Acceptable floor levels in modern methods of construction (MMC)

Some manufacturers of modern (steel and timber) construction products or systems wish to position ground floor joists either at or below external ground floor level. We currently require structural timber or steel to be located at least 150mm above external ground floor level, but noting manufacturers’ preferred approach we have conducted a review of our requirements in relation to ground floor levels and modern methods of construction (MMC).

This article describes the decisions we have made in relation to our warranty requirements.

Timber Framed Modular Build Systems

LABC Warranty’s stance on timber framed modular build systems, including hybrid systems which incorporate a steel frame with Structural Insulated Panels (SIPs) or Cross Laminated Timber (CLT) infill panels, remains unchanged. The requirements relating to timber frames given in Chapter 7.3 of the Technical Manual must be applied. Quoting the manual:

“All structural timber should be located at least 150mm above external ground level, except for localised ramping (incorporating satisfactory drainage and ventilation detailing) around door openings.”

It is also important to remember that all external wall claddings should be separated from the timber frame structure by a drained and ventilated clear cavity, provided with the minimum widths outlined in the table below.

Timber frame external wall cavity widths

Steel Framed Modular Build Systems

For steel frames, we have established a set of requirements which does allow for the level of the ground floor to be reduced, making it possible to include structural elements closer to external ground level.

These requirements are designed to ensure that all parts of the structure provide at least 60 years’ durability and manage the risk of water ingress and ventilation.

If a steel framed modular build system is to be located with the underside of the floor cassette less than 150mm above external ground level, the following criteria come into effect:

  • The internal finished ground floor level must be at least 150mm above external ground level
  • All steel which is less than 150mm above external ground level must be galvanised to a minimum code of Z450
  • A land drain is required to the perimeter of the building
  • A maintainable storm drain must be provided to the sub-floor void. Where site conditions permit and a ground condition report is provided, we may agree in advance that the drain be omitted
  • The sub-floor void must be ventilated in accordance with Chapter of the Technical Manual, to give a free cross flow of air. “External air bricks should be provided in two opposing walls [spaced evenly] and must meet the provision detailed in Table 5 [shown below].”

The Technical Manual also provides guidance on air brick types.

Cross ventilation provision

Please Note: Every care was taken to ensure the information in this article was correct at the time of publication. Any written guidance provided does not replace the reader’s professional judgement and any construction project should comply with the relevant Building Regulations or applicable technical standards. However, for the most up to date LABC Warranty technical guidance please refer to your Risk Management Surveyor and the latest version of the LABC Warranty technical manual.


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