Sarah Spink, CEO of the Liquid Roofing and Waterproofing Association (LRWA) provides specification advice on waterproofing inverted roofs, which are increasingly popular
The popularity of inverted roofs continues to grow with the construction of these systems offering many benefits. From a design perspective, inverted roofs enable additional space to be created on top of buildings that can be used for a variety of purposes such as a terrace for residents or a green roof to improve environmental performance.
Another major benefit is that an inverted roof – also known as a ‘protected membrane’, or ‘upside down’ roof – is constructed with the insulation on top of the waterproofing covering. Insulation boards are loose-laid on top of the membrane and then weighted down with paving slabs or gravel ballast. This protects the waterproofing membrane from extreme weather as well as UV degradation.
Most inverted roofs are installed on concrete decks, and a range of waterproofing systems can be used. Specifying a liquid-applied waterproofing system can, however, offer many advantages compared to other waterproofing methods.
Liquid technologies result in seamless and flexible membranes that fully bond directly to the substrate, providing excellent waterproofing performance with toughness, flexibility and adhesion over a wide range of temperatures.
They are also quick and easy to install, especially around any detailing, and root resistant grades are available, specifically designed for use on green roofs.
Correctly installed and maintained, liquids can guarantee that a roof remains watertight for more than 25 years. Some liquids such as hot melt monolithic systems, which are frequently used for inverted roofs, even have a British Board of Agrément (BBA) certified life expectancy of ‘the life of the structure it is applied to’, providing clients with reduced building maintenance, peace of mind, and whole life cost benefits.
In the last decade, the liquids sector has grown rapidly, which has attracted more manufacturers into the market. It is important for specifiers to select products, which have independent accreditations such as BBA, BDA and ETA certificates, and that also have the ISO 9001 standard.
Trusted trade associations such as the LRWA can also provide guidance and technical support to ensure the most appropriate product choice.
The importance of U-values
Another key consideration for specifiers – and something the LRWA continues to assess – is ensuring correctly calculated U-values are in place for every project. In inverted roof systems, the Design Lambda value should always be used, as this takes into account the rainwater cooling effect. Not using the declared Lambda value means the insulation will not meet the U-value requirements.
Changes to flat roof guidance
Architects should also be aware of recent changes to one of the key British Standards for roofing. BS 6229:2018 – ‘flat roofs with continuously supported flexible waterproof coverings’ – Code of Practice was published at the end of last year.
It describes best practice in the design, construction, care and maintenance of roofs with a flat or curved surface, at a pitch no greater than 10˚, with a continuously supported flexible waterproof covering.
The guidance reflects the fact the flat roof industry has evolved significantly over the last 15 years, with new challenges driven by Building Regulations and fire requirements, as well as new materials and systems.
With the progress of Part L of the Building Regulations and the requirement for more thermal insulation, the new standard recommends the use of warm and inverted roofs and advises against cold roofs. This is because of the difficulty of forming and maintaining an effective air and vapour control layer below the insulation and of providing sufficient cross ventilation above the insulation.
There is also a section providing advice on maintaining the thermal performance of inverted roofs. As Part L evolves, this is becoming more important, and long-term results will be under close scrutiny.
Sarah Spink is CEO of the Liquid Roofing and Waterproofing Association (LRWA)