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.

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 is 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.

The 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.

Two key areas of guidance are drainage and thermal performance.

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.

Thermal performance guidance includes the calculation of U-values (such as using the conventions of BR 443), 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.

Should designers and specifiers be familiar with BS 6229?

Even if your workspace isn’t home to a copy of BS 6229:2018, there’s a reasonable chance you already apply a lot of its principles in your work. Many of the recommendations become widely disseminated through blog posts, articles, training presentations and manufacturer literature.

It’s possible, therefore, that you’re more familiar with the standard than you realise. But issues can arise if the recommendations are not taken into account at the right stage of the design process.

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 not, the insulation layer may run straight into a door threshold, or there may not be space to accommodate the necessary height of upstand 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? We see a lot of flat roofs where a portion of the roof does not achieve the threshold U-value of 0.35 W/m²K (stated in BS 6229) to avoid surface condensation, because some compromise has had to be made in the design.

As an extruded polystyrene (XPS) insulation manufacturer supplying inverted roof systems, Polyfoam XPS strives to ensure that all technical support offered to designers and specifiers reflects the guidance of BS 6229:2018. We offer guidance on avoiding hybrid roof constructions, provide U-value calculations that accord with the recommendations, and apply the winter condensation risk adjustment as standard.

We can only do that, however, at the stage that we are involved in a project, which is sometimes too late to avoid compromises that affect the quality of projects. Design professionals should be familiar with BS 6229, and apply its principles from the outset.

About Polyfoam XPS

You can contact us at any stage of a construction project to discuss questions relating to inverted roofs and applying the principles of BS 6229:2018. Alternatively, download our inverted roof BBA certificate to gain an understanding of which of our products can be used in inverted roofs, and how.

We have also published an in-depth white paper looking at the assessment and certification of inverted flat roofs. It details aspects of BS 6229 that address how inverted roofs perform thermally, and discusses issues relating to different methods of assessing that performance.

For more on XPS insulation, browse our blog posts or find out more about us as a company.