Costs: Flat roofs

The key qualities that determine the durability of bitumen roof sheeting are:

• Ability to withstand movement. Daily thermal cycling for example may subject roof coverings to temperature ranges of about 100°C where the covering is directly above insulation, resulting in fatigue failure.
• Resistance to weathering, mechanical damage, puncturing and tearing.

Specification options

Confirmation of material quality and suitability for roofing application is provided by tests to BS 747 or by third party methods of assessment and testing. BS 747:2000 for reinforced bitumen sheets for roofing includes:

• Class 3 bitumen sheets with a glass fibre base, often used for sheds but not recommended for dwellings.
• Class 5 bitumen sheets, polyester fibre base with oxidized bitumen coating. These are high performance coverings with a fatigue resistance 10 times that of class 3.

Classes are further distinguished by type where used as top layers:

• Types 3B and 5B have a fine mineral surface and need a surface coating to be used as a top layer.
• Type 3E or 5E sheets include a granular surface.

BS 747 is due to be withdrawn at the end of July 2006 to be replaced by the European Standard for reinforced bitumen roofing sheets, BS EN 13707. A new standard, BS 8747, is being developed to give guidance on the selection and specification of reinforced bitumen membranes.

Modified bitumen roofing sheets

BS 747 does not cover modified bitumen roofing sheets. For performance assurance, specify products that have undergone relevant methods of assessment and have third party certification. Modified bitumens have a fatigue resistance 20 times better that BS 747 Class 5 sheets.

• SBS (styrene butadiene styrene) modifies the bitumen to give it greater flexibility. The cold flex temperature test gives a measure: the lower the better, products are tested to -25°C but values lower than this are achievable.
• APP (atactic polypropylene) modifies the bitumen to enable torch-on application as well as improving flexibility at low temperatures.

Direct comparison of modified bitumen products can be problematic as different tests may be used to appraise performance. The best test is to analyse performance in service.

Durability issues

Material choice and quality is only one factor in the long-term performance of built-up membrane roofing. Bitter experience has shown it is often design and workmanship shortcomings that have resulted in premature failures or expensive remedial work.

Good practice guidance is provided by:

• BS 8217:2005 for design, installation and maintenance of reinforced bitumen membranes for roofing.
• BS 6229:2003 is the code of practice for flat roofs with continuously supported coverings. It recommends these are inspected at least twice a year in spring and autumn and also after events that may have had a detrimental effect on the roof covering, such as violent storms.
• Workmanship is covered by BS 8000-4:1989 - but this standard is a bit dated and covers a limited range of issues.

Specifications for individual projects should draw on the relevant and applicable clauses from the standards and manufacturers' recommendations to ensure the roof system satisfies the project performance requirements.

Key design and workmanship issues

• Don't design flat roofs that are too flat: specify an incline of at least 1:40, to ensure achieving a 1:80 minimum fall, taking into account construction tolerances.
• Simplify the design and make use of preformed details where possible to reduce the risk of workmanship faults.
• Avoid penetrations and locating services plant on the roof covering where possible.
• Reduce the risk of interstitial condensation by specifying fully sealed vapour control layers or sufficient ventilation.
• Ensure the fastening-down method is suited for the following factors: the substrate, expected wind uplift, roof configuration and size and roofing sheet, especially at roof edges and upstands.
• Three-layer systems may be required to provide separation from a substrate, which retains some moisture, as well as offering increased movement capacity and improved puncture resistance.
• Consider specifying an inverted roof. The loading layer will protect the bitumen felt from weathering and foot traffic. A whole-life cost analysis can be used to appraise the benefits of expected increase in service life and reduced risk of failure against the increased capital cost and higher costs of repair.