Introduction

 

All light gauge steel framing must be located entirely above DPC level and a minimum 150mm above external ground level. However, this is often difficult to achieve for level access requirements in certain situations. As a result, we state the following in our Technical Manual:

 

Where level access requirements result in the floor joists, ring beams and base rails etc. being less than 150mm above external ground level, then these elements should be pre-galvanised to 600g/m2 zinc coating (Z). Level access arrangements should be kept to a minimum and up of 15% of the external perimeter of an individual building (e.g. a single plot in a row of terraced homes).

 

This article provides guidance where more than 15% of the external perimeter of an individual building (e.g. a single plot in a row of terraced homes) has:

 

• Level access arrangements, and/or,
• The external ground levels are less than 150mm from the lowest steel.

 

In all situations, the construction of the ground floor must meet the requirements of regional Building Regulations for structure, site preparation, and resistance to moisture and contaminants.

 

Please note, for the purposes this article, the term light gauge steel framing applies to both light gauge steel wall panelled framing and module construction where light gauge steel floor cassettes are utilised.

 

What we require to mitigate the risk

 

LGSF design requirements

 

Where more than 15% of the external perimeter of an individual building (e.g. a single plot in a row of terraced homes) has:

 

• Level access arrangements, and/or,
• The external ground levels are less than 150mm from the lowest steel.

 

The designer must provide factory applied corrosion protection to the LGSF system to achieve a service life of at least 60 years. This can be achieved:

 

• With the LGSF system pre-galvanised to 600g/m² zinc coating (Z), or,
• With the LGSF system pre-galvanised to 275g/m² with the addition of two coats of heavy duty bituminous paint, 200μm total thickness with appropriate quality control measures in place. All paint systems used should be specifically formulated for use on galvanised steel and applied in accordance with the paint manufacturer’s recommendations

 

In addition:

 

• Sheathing boards must be service class 3 in accordance with BS EN 13986,
• Drainage of the cavity (ground conditions must be considered where the cavity discharges below ground level).

 

Thermal modelling of the ground floor/base rail junction may be required to demonstrate that any issues with thermal bridging or interstitial condensation will not occur.

 

Voids below the LGSF structure

 

The ground below ground floor structure should be constructed to prevent moisture affecting the floor structure. This can be achieved by covering the ground with either:

 

• 50mm oversite concrete or 50mm fine aggregate on 1200 gauge (0.3mm thick) polyethylene membrane laid on 50mm sand blinding, or,
• 100mm oversite concrete on a compacted clean, inert hardcore bed. Where necessary, this concrete should be protected against sulfate attack.

 

Floors should have a 150mm minimum void below the floor. Where there is a potential for the soil to heave, the minimum void depth should be increased as follows:

 

• High potential – 150mm (300mm total).
• Medium potential – 100mm (250mm total).
• Low potential – 50mm (200mm total).

 

Sub-floor ventilation should be provided with ventilators on at least two opposing sides of the external wall with air bricks appropriately ducted in accordance with the manufacturer’s instructions. Sub-floor ventilation should be provided to whichever gives the greater opening area:

 

• 1500mm² per meter run of external wall.
• 500mm² per m² of floor area.

 

For light gauge steel frame systems that are less than 150mm above the external ground level a proposal for a suitable method of draining the floor void to a suitable outfall must be provided to prevent waterlogging and condensation.

 

Warranty stance

 

Extra care should be taken where more than 15% of the external perimeter of an individual building has either level access arrangements, and/or the external ground levels are less than 150mm from the lowest steel. This article provides some ways the enhanced risks may be mitigated.

 

In all circumstances, it’s important to discuss your site specific situation with your Risk Management Surveyor before associated works start on site.

 

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Every care was taken to ensure the information in this article was correct at the time of publication (September 2024). 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. For the most up to date Premier Guarantee technical guidance please refer to your Risk Management Surveyor and the latest version of the Premier Guarantee Technical Manual.
Ref: KM-3682