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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.
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.
Cross-wall system production and delivery driven by site requirements using lean construction planning
Inadequate storage of pre-fabricated units due to unforeseen delay in construction.
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.
Setting out of each unit surveyed and tolerances accounted for. Erection team specialist in delivery of particular volumetric system.
Volumetric units placed out of tolerance with limited flexibility designed in the building envelope.
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.
Setting out error so that foundation had to be extended to accommodate volumetric unit.
Foundation constructed out of tolerance so that timber panel bearing off-centre.
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:
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.
Drained and vented cavity in timber frame
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.
NHBC currently accepts a range of systems as shown on our list of accepted systems.
The first step is to contact NHBC’s Standards Innovation and Research department and submit technical information to NHBC for an initial review. We review the system to determine if it appears broadly in principle to be capable of meeting the performance criteria. We will assign a Technical Officer or Engineer to lead the review and provide an initial gap analysis indicating what further information would be required.
Please contact NHBC’s Standards Innovation and Research department as soon as you can so that we can commence a review of the MMC system. We can advise you if we are already reviewing this system and the status of this review to help you plan your project.
At present, NHBC delivers this service free of charge. The manufacturer may incur costs of employing consultants, technical approvals bodies and auditors to demonstrate the performance characteristics and quality procedures of the system.
This depends on the quality of information that is submitted. We endeavour to respond to all submissions within two weeks of initial contact so that the path to formal acceptance can be determined. Clearly, the more complete the information, the better.
The purpose of the MMC system review is to enable NHBC registered builders to use different construction systems. As such, we will prioritise the review of MMC systems that are proposed for use by NHBC registered builders.
A Buildmark warranty is only given on a completed home that has been built in accordance with NHBC Standards. A MMC system will form part of that home so while the system itself will have been reviewed, Buildmark warranty will only be given following the project design review and inspections of the registered plots.