction (MMC) are now used across the construction industry, and particularly to build new homes. MMC offer potential savings in time and materials, and can provide higher standards of quality than conventional construction methods. Off-site assembly means quicker erection on site, and the ability to achieve a weather-tight construction within a shorter period of time.

While off-site construction is generally well-managed and auditable, there are a number of additional factors that those using MMC must bear in mind. With traditional building methods, warranty providers have developed a good understanding of the associated risks. This is not the case with many new methods, and although the risk of systemic failure is low, the consequences could be severe. Insurers therefore seek to ensure that systems are correctly certified, and that they are manufactured and built in line with that certification.

This CPD will consider the risks that MMC present and how these can be appropriately managed throughout the construction process.

Types of MMC

MMC typically fall into the following categories.

• Volumetric or modular construction involves the off-site production of three-dimensional units. Modules may be brought to site in a variety of forms, ranging from a basic structural shell to complete rooms with internal and external finishes and services already installed. Any third-party approval would require evidence of quality control systems within the factory.

Volumetric construction can consist of timber frame, light gauge steel and concrete or composite constructions. External cladding may form part of the prefabricated system, with only localised sealing required on site. Alternatively, traditional masonry cladding may need to be constructed. In this case, specific detailing for the support of claddings, cavity barriers and DPCs must be pre-agreed and checked by site managers.

• Panelised systems involve the off-site production of panel units, which are assembled on site. The panels may consist of wall, floor or roof units, sometimes referred to as cassettes.

Closed panel units may be built of timber, steel frame or concrete panels. Lining materials and insulation are installed in the factory. Panels often include services, windows, doors and finishes.

Open panel systems do not include elements such as insulation, lining boards or vapour control layers. These are applied to the frame system on site, together with external cladding and internal finishing. Careful control of on-site finishing is therefore required, and the panels must be protected against the elements until weather-tight.

Conventional timber-frame panels are typically classed as open panel systems, and would normally arrive on site with the sheathing board fixed but without insulation or internal boards. For warranty purposes, these types of open-panel systems can normally be classified as established or traditional construction, providing that quality assurance systems are in place and registered either with the Structural Timber Association or TRADA BM. Bespoke timber-frame open-panel systems that do not have such procedures will require either third-party accreditation or the supervision of an independent structural engineer to monitor installation, erection and completion.

• Hybrid or semi-volumetric systems are manufactured off-site and combine both panelised and volumetric approaches. Examples would be student accommodation or hotel pods.
• There is another category of sub-assemblies and components within an otherwise traditionally built structural form. Typical examples would be floor or roof cassettes, precast concrete foundation assemblies and preformed service installations and cladding systems.
• Site-based structural systems, such as insulated concrete formwork, are not considered to be manufactured off-site. These systems require third-party approval to secure a warranty, and their acceptability relies heavily on the procedures in place to ensure correct installation on site.

Design considerations

With MMC, the construction, design and layout of a typical system is planned in advance and a “design freeze” should be imposed before factory production begins. Rigorous project management is essential to avoid last-minute changes.

MMC are based on standardised construction, and may not be adaptable for complex architectural or planning requirements. Additional testing may be needed to ensure that standards for durability and weather-tightness can be achieved - for example incorporating flat roof drainage outlets through closed panel parapet extensions.

To obtain warranty approval, MMC products or systems must:

• Meet the requirements of relevant British standards or codes of practice, or European equivalents current at the time of application.
• Be covered by a current approval from an independent third-party technical approval body. Details of the testing body’s accreditation will need to be supplied to the warranty provider, together with the certification document.
• Have been subject to independent third-party testing that recognises UK Building Regulation requirements and additional warranty standards. Details of performance and the limitations of use must be provided.
• Bear a CE marking in accordance with the Construction Products Directive. This should be supported by evidence of testing.

Third-party testing

MMC manufacturers should carry out quality assurance on the manufacture, delivery and erection of their product. In addition, systems should always be tested by a body accredited by the United Kingdom Accreditation Service (UKAS) or a European equivalent. The testing must be relevant to the intended use of the building and application of the MMC, and third-party approvals must be current and up to date. Where there is no test data or certification available for a system, the building is very unlikely to secure a warranty.

There are a number of risks associated with third-party testing, such as:

• MMC components are tested individually and rely on other products to perform adequately.
• Additional products may not be compatible, or may lack third-party certification themselves.
• Product testing does not take UK weather conditions and climate into account.
• There are often caveats included within a certification. For example, the building is said to be waterproof if it is bolted together correctly, but there is no reference as to how this should be done.
• Often the design of the MMC can exceed the original parameters of the test - for example, where a system is approved for the construction of individual homes but is used for a block of flats.
• It is often difficult to measure the long-term durability of a new or innovative product. Requirements from the Council of Mortgage Lenders expect structural components to have at least a 60-year durability. Individual components and assemblies, not integral to the structure, may have a lesser durability, but not less than 15 years.

Even when MMC products or systems have achieved third-party approval, they will still need to be structurally approved on a site-by-site basis depending on the layout and loading of the component. Thermal properties and measures to prevent condensation will also require specific assessment depending on factors such as exposure or orientation.

On-site considerations

Although MMC are generally manufactured off site, there is always an element of on-site workmanship. The quality of the final product will depend on accurate assembly by factory-trained or authorised specialist contractors, and any alterations made on site may invalidate a system’s guarantee or affect its performance. The following risks should therefore be addressed.

• Approved contractors Most manufacturers have a process for approving specialist contractors - but project managers should check that this is suitably robust, and determine where liability for design and workmanship lies.
• Foundations There is very little tolerance for error with factory-built systems. Where only the superstructure is to be delivered via MMC, the foundations and substructure must be extremely square and level. The project manager must ensure that the contractor carrying out these works understands the level of accuracy required.
• Penetrations All buildings require some form of penetrations to the structure during the installation of services. In most cases, these trades are outside the scope of an approved contractor.
• Site planning MMC, particularly modular systems and large panel systems, will require detailed advance planning of site access, off-loading, installation and possibly storage.

Although MMC do present unfamiliar risks, this is not to say that warranty providers do not support their use, as long as the risks are appropriately managed. LABC Warranty considers that such systems are a good option for development when supported by correct, relevant third-party certification, and will accept any MMC that meets the requirement of its technical manual.

LABC Warranty also works in partnership with LABC (Local Authority Building Control) to provide an integrated “Registered Details” approval process, which encompasses both building control and structural warranty approvals. This allows a building or product to be accepted by more than 300 local authorities across England and Wales without a separate structural warranty application.