Light steel framed walls and floors

6.10.16Moisture control

The structure shall be adequately protected from the effects of moisture. Details for LSF at low level shall fully consider the durability of materials, protection of the building from moisture ingress and thermal bridging. Issues to be taken into account include:

  1. cavities in external walls
  2. protection of steel at low level
  3. DPCs, DPMs and cavity trays.

Cavities in external walls

A clear cavity in accordance with Table 8 should be provided between the cladding and insulation.

Table 8: Minimum cavity widths

CladdingCavity width
Render on board background25mm
Vertical tile hanging without underlay No vertical cavity required where a breather membrane is provided
Other cladding(1)15mm


1 See Chapter 6.9 ‘Curtain walling and cladding.’

The cavity should:

  • extend at least 150mm below the DPC
  • be kept clear to allow drainage
  • be provided with weep holes or other suitable means of drainage where necessary.

Protection of steel at low level

The base rail of LSF should be kept a minimum of 150mm above the external ground level (or waterproofing layer of a flat roof, balcony or terrace) and cavity fill.

Locally raised ground levels (up to the internal floor finish) to less than 15% of the external perimeter (of an individual building, e.g. row of terraced homes, apartment blocks and detached garages, measured on plan) to accommodate level thresholds can be accepted. The cavity should be kept clear and allow drainage. Wall insulation should overlap the base rail by a minimum of 150mm.

Alternatively, where the base rail or lowest steel is less than 150mm above ground level (or waterproofing layer of a flat roof, balcony or terrace), the design should consider the following:

  • factory-applied protection to the steel frame to achieve a design life of 60 years. This may be LSF: – Galvanised to 600g/m², or – Galvanised to 275g/m² with the addition of two coats of heavy duty bituminous paint, 200μm total thickness
  • sheathing, or backing boards to waterproofing, used below 150mm, should be service class 3 in accordance with BS EN 13986
  • drainage of the cavity (ground conditions should be considered where the cavity discharges below ground level)
  • insulation to limit thermal bridging and interstitial condensation. NHBC may ask for thermal modelling of the junction to demonstrate that these issues have been sufficiently limited.

Where levels are raised above the base rail or lowest steel:

  • local areas of LSF walls (less than 15% of the external perimeter) retaining up to a maximum of 600mm of ground can be acceptable, subject to appropriate waterproofing design
  • in addition, where more than 15% of the external perimeter has ground levels above the internal floor finish (up to a maximum of 600mm), the structure should be designed by an engineer in accordance with Technical Requirement R5.

Waterproofing should be designed by a waterproofing design specialist and be in accordance with Chapter 5.4 ‘Waterproofing of basements and other below ground structures’.

The manufacturer of the waterproofing system should confirm compatibility between the waterproofing and sheathing board, which should be supported by test evidence.

DPCs, DPMs and cavity trays

DPCs, DPMs and cavity trays should:

  • be provided at openings to prevent rain penetration
  • be installed underneath the full width of the lowest section of framing, e.g. all ground floor walls and internal partitions, to protect the steel from corrosion due to moisture

Materials acceptable for use as DPCs include:

  • BS 6515 polyethylene
  • BS 6398 bitumen
  • materials assessed in accordance with Technical Requirement R3.