MMC Guidance

Providing support on common queries associated with MMC

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Top tips FAQs

Since our inception in 1936, NHBC has worked closely with house builders in the development and review of new forms of construction. We have a unique insight into the benefits of differing construction types as well as common pitfalls that have prevented new technologies from becoming mainstream.

As part of the technical review, the Innovation Team will be pleased to share their experience of working with similar MMC systems and our “top tips” highlight some common hazards that we have seen in MMC systems.

1. Plan for site delays

Recognise that delays can easily occur in construction due to weather, unforeseen ground conditions, available labour and transport. Provision should be made for storage of manufactured systems and to align manufacturing output with site progress.

Site delays
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Cross-wall system production and delivery driven by site requirements using lean construction planning

Site delays
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Inadequate storage of pre-fabricated units due to unforeseen delay in construction.

2. Determine allowable tolerances

Given their size, volumetric units require erection to relatively tight tolerance so the method of lifting and erection should be planned and proven and the detailing between units should be addressed.

Determine allowable tolerances
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Setting out of each unit surveyed and tolerances accounted for. Erection team specialist in delivery of particular volumetric system.

Determine allowable tolerances
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Volumetric units placed out of tolerance with limited flexibility designed in the building envelope.
 

3. Check line and level of foundations

Systems manufactured off site will often be built to tight tolerances. A common error is the lack of consideration of the tolerances that are achieved on site, particularly at the interface with foundations. Allowable tolerances should be specified and the as-built elements should be surveyed prior to delivery of the MMC system.

Check line and level of foundations
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Setting out error so that foundation had to be extended to accommodate volumetric unit.
 

Check line and level of foundations
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Foundation constructed out of tolerance so that timber panel bearing off-centre.

4. Confirm the material is suitable for its intended purpose

The components used in MMC systems are often certified by approvals bodies that set out the limits of their use and scope of application. The conditions set in these certificates are critical to the performance of the MMC system and must be adhered to. Common items that are overlooked are:

  • Limitations to building height
  • Substitution of products that affect performance in fire and durability
  • Limitations to location, e.g. below ground
  • Requirement for project specific structural design (e.g connections)

5. Design for resilience, particularly to account for water ingress

Whether through poor construction practice, long term exposure or accidental damage, it is possible that water will penetrate the building envelope or other first-line barriers. Details should be designed to allow moisture to drain or evaporate from sensitive parts of the structure.

Design for resilience, particularly to account for water ingress
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Drained and vented cavity in timber frame

6. Consider the likely topography on site

The construction type, particularly of off-site manufactured panelised or volumetric MMC may not be compatible with the features or characteristics of some development sites. Frequently, a design is proposed assuming a level site or regular layout on plan whereas the MMC system should provide flexibility to address vertical steps and horizontal staggers.

Consider the likely topography on site
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