Modern methods of construction pitfalls

The more common pitfalls are as follows.

• Incomplete design:

  The rigour required to complete the detailed design of an MMC system is often overlooked. The drawings and specification of all components go beyond what is required in conventional construction, principally because manufacture is often carried out by unskilled operatives. Take the example of a mastic joint; good practice in one MMC manufacturer's specification sets out the colour, location, length, width and allowable tolerance as well as the temperature at which it can be installed and how frequently it is inspected. The detail and accompanying procedures for all elements should not be underestimated.

• Clearly defined scope:

  No single MMC system is suitable for all sites, building shapes or planning requirements though these restrictions are often not defined by the manufacturer. However, the best designs seem to come from a simple, core product to which further features may be added. When MMC systems have a clearly defined scope, the limits of applicability and permitted environmental conditions can be fully specified. This then allows any variations to be fully designed and detailed.

• Durability:

  Basic principles are being ignored in many MMC designs, particularly with respect to water ingress. The leaky condominium crisis in British Columbia, Canada between 1985 and 2000, when 50,000 homes were affected by damp, is an important history lesson in this respect. Similar problems have been reported in other countries including the USA, Sweden and New Zealand leading to the development of the so-called four Ds of moisture management: deflect moisture away, using overhangs and flashings; drain moisture when it gets past the cladding; dry the structure by providing ventilation; and choose durable materials that do not degrade when wet.

• Sequence of assembly:

  Most MMC systems involve assembly of elements off site to form a panel or volumetric unit. The design of the joint between modules is complex because it needs to transfer structural load as well as retaining the integrity of fire protection, moisture and air barriers. Good practice will allow for lapping of these critical membranes, visible inspection of the integrity of joints and consideration of how these connections are formed on site.

• Specification of products:

  All materials used in construction should be fit for their intended purpose and certified as such. This means that, for many MMC systems, product specification and procurement needs to be completed well in advance of manufacture, and often before work starts on site. Good-quality MMC system manufacturers understand this, and their design process and programmes recognise the detail required to generate complete product specifications at an early stage.

• Spread of flame:

  Volumetric MMC systems - that is, 3D modules delivered to a site - introduce voids between the walls of modules and intermediate floors. Factory-fitted fire barriers are then often hidden from view so the inspection of this safety-critical element needs to be considered at design stage. Another common issue concerns the subfloor void. Some volumetric modules sit on foundations supported only at the corners, creating a void in the subfloor beneath the party wall. Where a compartment wall sits above these foundations, designers should consider how the void is subdivided on the party wall line.

• Setting out and site works:

  The risks associated with installing MMC on site are well documented. While it is often appreciated that the tolerances in groundworks or podium construction need to be tighter to accept highly engineered, factory-assembled modules, MMC designers also need to understand the realities of site conditions and to build in greater tolerance, to allow their systems to be effectively and efficiently assembled. Commonly, designers will address the interface between the MMC systems and foundations but will not pay sufficient attention to incoming services - how the drains, water, electricity and telecommunications are to be connected.