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

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.

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.

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