Flat roofing

Numerous liquid applied roofing systems have been developed and specified for use in both new-build and repair situations. The performance of some coatings has not been satisfactory, but mainstream products are the result of considerable investment and research by producers and now offer considerable benefits over more traditional roofing systems or prefabricated single ply membranes.

Most of the new roofing materials are polymeric in nature meaning that they are made up of many ('poly') molecules ('mer') that are linked together in a chain; in effect forming very large molecules of different chemicals, for example, poly(propylene) or poly(isoprene). Liquid applied membranes offer certain advantages over the traditional membrane materials such as asphalt and built-up felt systems. They are lightweight and flexible and can be worked around complicated details that would otherwise be difficult to achieve. However, their convenience can lead to a risk of poor application or selection with the inevitable result that the roof performs unsatisfactorily.

In broad terms there are 2 types of membrane - those that are applied cold and those that are applied hot. Generally, the market for cold applied membranes lies in repair and maintenance - they are particularly suitable for application over pre-existing roof coverings without having the need to strip the old roof. Hot applied membranes are often used for new-build applications and require some form of surface protection.

Like all roofing materials, care in selection of the product and proper attention to detailing and surface preparation are key for ensuring long-term performance. Ease of application is no substitute for proper application.

European Technical Approvals

As a requirement of the Construction Products Directive, individual EU member states were required to prepare harmonised standards for liquid systems. This was achieved with the production of European Technical Approvals.

In the UK, a parallel system of certification is provided by the British Board of Agrément (BBA), with many certificates now having been issued on a variety of liquid roofing products - some suggesting an effective life of 25 years.

European Technical Approvals (ETAs) exist for:

• polymer modified bitumen emulsions and solutions;
• glass reinforced resilient unsaturated resins;
• flexible unsaturated polyester resins;
• hot applied polymer modified bitumen;
• polyurethanes;
• bitumen emulsions and solutions; and
• water dispersible polymers.

Solvent-based acrylic roofing systems are also available. These are not covered by ETAs and are often used for emergency repairs.

Most of the above products are cold-applied, but polymer-modified bitumens are very popular for new-build applications, while cold applied materials are generally (but not exclusively) used for remedial works. Both have good performance characteristics and if properly laid a life expectancy in excess of 30 years would not be an unreasonable expectation, although as with most materials this is highly subject to workmanship, protection, maintenance, etc.

Hot-applied rubberised bitumen membranes

By dispersing virgin or reclaimed rubber in asphalt together with limestone powder, oil and styrene butadiene-styrene (SBS) it is possible to manufacture a very flexible roofing membrane that can form a seamless membrane about 3-4.5mm thick. Some formulations include an additional spun bond polyester fabric interlayer with a further 3mm layer to form a roofing system. However, these materials are not usually resistant to UV light and so it is important to add a bonded membrane or alternatively a ballasted insulation layer to provide mechanical and solar protection.

The materials are usually supplied as cakes encased in polyethylene wrapping, which are melted in a heater to around 200°C. The melt is discharged from the heater into a suitable container and applied to the roof using long-handled, rubber-bladed squeegees.

Prior to application, the roof surface must be free from contaminants. Liquid membranes depend on a bond to the surface, so all traces of dust, grease and oil must be removed. Concrete surfaces are usually primed with a suitable asphaltic solution.

Cold-applied liquid membranes

These membranes are polymeric compositions with one or more ingredients, such as modified asphalt or coal tar pitch, or various resins or elastomers, such as polyurethane, dibromobutadiene, silicone or acrylic. The bitumen-based solutions are usually applied as emulsions, brushed or rolled in place.

Resin-based systems can often be supplied in different formulations to give 'guaranteed' life expectancies of up to 25 years. These materials can also be dressed with sand or mineral granules to provide slip resistance, and so are often used for waterproofing car-park decks, where a degree of friction is desirable and where various colour pigments can be used to advantage.

Cold-applied membrane showing excessive adhesion failure: probably laid over poor felt roofing

Cold-applied systems tend to be used for remedial operations, with the clear advantage that heat-producing appliances are not required and the membrane can be sprayed over existing finishes without the need for a temporary roof. The ability to form complicated weathering details with the minimum effort places the materials at a distinct advantage over BUR systems. Like hot-applied systems, it is usual to reinforce the membrane with a polyester fleece sandwiched between 2 layers of waterproofing. Some coatings require periodic refresher coats.

Limitations

While both hot and cold applied systems are relatively easy to lay, this does not mean that they tolerate lower levels of workmanship. Most systems require good substrate preparation - clear of sawdust, swarf, dirt, hydrocarbons or organic materials, while the maintenance of the correct dry-film thickness is important to ensuring future durability. Similarly, many products have only a limited shelf-life before they deteriorate (6 months or so).

From a designer's point of view, a failure to consider the usual principles of flat roofing design, rainwater disposal and the design of roof penetrations will lead to failure in much the same way as it would using more traditional methods. Similarly, because one of the great benefits of liquid membranes is their ability to bond to the substrate (and hence limit the propensity of water to travel through a sheathing layer) it is vital to ensure that the correct level of preparation is used and that the correct primers are employed. Compatibility with the substrate (and any likely contamination of the surface) must be established at the outset.

Incompatibility between substrate and coating has led to extensive peeling and delamination

One of the benefits of liquid roofing is its ability to accommodate difficult detailing, but it is no substitute for a failure to design details properly initially

Typical defects

• Embrittlement of membrane due to loss of plasticiser. Regular pattern of fine cracks, material loses flexibility: Possibly due to lack of UV protection (hot applied system) or contamination.
• Tearing, ruckling or splitting: Inability to deal with tensile stresses at places of concentrated movement, initiated, for example, by shrinkage in low temperature or creep during hot weather.
• Splitting of membrane, lack of sufficient elasticity: Ability to bridge cracks - particularly those that open and close cyclically as a result of changes in temperature or structural movements. If a membrane is fully bonded, the percentage elongation over, for example, a 3mm crack is very high.
• Puncturing: Debris or sharp materials dropped on roof, foot traffic, puncturing by plant growth.
• Biological attack: Colouring/mottling, softening and degradation.
• Loss of adhesion: Contamination of substrate with water, oil, sawdust, bitumen, etc. Incompatibility of background material.