Whole life costing: Liquid roof waterproofing

Peter Mayer

Liquid roof waterproofing systems offer
a versatile range of options.
Peter Mayer of Building LifePlans reviews
these and their whole life costs.


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 are also used for new roofs. Systems which can withstand heavy traffic may be specified for balcony surfaces and walkways. Expected service lives vary from between 5 and 30 years.

European Technical Approval Guideline ETAG 005, identifies seven generic liquid-applied waterproofing kits and sets out tests related to durability: 5, 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, American 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 - 6mm 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 – 3mm 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 0.6 – 2mm. Expected service lives are between 10 and 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:
• Ensure 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

• 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.


Specification options


Liquid applied roof waterproofing systems Capital cost
Net present value for 60 years £/m2 Service life
Modified bitumen      
Liquid modified bituminous system minimum 3mm thick, surface reflective coat. 40 141 10 - 20
Inverted roof: hot applied bitumen liquid membrane in two coats minimum 6mm thick with reinforcement, protective sheet, insulation board, separation layer and 50mm thick concrete paving slabs. 90 131 60
Polyester based system      
Polyester based system, minimum 1.5mm thick applied in two layers with pigmented top coat. Third party assurance for 20 years life expectancy 42 116 20
Polyester based system, minimum 1.5mm thick applied in two layers with pigmented top coat. Third party assurance for 25 years life expectancy 48 113 25
Polyurethane based system      
Polyurethane based system, minimum 1.4mm thick two layers. Third party assurance for 15 years life expectancy 40 131 15
Polyurethane based system, minimum 2mm thick, three layers. Third party assurance for 25 years life expectancy 50 114 25


Table notes

• A discount rate of 3.5% is used to calculate net present values.

• Liquid-applied roof waterproofing systems in the cost table have third party certification. Where a service life range is given, an average is used in the whole life cost calculation.

• Costs are based on rectangular 50m2 flat roof for new work, with allowances for outlet, overflow pipes and upstands, liquid cold applied, unless other wise stated. No allowances for access. Costs for remedial work are very variable relating to the nature of the roof to be covered so have not been included in this analysis.

• Maintenance includes for inspections, recoating surface coats where applicable and an allowance for minor repairs.

• A cost analysis based on project specific information is essential for a realistic best value appraisal.

First published in Building 2006


Further information

BLP provides latent defect warranties for buildings www.blpinsurance.com

Further information contact peter.mayer@blpinsurance.com or telephone: 020 7204 2450