Flat roof coverings
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Single ply membrane: TPO (Thermoplastic Polyolefin) |
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Single ply membrane: EPDM (Elastomeric membranes) |
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Single ply membrane: PVC membranes |
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Mastic Asphalt |
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Built-up felt |
Key issues
• Embodied energy - there's a wide scale of embodied energy from wood shingles at the low end of the range to aluminium at the high. The main energy components are those derived from manufacture and transport. As with other materials, sourcing a product in the UK (and close to site if possible) can dramatically reduce its embodied energy.
• Material efficiency - Selecting a material can be made more complicated once its efficiency is considered. For example in some circumstances it can be argued that a strong, lightweight material used over a large span can have a lower environmental impact than a heavier, more sustainable material once the degree of structural support is introduced into the equation.
• Durability - The longer lasting roofing material has a lower environmental impact.
Materials
Single-ply membrane: TPO (Thermoplastic Polyolefin)
Closely-associated with other thermoplasic elastomers, thermoplastic polyolefin elastomers (TPOs) are two-component elastomer systems consisting of an elastomer (such as ethylene-propylene-diene monomer or EPDM) finely dispersed in a thermoplastic polyolefin (such as polypropylene) where thermoplastic polyolefin is usually the major component.
TPO's usage has spread from the car industry and since around 1990 had been making steady inroads in construction until the last few years when it's expansion accelerated when its superior environmental credentials became more recognised. TPO roofing membranes differ from EPDM membrances in that they are easily heat-weldable and, allegedly provide greater resistance to chemical and biological attack
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Reusable, particularly if mechanically fixed or better still ballasted with water or pebbles |
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Some products include recycled content |
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Durable - high resistance to UV, ozone and chemical exposure |
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Does not pollute rainwater run-off |
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Comparatively new product, relatively untested over long periods |
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Not as flexible as PVC |
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Not easily recyclable |
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Derived from oil, a non-renewable resource |
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Pollution associated with the extraction and refining of oil |
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Like all single-ply membranes - prone to punctures |
Thermal conductivity
unknown
Density
Typically: Density 890-910 Kg/m3 (2)
Embodied energy
Synthetic rubber generally: 120 MJ/kg (1)
Design life
20 years
Single-ply membrane: EPDM (Elastomeric membranes)
Elastomeric systems are dominated by EPDM. Ethylene propylene diene monomer is a type of synthetic rubber made through the polymerization of ethylene and propylene in combination with ethylidene norbornene.
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Reusable if it has been laid loose or mechanically fixed |
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Highly durable - particularly resistance to UV light and ozone |
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Does not pollute rainwater run-off |
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Not easily recyclable |
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Derived from oil, a non-renewable resource |
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Pollution associated with the extraction and refining of oil |
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Like all single-ply membranes - prone to punctures |
Thermal conductivity
unknown
Density
unknown
Embodied energy
Synthetic rubber generally: 120 MJ/kg (1)
Design life
20 - 30 years (though 40+ years have been reported)
Single-ply membrane: PVC
PVC is made by combining ethylene and chlorine to produce ethylene dichloride (EDC) which is then processed to produce vinyl chloride monomer (VCM). The VCM gas is polymerised into vinyl resin. Finally the resin is 'compounded' through the adding of plasticizers for flexibility, stabilisers for durability and pigments for colour.
PVC continues to have a dominant role in the single ply market. Conventionally, PVC has offered a tried-and-tested and durable roofing solution, but in recent years, concerns about the role of PVC in the environment have witnessed moves away from the material to alternative membranes that are felt to present a lesser risk.
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Durable |
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Some manufacturers provide recycling schemes |
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Good resistance to pollutants |
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For many designers, the threat to the environment posed by the manufacture, use and disposal of PVC, renders it unacceptable. |
Thermal conductivity
PVC generally: 0.16 W/mK
Density
PVC generally: 1380 Kg/m3
Embodied energy
PVC generally: 77.2 MJ/kg (1)
Design life
20 - 30 years
Mastic Asphalt
Mastic asphalt is a blend of asphalt, limestone powder and limestone aggregate. The bitumen is derived from the distillation of crude oil or from natural deposits.
Polymer-modified mastic asphalts, usually containing styrene butadiene styrene block copolymers, provide enhanced flexibility and a reduced susceptibility to the effects of temperature.
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Long track record, it is a well-understood roofing system |
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Durable where provided with firm continuous support |
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Where derived from oil, pollution associated with extraction and distillation |
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Where derived from natural deposits, degradation of the landscape |
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Brittle when set |
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Needs thermal protection |
Thermal conductivity
1.15 W/mK (3)
Density
2330 Kg/m3 (3)
Embodied energy
2.6 MJ/kg (1)
Life expectancy
60 years
Built-up felt
Built-up roofing consists of two or three layers of bitumen sheet bonded together through adhesive or hot bitumen.
The bitumen is produced through the distillation of crude oil.
Reinforced bitumen sheets are manufactured using a base layer consisting of one of a number of materials including organic fibres, glass fibre or bitumen-saturated polyester, though in practice, organic felts rarely meet performance criteria. The material is passed through a bath of molten bitumen and then coated with sand to prevent adhesion.
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High material efficiency |
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Tried and tested waterproofing ability |
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Un-recyclable and un-reusable |
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Ultimate performance depends to a high degree on the quality of installation |
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Bitumen is refined from a non-renewable resource |
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Pollution associated with extraction and refining of oil |
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A by-product of polyester fibre production is the toxic Methyl Bromide |
Thermal conductivity
0.16 W/mK (3)
Density
960 Kg/m3 (3)
Embodied energy
unknown
Life expectancy
15 - 30 years depending on type and quality of installation
References
1 Inventory of Carbon & Energy (ICE) - Version 1.6a - Hammond & Jones, Univ Bath
2 Sandene Ltd.,Safety Data sheet
3 CIBSE Guide A 1999
Flat roof covering membranes on GreenSpec
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