Designers and installers of flat roofing solutions are often told what they ‘should’ know about products and standards. In this ongoing series Rob Firman, of Polyfoam XPS, looks at different aspects of flat roofing design and construction and helps to demystify them. This month’s focus is the British Standard for flat roofing, BS 6229: 2018.
What is BS 6229?
To give it its full title, BS 6229: 2018 ‘Flat roofs with continuously supported flexible waterproof coverings. Code of practice’ describes best practice for the design, construction and maintenance of flat roofs.
A flat roof is defined as having a pitch of no greater than 10 degrees to the horizontal, but the recommendations in BS 6229 can be applied to roofs with a steeper pitch.
Its guidance applies to roofs with a timber, concrete or metal deck. The waterproofing types addressed by the standard are hot and cold liquid applied systems, reinforced bitumen membranes, plastic and rubber sheets, and mastic asphalt.
The standard is not an adopted European or international standard, and its contents are guidance and recommendations only. But as a code of practice developed by the UK flat roofing industry, BS 6229 carries weight.
What are the key areas of guidance in BS 6229?
The standard recognises four types of roof system: warm, inverted, cold and uninsulated flat roofs. It also emphasises the heightened risk of condensation caused by hybrid roof constructions.
A particularly significant section of the recommendations concerns drainage. Advice on minimum falls sets out how to ensure that a roof drains properly, including avoiding back falls in a zero falls flat roof. To aid detailing, BS 6229 includes typical level access arrangements for the different roof systems.
Meanwhile, thermal performance guidance includes: the calculation of U-values; how to treat point thermal bridges that prevent the installation of continuous insulation; and what correction factors to apply to inverted roofs. There are also short sections covering green and blue roofs.
There is limited guidance on condensation risk analysis, as the authors of BS 6229 agreed that the topic is better covered by BS 5250 Code of practice for control of condensation in buildings. There is some specific guidance relating to winter condensation risk in flat roofs, however.
The rest of BS 6229 lists commonly used flat roofing materials and relevant standards, highlights issues relating to workmanship, and describes how roof inspections and maintenance can ensure the system achieves its desired service life.
How familiar should you be with the recommendations of BS 6229?
Many of BS 6229’s principles and recommendations are widely communicated across the flat roofing sector, so you might be more familiar with the standard than you realise. Issues occur, however, when its recommendations are not taken into account at the right time.
For example, it’s not uncommon to see detailed drawings where the correct minimum fall has not been allowed for. A project may be at detailed design stage or, even worse, already on site, and it comes to light that the falls need to be adjusted. If they’re not, the insulation layer may run straight into a door threshold, or there may not be space to accommodate the necessary upstand height above the roof finish.
The question is therefore asked: is there a thinner insulation product we can use? What is the minimum thickness of insulation we can get away with?
In some cases, no amount of helpful advice can overcome the issue. As a consequence, the design has to be compromised and a portion of the roof does not achieve the worst-case U-value of 0.35 W/m²K – stated in BS 6229 – to avoid surface condensation.
Anybody involved in flat roofing should, therefore, be familiar with BS 6229, and apply its recommendations from the outset of a project.
For more information about BS 6229 and the certification of inverted flat roof solutions, download a copy of our new white paper at polyfoamxps.co.uk.