Durability - High performance roofing
High
performance built-up bitumen coverings have expected service lives in
excess of 20 years. Peter Mayer of Building LifePlans
considers some options and whole-life costs for flat roofs.
Introduction
The key qualities which 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 around 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 or 5B have a fine mineral surface and need a surface
coating to be used top layers.
• 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BS 747 is due to be withdrawn at the end of July 2006 to be replaced by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 flexibly 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 the 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 as a minimum and also after events which 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 only 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 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 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
Specification options
| Capital cost £/m2 | Net present value for 60 years £/m2 | Service life years | |
| Modified bitumen | |||
| Three layer SBS modified bitumen system, top layer 4mm thick, polyester fibre base 250g/m2 | 51 | 92 | 20 - 30 |
| Two layer APP modified bitumen system, top layer 4.5mm thick, polyester fibre base weight 180g/m2 | 42 | 77 | 20 - 30 |
| BS 747 bitumen | |||
| Three layer system, BS 747 Type 3E top layer, weight 2.8kg/m2, glass fibre base weight 55g/m2 | 16 | 53 | 7 - 15 |
| Two layer system to insulation sub-base, BS 747 Type 5E top layer, weight 4.5kg/m2, polyester fibre base weight 330g/m2 | 30 | 70 | 15 - 25 |
| Two layer system to insulation sub-base, BS 747 Type 5B top layer weight 4.0kg/m2, polyester fibre base weight 330g/m2, with solar reflective paint coating | 36 | 82 | 15 - 25 |
| Three layer system, BS 747 Type 5E top layer, weight 4.5kg/m2, polyester fibre base weight 330g/m2 | 40 | 80 | 15 - 25 |
| Three layer system, BS 747 Type 5B top layer, weight 4.0kg/m2, polyester fibre base weight 330g/m2, with solar reflective paint coating | 42 | 90 | 15 - 25 |
Table notes
• A discount rate of 3.5% is used to calculate net present values.
• Costs are based on covering a simple rectangular 500m2 concrete
flat roof. Covering system, eaves, verges and abutments only are included.
Service lives based on average life expectancy. Whole-life costs include
allowance for inspections, cleaning, minor repairs and repainting solar
coating where applicable.
• A cost analysis based on project specific information is essential
for a realistic best value appraisal.
First published in Building 2006
Further information
Building LifePlans provides latent defects warranty
for all buildings.
BLP Construction Durability Database at www.componentlife.com
provides durability information for building components.
Further information contact peter.mayer@buildinglifeplans.com
or telephone: 020 7204 2441.
