British Board of Agrément or other third-party-approved polymer-modified asphalt, supplied in blocks similar to traditional asphalt, or in molten form in purpose-built transporters. Applied to low-traffic roofs or accessible roofs with additional protection (for example, tiling or paving). Traditional asphalt application and detailing in accordance with BS 8218.
Capital cost £30/m2
Net present value for 60-year life £108/m2
Service life 50 years
Asphalt minimum thickness 20 mm applied in two layers horizontally or up to 30° pitch, three layers vertically or more than 30° pitch with vapour barrier below. Three-layer work applied to bitumen-primed stainless steel lathing. Asphalt to BS 6925 Type R988.
Capital cost £25/m2
Net present value for 60-year life £104/m2
Service life 40 years
Single-ply polymer
Single-ply polymer elastomeric membrane with BBA or other third-party assurance installed in strict accordance with manufacturer’s details.
Capital cost £23/m2
Net present value for 60-year life £31/m2
Service life 25 years
Single-ply polymer thermoplastic membrane with BBA or other third-party assurance installed in strict accordance with manufacturer’s details.
Capital cost £30/m2
Net present value for 60-year life £40/m2
Service life 25 years
Built-up bitumen membrane systems
Modified bitumen cap sheet having BBA certificate with underlayers and bonding in accordance with cap sheet manufacturer’s recommended specification/BBA certificate requirements (or other third-party assurance).
Capital cost £33/m2
Net present value for 60-year life £44/m2
Service life 25 years
Polyester-based cap sheet to BS 747 Type 5 with underlayer and bonding in accordance with cap sheet manufacturer’s recommended specification
Capital cost £30/m2
Net present value for 60-year life £44/m2
Service life 20 years
Glass fibre-based cap sheet to BS747 Type 3 with underlayer and bonding in accordance with cap sheet manufacturer’s recommended specification.
Capital cost £21/m2
Net present value for 60-year life £30/m2
Service life 15 years
Factors included in whole-life cost over 60 years
Initial supply and installation, removal and replacement at end of service life. Recoating solar reflective paint on asphalt every three years. Costs are discounted to net present values at a rate of 6%.
Factors affecting durability
Exposure
Bitumen is affected by direct exposure to solar radiation. It becomes more brittle, leading to greater risk of mechanical damage.
Thermal stressing
Asphalt, and to a lesser extent bitumen membranes, should be protected from extremes of temperature, particularly on sloping or vertical surfaces. This can be achieved by using solar reflective paint or stone chippings, or by covering the asphalt with insulation.
Maintenance
Solar reflective paint requires renewal every three years. Reflective stone chippings should be inspected and rebonded in areas subject to “wind scour”.
Choice of material
Bituminous materials have been improved tremendously over the past 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 bases of unmodified bitumen, organic material, asbestos and glass should be avoided.
Use
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.
Detailing and workmanship
Poor detailing and workmanship are the most common causes of flat roof failure. Ensure that manufacturer’s and good practice recommendations are followed, particularly in the use of angle fillets, isolating membranes and the correct grades of bitumen. Single-ply membrane roofs are reliant on good workmanship, but are generally fixed by contractors licensed by the manufacturer.
Durability tips
- Maitain solar protection. Recoat solar reflective paint every three years. Regularly inspect and replenish chippings as necessary.
- Use polymer-modified asphalt and high-performance bituminous membrane materials.
- Inspect coverings and repair all mechanical damage annually. Use of stainless steel or copper-faced bituminous membranes will extend service life by up to 10 years.
- Where regular foot traffic is expected, protect surfaces with additional applied membranes or tiles.
- Use of thicker single-ply membranes will result in increased durability.
- Reduce the number of designed perforations through the roofing membrane.
- Wherever possible, an inverted roof system, where the waterproofing layer is protected by insulation, should be used.
Modes of failure
Cracking
In asphalt, cracking can be related to thermal stressing where movement is locally restricted. Cracks in bitumen membranes can be related to substrate movement or brittleness induced by solar radiation.
Slumping
With asphalt, slumping is related to inadequate keying on vertical and sloping work. It is exacerbated by a lack of solar protection.
Blistering
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.
Joint failure
This is characterised by “fish mouthing” or “rucking” at joints and is caused by an inadequate amount of bitumen at joints or the wrong grade of bitumen.
