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Liquid-applied waterproofing systems are either based on natural bitumen, artificial polyester or polyurethane. They are typically used for the remedial covering of existing flat and pitched roofs, and their advantage is that they can be applied without removing the existing roof covering. They can also be used for new roofs. Systems that can withstand heavy traffic may be specified for balcony surfaces and walkways. Expected service lives vary from between five and 30 years.

European Technical Approval Guideline, ETAG 005, identifies seven generic liquid-applied roof waterproofing kits and sets out tests related to durability: five, 10 and 25-year working lives. The longer the working life, the more demanding the test criteria, which include resistance to: wind loads, mechanical damage, dynamic and static indentation, fatigue movement, low and high surface temperatures, ageing media such as heat, ultraviolet radiation and water and plant roots. Other tests cover resistance to slipperiness, fire and water vapour.

In practice many liquid roof waterproofing systems are certified by third parties using tests based on a range of British, German, European, US and MOAT standards. Not all product options have individual certification. When specifying, it is important to be clear which specific product and grade is wanted.

Specification options

Typically, systems include a primer, layers of the waterproofing liquid, a reinforcing fibre layer and a protective top layer. Systems may comprise one or several coats. Coloured systems will fade with exposure to light. Waterproofing systems are predominantly based on three materials, bitumen, polyester and polyurethane, as detailed below. Other liquid-applied waterproofing systems include formulations based on rubber, acrylics and silicones.

Modified bitumen

Bitumen is modified by substances such as natural rubber or styrene to make it more resistant to fatigue movement failure. Two or more bitumen solution or emulsion layers are applied to a total thickness of 3-6 mm or greater. Expected service lives vary between 10 and 20 years.

Polyester-based systems

These are glass-reinforced fibre systems. "Resilient" types are applied to plywood or similar substrate. "Flexible" types are applied to existing substrates. The system is 2-3 mm thick. Expected service lives are between 20 and 30 years.

Polyurethane-based systems

Some polyurethane systems may be more tolerant of moist laying conditions. Thickness of system is 0.6-2 mm. Expected service lives are between 10-25 years.

Durability issues

The material factors that influence durability are complex and related to the chemical composition of the liquid constituents, the thickness of the system as well as the type and density of the reinforcement.

Central to the long-term success of liquid-applied waterproofing systems is preparation of the substrate and achieving the required membrane thickness.

Typically, manufacturers only guarantee their products when applied by approved and trained installers. This is not surprising as standards of application by squeegee, roller, brush or spray are very demanding; layers are measured to tenths of a millimetre and avoidance of air holes is essential. The weather conditions in which the liquid is laid are critical as are the times between application of coats.

Other key durability issues include:

• Ensuring the substrate construction and condition is free from material which may impair the bond, create thin spots or penetrations in the coating.
• Provision for moisture removal of damp substrates.
• Ensuring there are no point loads on the roof.
• Specifying a system suited to the expected traffic load.
• Appropriate detailing at edges, verges, upstands, outlets, movement joints and day joints.

Liquid-applied coating that forms the waterproofing layer of an inverted roof should not require replacement during the life of the building, provided it is fully protected from ultra-violet radiation and extreme temperature variations by a loading layer above it.

When the roof covering is installed tests may be carried out to confirm its waterproofing capacity.