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How to design tapered falls to maintain the thermal performance of flat roofs

Lisa Stephens, Product Manager – Building Envelope
Lisa Stephens, Product Manager – Building Envelope
December 1, 2022

Designing tapered falls to maintain thermal performance for flat roofs

It’s no wonder flat roofs have become the go-to roofing structure for commercial buildings. They’re versatile, easy to access and maintain, and are a great design choice. And when it comes to a roof’s fundamental purpose – sheltering from the elements – flat roofs offer a cost-efficient solution without compromising on performance. 

But despite their name, flat roofs often aren’t truly level. Most of the time they are designed to have a bit of a slope – a taper – so rainwater can fall to one side and protect the roof’s longevity. And because structural falls are not always viable for a building’s design, a common way to create a fall in the roof is through a tapered system. Tapered falls are ideal for both new builds and refurbishments as they can simplify construction, free up internal space and save money. 

There are, however, certain challenges that specifiers and architects need to be aware of when designing tapered falls, including standards for thermal performance. Lisa Stephens, Product Manager - Building Envelope at ROCKWOOL UK, explores the benefits of tapered falls and how designers can ensure consistent insulation performance across the varying levels of thickness in a flat roof system.

Compliance and best practice for flat roofs featuring tapered falls

Flat roofs featuring tapered falls must comply with building regulations requirements and should follow both the regulatory guidelines set out in Approved Document B (ADB) and sector-specific guidelines, such as BB93 for education. This is in addition to complying with the requirements of the Building Regulations. Aside from the statutory requirements, other codes of practice also need to be considered when designing tapered falls, such as those detailed in BS 6229:2018. 

Tapered falls are fast-drying and a preferred strategy for rainwater management – they can prevent ponds from forming and help to avoid expensive associated damages, such as:

  • Accelerated degradation of roofing materials
  • Added stress to load-bearing elements
  • Slip hazards due to the growth of moss and algae 

To be effective, tapered insulation needs to be installed at a certain angle.

The BS 6229:2018 standard states that the recommended minimum angle for a slope is 1:80 (0.72⁰) and that to achieve this drop, tapered falls should be designed to a slightly steeper angle of 1:40 (1.44⁰). Factoring in surface deviations, material deflection and on-site inaccuracies, aiming for a 1:40 drop is a way to make doubly sure that the finished construction will result in a fall of at least 1:80.

Key considerations when specifying insulation materials

A key function of the roof is to protect from fire, water, cold weather and noise pollution. And with evolving fire safety, acoustic and thermal performance targets, the insulation materials should be chosen with greater care, since not all of them may be fit for purpose.

Thermal efficiency (U-values)

When it comes to insulation, low U-values indicate solutions which offer excellent thermal performance capable of reducing heat loss, improving energy efficiency and promoting indoor comfort.

As per BS 6229:2018, the roofs of heated buildings “must be insulated to provide a U-value that does not exceed 0.35 W/m2K at any point” – a key consideration at gutter details, where insulation will typically be at its thinnest.

Designers must then calculate the overall U-value of the roof factoring in these gutter details as well as the tapered fall, which is not simply an act of averaging the insulation thickness across the roof. 

To correctly measure thermal efficiency, designers need to use the area-weighted average method given in Annex E of BS EN ISO 6946, ‘Building components and building elements — Thermal resistance and thermal transmittance — Calculation methods’. This method accounts for varying levels of insulation thickness and is a more accurate measure of thermal performance.

Acoustic performance

BS 6229:2018 and BS 8233:2014, ‘Guidance on sound insulation and noise reduction for buildings’ also refer to the acoustic requirements of flat roofs. With tapered falls, designers must take into account the impact varying insulation thickness has on acoustics and they may require materials that deliver additional acoustic performance to meet their targets.

Special attention should be paid to the link between tapered falls and gutter details

According to best practices stated in BS 6229:2018, all flat roof surfaces, including gutter beds, should be designed with a fall of 1:40 to achieve finished drainage falls of 1:80. This way, water can flow more easily to rainwater outlets – but it also introduces insulation considerations.

The point where the gutter detail meets the roof has the thinnest insulation layer in order to maintain the slope required for drainage – and this must be accounted for when measuring the thermal performance of the roof system as a whole. 

The 0.35 W/m2K maximum U-value stated in BS 6229:2018 is in place to reduce the risk of condensation – especially relevant for places like swimming pools – and the document goes on to state that care should be taken to ensure continuity of insulation at junctions and penetrations. 

Maintaining thermal performance and continuous insulation will help designers to address condensation – but condensation is not the only risk associated with flat roofs.

Flat roofs and reducing fire risk

Whether used as a means of escape or a place of refuge, flat roofs are a key part of a building’s fire safety strategy. But while the combustible cladding ban prohibits combustible materials on upstands, balconies and terraces, it stops short of requiring all areas of roof insulation to be non-combustible. Given the absence of an outright ban, designers must ensure that they are familiar with the relevant standards for flat roofs.

Notably, section 8.26 in ADB Volume 2 covers the importance of junctions between the roof and compartment walls. It is essential there aren’t any weak points from where fire can enter new zones or spread within the roof structure itself. 

Knowing this, an increasing number of designers are using non-combustible insulation across the whole roof including tapered falls to avoid zoning concerns and streamline the installation.

The benefits of specifying non-combustible materials for tapered falls

During roof construction, creating the fall and installing insulation can be achieved in a single step by specifying a tapered system from the start. This simplifies the installation, reduces site traffic and cuts down on costs. 

Using non-combustible materials within tapered falls is highly efficient as it helps meet thermal requirements, acoustic standards and fire safety regulations all in one go. It’s not just best practice for risk management – it’s a way of preparing buildings for tomorrow’s regulatory and client demands.

If you’d like to save time and simplify construction, visit ROCKWOOL’s Flat Roof Resource Hub – it brings together all the key information you may need in designing, specifying and installing a safe and effective flat roof system.

Article History

Our experts continually review and update our articles when legislation changes or new information becomes available. 

Current version: 1

First published: 1/12/2022