Lifetime costs: flat roofs

Flat roof coverings have an unfortunate reputation for premature failure and absorbing a large proportion of the maintenance budget. This view of flat roofs is now somewhat dated as the manufacturing of flat roofing materials has improved significantly since the 1970s. Longevity depends as much on correct design and detailing, specification, installation and maintenance as the quality of materials used.
Experience suggests that causes of premature failure are mainly due to workmanship; a smaller proportion of early failures are attributed to design deficiencies, and only a small percentage are attributed to material defects. Nevertheless, the combined effects of ultraviolet radiation, moisture and temperature variations, plus oxidation, can cause embrittlement, cracking and ultimately water penetration. Selection and specification of flat roof coverings should be carried out with the knowledge of the options available and the implications for longevity.

<B>

• Factors affecting durability</b>
• <B>Exposure</b>
• Bitumen is affected by direct exposure to solar radiation, which makes it brittle and leads to the increasing risk of mechanical damage.
• <B>Thermal stressing</b>
• Asphalt, and to a lesser extent bitumen membranes, should be protected from extremes of temperature, especially on sloping or vertical surfaces. This can be achieved by the use of solar reflective paint or stone chippings or by placing insulation over rather than under the membrane. Thermal movement of insulation under the membrane can induce stress cracks in the membrane itself. Slip membranes can overcome this, as well as partial bonding and specifying insulation with a low thermal co-efficient.
• <B>Maintenance</b>
• Solar reflective paint may require renewal every three years. Reflective stone chippings should be inspected and re-bonded in areas subject to "wind scour".
• <B>Usage</b>
• Where regular traffic is expected, compressible insulation under the weathering layer should be avoided. Alternatively the load can be spread using paving tiles or flags.
• <B>Choice of material</b>
• Bituminous materials have improved tremendously over the last 25 years with the introduction of polymer-modified bitumen and bases that are more able to resist thermal stressing, mechanical damage, ultraviolet exposure and substrate movement. For maximum durability, the use of unmodified bitumen and organic, asbestos and glass bases should be avoided.
• <B>Detailing and workmanship</b>
• Poor detailing and workmanship is the most common cause of flat roof failure. Ensure that good practice and manufacturer's recommendations are followed, especially the use of angle fillets, isolating membranes and the correct grades of bitumen. Single-ply membrane roofs are especially reliant on good workmanship, but are generally only fixed by contractors licensed by the manufacturer.
• <B>
• Modes of failure </b>
• <B>Cracking</b>
• In asphalt, cracking can be related to thermal stressing where movement is locally restricted, for example by plant bases. Cracks in bitumen membranes may be related to substrate movement; or brittleness induced by solar radiation; or differential thermal or moisture movement.
• <B>Slumping</b>
• With asphalt, slumping is related to inadequate keying on vertical and sloping work. It is exacerbated by lack of solar protection.
• <B>Blistering</b>
• In asphalt or sheet materials, blistering can be caused by trapped constructional moisture in the substrate and the omission of an isolating membrane or ventilating membrane. Lines of blisters may appear where the isolation layer has not been lapped but laid with gaps.
• <B>Joint failure</b>
• Characterised by "fish mouthing" or rucking at joints, this is caused by inadequate amounts of bitumen at joints, or the wrong grade of bitumen. Joint failure may also be caused at upstands due to thermal stress induced where membrane is laid over or under insulation on the roof surface, but no insulation is used at the brick or concrete upstand. Failure may also occur where insufficient allowance has been made for movement joints.
• <B>
• Durability tips</b>Maintain solar protection to minimise deterioration caused by thermal chemical reactions, ultraviolet degradation and thermal movement.
• Recoat solar reflective paint every three years. Regularly inspect and replenish chippings as necessary.
• Chippings generally offer a more effective solar protection on the flat surfaces of asphalt roofs. Solar reflective paint comes into its own for upstands.
• Use polymer-modified asphalt and bituminous membrane materials. Styrene butadiene styrene (SBS) enhances low-temperature performance. Atactic polypropylene (APP) increases high-temperature stability. When roofing in cold conditions – that is, at temperatures below 5°C – use the more flexible SBS-modified bitumen membranes. APP-modified bitumen membranes can offer improved weathering characteristics.
• Inspect coverings annually and repair all mechanical damage directly.
• Use of stainless steel or copper-faced bituminous membranes will extend service life by up to 10 years.
• Protect surfaces with additional applied membranes or tiles where regular foot traffic is expected.
• Thicker single-ply membranes will result in better durability.
• Keep number of designed perforations through the roofing membrane to a minimum; ideally, design out perforations.
• Use an inverted roof system wherever possible; the waterproofing layer is protected by insulation.
• Where a roof covering shows signs of wear or deterioration, correct diagnosis of cause is essential to ensure that the covering is not replaced prematurely or that an effective repair is specified.
• Consider over-roofing as an option, provided the roof structure can tolerate the increased load and good practice details can be maintained – at upstands, for example. Typically, a built-up roofing system or a single-layer torch-on system may be applied on to a built-up felt or mastic asphalt roof.
• Specify falls greater than the absolute minimum recommended – 1:40 rather than 1:80 – to allow for variations in construction and to avoid ponding.
• Avoid complicated roof designs; minimise steps, corners and junctions.