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Pitched Roofing - Part C of the Building Regulations by Chris Thomas

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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 last year come into force in December 2004.

To help you understand the scope and detail of this document this paper summaries the requirements of the Regulations as it relates to pitched roofing.

Part C
Approved Document C2 of the Regulation states “The floors, walls and 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 requirement of this regulation is for the roof to be weather-tight. In the case of pitched roofs the document states that any roof will meet the requirement if it has overlapping dry joints, is impervious or weather-resisting, and is backed by a material which will direct precipitation which enters the roof towards the outer face (as with roofing felt).
Impervious materials include metal, plastic and bituminous products, weather-resisting materials include natural stone or slate, cement based products, fired clay and wood.
The regulation goes on to state “Dry joints between roofing sheets should be designed so that precipitation will not pass through them, or the system should be designed so that precipitation which enters the joints will be drained away without penetrating beyond the back of the roofing system. Note: Whether dry joints are suitable will depend on the design of the joint or the design of the roof system and the severity of the exposure to wind driven rain. This would imply that dry joints between roof tiles, slates and roofing components need to be suitable for the situation, location and with other components.
Perhaps the most important clause states “Each sheet, tile and section of roof should be fixed in an appropriate manner. Guidance as to appropriate fixings methods is given in BS 800-6 1990.” As BS 8000-6 refers fixing of slates and tiles to BS5534, this makes the need to use the wind uplift and resistance calculations of BS 5534 a requirement of the Building Regulations, which it has never been before. While it does not state that proof of calculations is required, this would appear to be the logical conclusion.
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 meet the requirements of the regulation one of the following systems should be employed:
Cold Roof
To ventilate the roof from eaves to eaves or from eaves to ridge with 10 000 mm˛/m run of low level ventilation and 5 000 mm˛/m run of high level ventilation. Cold Roof with ventilation
Warm Roofs
To ventilate between the insulation and the underlay where the insulation is parallel with the roof covering through a min 50mm cavity from low level to high level. Low level ventilation should be equivalent to 25 000 mm˛/m run and the high level ventilation should be equivalent to 5 000 mm˛/m run. Warm roof with insulation between the rafters and ventilation
Alternatively where the insulation is located above the rafters a vapour permeable underlay can be used without the need for additional ventilation. Warm roof with insulation above the rafters with VP underlay and no ventilation
Whilst the regulation refers to BS 5250 2002, it has been written such that when the next major revision to BS 5250 is published, the levels of ventilation when using VP underlay with cold roofs, and warm roofs with insulation between the rafters, will change with minimal alterations to the text.
The Regulation recommends the use of Robust Details as produced by the Building Research Establishment, including the elimination of gaps, cold bridges, sealing around pipes and other penetrations in areas of high humidity, and a maximum U Value of 0.35 W/m˛K at any point in the roof construction.
Building Regulation Approved Document C2 now requires that all tiled and slated roofs are designed and constructed to keep out wind driven and deluge rainfall or have a means of draining away safely water that does enter the roof system. It also requires that roof coverings are fixed correctly. In accordance with the guidance in BS 8000 part 6, which in turn direct users to the wind-uplift calculations in BS 5534. This will inevitably result in fixing calculations being required and complied with for all roofing projects covered by Building Regulations. In many instances the tile or slate supplier will be able to provide a fixing calculation service. For those that do not provide such a service, Roofconsult can help you.
The new requirement for the control of condensation will cover systems that do not rely upon ventilation, such as vapour permeable underlay, when the present BS 5250 revisions are published, the Approved Document will only require a minimal revision to its text. By requiring compliance with BS5250 it will prevent the anomaly whereby previously the British Standard and the Building Regulations had slightly different requirements. 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
Thanks to Klober Limited for the illustrations.
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