WHAT IS A ZERO FALLS FLAT ROOF?

‘Zero falls’ is a term generally used to describe a flat roof designed to be as close to level as possible. That is extremely difficult to achieve in reality, for which reason zero falls has long been a topic of debate within the flat roofing sector.

Designing for zero falls may be desirable on a roof terrace or roof garden, where the flat roof acts as amenity space for people to use. It’s also an approach commonly used with blue roofs. Whatever the fall of a flat roof, however, it must still achieve the expected standard of rainwater drainage.

What is the definition of a zero falls flat roof?

The issue of zero falls has tended to be tied particularly to inverted warm roofs. The 2003 version of BS 6229 included a note to table 7.2 saying that inverted roofs could be designed to the manufacturer’s documented advice and BBA certification – which often said that systems could be used at zero falls.

Despite seemingly widespread opposition to zero falls, 2012 saw the BBA publish Information Bulletin No.4. The document addresses drainage, and corrections to U-value calculations, for inverted warm roofs. It defines zero falls (or “zero pitch”) roofs as having a slope between 0 and 0.7 degrees (which is a fall of about 1 in 80).

In the years since, there has been a general acceptance of the concept of zero falls, coupled with a rise in awareness of the importance of designing to minimum falls and ensuring water still drains freely from the roof.

BBA Information Bulletin No.4 says that zero falls doesn’t necessarily mean flat, and that adequate drainage is still a requirement.

When BS 6229 was revised and reissued in 2018, it sought to clarify guidance around zero falls roofs. The definitions section makes clear that a zero falls roof has a slope between 0 and 1:80. However, in order to ensure that such a roof drains correctly, the standard is clear in recommending that it be designed to a minimum fall of 1:80, to allow for settlement of the roof structure and to avoid back falls.

Why is it important to achieve a minimum fall on a zero falls roof?

Designing a roof for one fall in order that it achieves another when constructed is not unique to zero falls roofs. Standards and guidance on drainage for flat roofs give minimum falls that designers should achieve in all cases.

However, this blog post is not about general roof drainage, so we are only focusing on zero falls for the moment.

Roofs can fail to achieve their intended fall for a variety of reasons. Designers may not include sufficient allowance for achieving falls. Contractors may omit a layer, such as a screed laid to falls, that would achieve the designer’s vision. Structural engineers may not account for sufficient load, which causes the roof deck to deflect more than expected.

Any of these things can happen accidentally, or through a lack of awareness of best practice, or because of ‘value engineering’. Whatever the reason, the onus then falls on the insulation manufacturer to be able to offer a system that works at zero falls.

A roof design that doesn’t drain adequately – whether through poor drainage or through roof deck deflection due to structural loads – increases the risk of ponding, which increases the weight of water, which imposes further loading and further deflection, and so on. At too shallow a pitch, that deflection causes back falls, or negative falls, which causes water to flow away from designed drainage outlets.

Inverted roof systems are designed and tested to work with roof falls and appropriate drainage. Back falls introduce a risk of water flowing under the laps of the loose laid water control layer, increasing the proportion of rainfall reaching the waterproofing layer and worsening the thermal performance of the roof.

Achieving a successful zero falls flat roof

For any project where a zero falls roof is desirable, the guidance is clear: design for a minimum fall of 1:80, in order to allow the roof to settle towards zero without causing back falls.

As a supplier of inverted roof insulation and water control layers, Polyfoam XPS will always strive to offer the best advice to help specifiers and installers get the most out of our system. However, even the best products and advice will struggle to overcome a poorly designed or constructed roof that does not follow relevant codes of practice.

Communication throughout the project team is vital from an early stage to make sure the finished roof achieves all of its intended performance criteria, including thermal performance, waterproofing and drainage. Contact us for advice on achieving a successful inverted warm roof, whatever the fall.

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