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Profiled Metal Sheeting Part C of the Building Regulations by David Roy

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The new Part C of the Building Regulations "Site preparation and resistance to contaminants and moisture" which was published in June this year comes into force in December (2004). Picture 1
This paper summaries the recommendations of the Regulation as it relates to metal roofing.
Part C2 of the Regulation states that the roof of the building shall adequately protect the building and people who use the building from harmful effects caused by: 
  • Precipitation and wind-driven spray
  • Interstitial and surface condensation
Section 6 gives guidance on technical solutions to achieve the requirements. Note this section replaces Approved document F2 - Condensation in roofs.
The first and obvious requirement is that the roof shall be watertight, in the case of metal cladding the following documents are referred to
  • BS CP 143 Code of Practice for sheet roof and wall coverings
  • MCRMA Technical Paper 6 Profiled metal roofing design guide, revised edition 1996
  • MCRMA Technical Paper 9 Composite roof and wall cladding panel design guide, 1995
Note that the MRCRMA Technical Paper 6 was revised in June 2004 for more information and to download the paper go
Interstitial Condensation
Three documents are referred to:-
  • BS 5250:2002 Code of practice for the control of condensation in buildings
  • BS EN ISO 13788:2001 Hygrothermal performance of building components and building elements. Internal surface temperature to avoid critical surface humidity and interstitial condensation. Calculation methods
  • BRE Report BR 262 Thermal insulation: avoiding risks, 2002
To summarise the advice given in the above publications.
Site assembled built-up systems
  1. Provide a vapour control layer, either the liner with laps sealed or separate vapour control layer minimum 500 gauge polyethylene. Ensure continuity of vapour control at penetrations and junctions.
  2. Use ventilated filler strips at eaves and ridge to allow ventilation of rib voids. (Ventilated fillers should have an opening of not less then 5% of the rib void)
  3. For building with internal high humidity such as swimming pools, give consideration to the use of a breather membrane on top of the insulation. (Note that the membrane should drain externally). 
The BRE report states that a breather membrane is recommended where there is a calculated risk of condensate forming on the underside of the out sheet and subsequently dripping. 
Composite systems
Composite panels should be capable of being sealed at side and end laps, to prevent moist air entering the joints between panels
Surface Condensation
To minimise surface condensation the roof shall be designed and constructed so that the thermal transmittance (U-value) does not exceed 0.35 W/m2K at any point
The junctions between elements and the details of openings, are designed in accordance the MCRMA Technical Paper 14 Guidance for the design of metal roofing and cladding to comply with approved document L2:2001, 2002.
Details can been analysed using a 2D or 3D finite element thermal modelling programs. These programs will give internal surface temperature at any point in the detail.
Picture 2 Picture 3
The minimum temperature factor (fmin factor) indicates the risk of surface condensation, the lower the value the greater the risk.

Picture 4

ts = minimum surface temperature
te = external surface temperature
ti = internal surface temperature

The minimum values of the f-factor are
  • Storage buildings - 0.3
  • Offices, retail premises - 0.5
  • Sports hall, kitchens - 0.8
  • Swimming pools, laundries, breweries - 0.9
A number of calculations are likely to be required for a building to comply with these regulations, minimum U-values, condensation risk analysis and minimum temperature factors. For help with these and other design calculations please contact me here
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