Surveying A Flat Roof | Materials And Their Properties

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Part three of our guide explores the various flat roof materials you will encounter when surveying a flat roof.

It is of course imperative that you have a good understanding of the different flat roof waterproofing systems which are available and also those which you may find on the roof you are surveying. You need to understand their features, strengths and benefits – this will help you decide what system should be used and will give the best value.

Flat Roofing Systems

The below are common flat roofing materials which are used for refurbishment projects.

Single Ply Membrane

There are a large number of single ply systems, the most popular being EPDM rubber and PVC. They can offer an economical and fast track solution on large featureless roof areas and are frequently used in the new build environment. Because there is only one layer of waterproofing, it can be easily punctured, and even a small failure can have significant consequences. This can be a particular problem where future access is required for recreation or maintenance. People who are unaware of the characteristics of the membrane often abuse it. This risk is made higher when incoming tenants are free to install new plant and roof top equipment.

Most single ply membranes come to site in large rolls to reduce the number of joints and overlaps. The rolls have to be cut and joined together on site and the membrane has to be cut around all details and features. The laps and joints are the greatest weakness in these systems. Different systems use different methods to weld the joints – some use mechanical fasteners, others are self-adhesive, some use tape, others are glued but by far the most common method is to heat weld the two surfaces together. Although these seams are sealed, they are more vulnerable to leaks than other roofing systems. The welded joints require a three-part weld and any imperfections which compromise the adhesion, such as trapped moisture, dust, variances in the level of heat used, or small imperfections in workmanship, can result in failure.

The systems are usually installed using hot air guns, and whilst this is undoubtedly a lower fire risk than the use of naked flames, the hot air guns are required to weld at extremely high temperatures (typically between 300 – 600 degrees) creating a significant fire risk.

High Performance Built Up Roofing Felt Systems

Usually referred to as ‘roofing felt’ these systems provide a time honoured, tested and reliable solution. The material is delivered to site in rolls and heat welded with bitumen. The lap seals are less dependent on the exacting conditions required for single ply membranes. The membranes are usually laid as a build-up of several layers and because the overall system is thick and durable they are far less likely to be punctured by occasional access or maintenance.

In the early years of their development 100 years ago, roofing felts were poor in quality but modern roofing felts are manufactured with polyester reinforcement carriers and with modified bitumen, which greatly enhance their performance, allowing them to cope well with movement, freeze thaw cycles and ultraviolet light. This, coupled with a greater understanding of flat roof design, means that high performance felt systems are at least as durable as anything else on the market.

Most roofing felt systems rely upon the use of naked flames or heat for their installation. However, roofing felt systems have now been developed where the use of heat is minimised or removed completely. This is particularly important at junctions with cladding or pitched roofs.

The NFRC Safe2Torch campaign has been widely adopted by the roofing industry and reduced this risk even further.

These systems are generally more economical than any other.

Cold Applied Liquid Waterproofing Systems

Liquid applied waterproofing systems are applied without the use of heat. They are versatile and adaptable and can be applied under low-lying plant and around complex and detailed roof areas. The material seals to itself and so the finished covering is seamless, with no joints which lends its use to complex detailed roofs.

This characteristic also makes it easy to repair or adapt if new plant is installed or removed at a later date.

The material is delivered to site in small drums so that it can be transported to the roof and stored easily. This can enable existing services such as lifts to be used to transport material to the roof, rather than requiring the use of external scaffolding and hoists.

Where there is concern over the use of heat or naked flames, cold applied applications provide an excellent, safe alternative.

There are different types of liquid systems and some have been developed for use on car parks, balconies and walkways.

Liquid systems are generally one of the more expensive options.

Mastic Asphalt

Mastic asphalt is a liquid applied system, but it is applied hot after being melted at up to 230 degrees centigrade. As well as the obvious disadvantages that hot systems embody, this also reduces its practical use for application under low-lying plant or narrow gutters; the heat simply makes it impossible to use in confined areas.

However, like other liquid applied systems, it is adaptable and can be used to form difficult details. However, unlike them, because it is unreinforced, it does not cope well with movement and is best used on stable substrates where there is unlikely to be any significant movement.

Mastic asphalt combines an aggregate within it, so it is ideal for use where there will be occasional access. It can also be used for car parks and walkways. However, in hot conditions it can become very soft and anything left lying on the surface will gradually sink into it and this can cause punctures. Therefore, contrary to popular belief, mastic asphalt also requires protection or adaptation if it is to be trafficked regularly.

Mastic asphalt can be badly degraded by ultraviolet light and heat. Therefore, to reduce the risk of premature failure, it should be coated regularly (every 3 – 4 years) with white solar reflective paint. This creates a maintenance liability.

Whilst older rock asphalt had a long life span the life span of mastic asphalt is in the region of 7.5 – 15 years, depending on exposure. Mastic asphalt is one of the most expensive flat roof options.


Fibreglass is also a liquid applied membrane. Many cold applied liquid systems incorporate fibreglass. However, the resins used to form fibreglass products like boats have different qualities and do not lend themselves for use on large roof areas. Therefore, we have only ever seen these products used on small roof areas, such as domestic extensions or porches and they have not broken into the commercial roofing market.

Our next article will explore further aspects of materials used in flat roofing, such as insulation, as well as look at the many types of roof decks.

This article, as with all other articles we produce, is for guidance purposes only. It does not constitute formal advice and should not be relied upon as such. For bespoke, unbiased advice relating to your commercial roofing project please contact us and we would be pleased to assist.

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