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Flat Roofing - 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 last year comes into force in December 2004. Part C
This paper summaries the recommendations of the Regulation as it relates to flat 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 flat membrane roofing then the document states the any roof will meet the requirement if it is jointless or has sealed joints, and is impervious to moisture.
Impervious materials include metal, plastic and bituminous products
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.
Warm Roofs
Warm Roof
A vapour control layer is required below the insulation as shown above.
Guidance for the calculation of the interstitial condensation risk is given in BS 6229:2003 Flat roofs with continuously supported coverings Code of practice. The calculation method is detailed in BS EN ISO 13788 :2002 .
The method given in BS EN ISO 13788 involves a monthly assessment of condensation taken over the whole year. For flat roofs BS6229 recommends a simplified approach based on two calculations only, a winter period and a summer period, both of 60 days.
Cold Roof
Cold Roof
A continuous opening along two opposite roof edges with an unrestricted path for ventilation between the two edges is required.
For roofs up to 5 metre span, continuous vent openings of 25mm and the space between insulation and underside of roof deck must be 50mm minimum. Between 5 meter and 10 metre span continuous vent openings of 30mm and the airspace must be 60mm minimum. Above 10 metres adequate cross ventilation can be difficult to achieve and both the openings and airspace should be substantially increased.
3-4 mm meshes will prove necessary to cover the ventilation openings to prevent entry of birds and insects, the size of the mesh must be taken into account when calculating the ventilation area.
Note that in the Building Standards (Scotland) Regulations, the Scottish Office considers that cold roofs should not be endorsed for the climatic conditions in Scotland. Although the Regulations do not prevent the use of cold roof constructions, warm roof constructions are recommended.
BRE Report BR 262 states that the cold roof is considered a poor option in the temperate, humid climate in the UK.
Inverted Roofs
Inverted Roof
With the inverted roof, the waterproofing also acts as the vapour control layer so normally no separate vapour control is required.
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, such as windows, are designed in accordance with the recommendations in the report on robust construction details - Limiting thermal bridging and air leakage: robust construction details for dwellings and similar buildings, TSO, 2001
Details can also be analysed using a 2D or 3D finite element thermal modelling programs. These programs will give internal surface temperature at any point in the detail.
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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