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Flat roofs, or more specifically roofs with a fall of less than 10 degrees, form a significant part of the commercial roofing market. There is a wide range of choices for the specifier and many compete head to head in terms of specification, cost and performance.

1. Built-up

The built-up, multilayer felt roof dates from the 1940s and uses several layers of bitumen-impregnated felt. It is a practical solution, being relatively cheap and long-lived, as well as being easy to repair and maintain.

Today the basic bitumen sheet has been improved with the use of electrometric coatings and by interlacing the bitumen with a polymer weave. This gives it better movement properties and greater strength. It is usually installed as a three-layer system consisting of a base layer bonded to the substrate or insulation, an intermediate layer providing primary waterproofing and a top sheet that may have a decorative solar reflection and slip reduction coating. The top sheet can come in a number of colours. Variations include using electrometric mineral sheets in two-layer or even one-layer systems.

A downside of built-up roofs is that they do not look as neat as other systems. This is because bitumen bleeds from the joints when the roof is laid; in fact, this is encouraged to show that a good bond has been achieved. Additionally, if the fall is relatively small the build up at joints can cause areas of standing water.

The material is also rather thick - each sheet is between 3 mm and 4 mm - and consequently difficult to mould.

2. Single-ply

Developed more than 40 years ago, single-ply plastic membranes made from PVCu and its variants are now a significant force in the roofing market. Despite initial market resistance, thousands of installations have proved that single-ply roofs demonstrate long life, low maintenance and an aesthetically pleasing result.

The market-leading membranes are still based on PVC with many less popular materials also available. To satisfy environmental concerns about PVC, most makers offer a non-PVC variant, including thermoplastic olefin or polyolefin made from ethylene propylene rubber. These are commonly called "TPO".

The finished result of PVC-based single-ply membranes can look very neat and tidy. They are available in a range of thicknesses and colours, and can be provided in bonded and mechanically fixed versions. Fleece-backed versions are also available. It is important to minimise traffic on the completed roof. All maintenance paths and areas should have purpose-made protection consisting of promenade tiles made from recycled stock of the material.

3. Asphalt

Mastic asphalt is one of the oldest forms of roofing and still one of the most reliable - two-layer roofing grade asphalt is hard to beat for performance. Being a poured material, it can form complex shapes, fit in tight corners and be formed to suit the fall and outlets. However, vertical surfaces, even with reinforcement, may slump when subjected to the summer sun, although most applications are finished with a reflective coating to minimise this. Mastic asphalt is easily repaired and refurbished. For best results it must be laid on a solid substrate. A big disadvantage is that its application requires hot working with all the consequent risks of fire.

4. Metal

Flat metal roofs are formed by making trays and joining them together. These have been around for centuries - the lead roofs on many churches are probably the most common example. Highly durable and relatively reliable these were once the highest quality roof. Today they are used mostly for heritage or aesthetic reasons, rather than for their performance.

Lead is the oldest form of flat roofing, followed by copper and zinc.

5. Insulation

It is important to match the performance of the roof with the appropriate insulation. Mineral rock fibre, blown plastic foam and extruded boards are the most common types. The cheapest and most common is blown plastic board. This is relatively durable but is flammable. Variants with fire retardants do satisfy some standards: the specifier should read the small print carefully.

Mineral rock fibre is less of a fire risk, but do check its density, as it is available in a number of different kinds. Some will allow foot traffic on the finished roof but others will go spongy if walked on.

It is also important to ensure that a vapour control layer is installed, irrespective of a building's use, in case the use changes to one where greater levels of vapour are present. This layer can also double up as an air infiltration seal.

6. Protection

If access to the roof is necessary, for example to inspect plant, then protection must be installed. It may be worth considering installing an inverted detail around plant areas, using paving slabs as the working surface.

An alternative is to use an appropriate separating membrane under the plant base but adequate protection around plant areas is still necessary.

7. Maintenance

Flat roofs will require regular and proper maintenance to maximise their life. Ensure that the documentation contains the maker's recommendations, and that a clear understanding of maintenance is demonstrated. Many guarantees are void unless maintenance inspections are undertaken.

8. Safety

Safety and maintenance go hand in hand as the roof that requires minimal maintenance is a safer roof. A roof that avoids ponding and does not promote the growth of lichen or algae is also safer.