Flat roof coverings: Membrane & Asphalt

Single ply membrane: TPO (Thermoplastic Polyolefin)
Single ply membrane: EPDM (Elastomeric membranes)
Single ply membrane: PVC membranes
Mastic Asphalt
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

Pro

Reusable, particularly if mechanically fixed or better still ballasted with water or pebbles

Pro

Some products include recycled content

Pro

Durable - high resistance to UV, ozone and chemical exposure

Pro

Does not pollute rainwater run-off

Con

Comparatively new product, relatively untested over long periods

Con

Not as flexible as PVC

Con

Not easily recyclable

Con

Derived from oil, a non-renewable resource

Con

Pollution associated with the extraction and refining of oil

Con

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)

roof
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.

Pro

Reusable if it has been laid loose or mechanically fixed

Pro

Highly durable - particularly resistance to UV light and ozone

Pro

Does not pollute rainwater run-off

Con

Not easily recyclable

Con

Derived from oil, a non-renewable resource

Con

Pollution associated with the extraction and refining of oil

Con

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.

Read more: Toxic Chemicals: Polyvinyl chloride (PVC)

Pro

Durable

Pro

Some manufacturers provide recycling schemes

Pro

Good resistance to pollutants

Con

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.

Pro

Long track record, it is a well-understood roofing system

Pro

Durable where provided with firm continuous support

Con

Where derived from oil, pollution associated with extraction and distillation

Con

Where derived from natural deposits, degradation of the landscape

Con

Brittle when set

Con

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.

Pro

High material efficiency

Pro

Tried and tested waterproofing ability

Con

Un-recyclable and un-reusable

Con

Ultimate performance depends to a high degree on the quality of installation

Con

Bitumen is refined from a non-renewable resource

Con

Pollution associated with extraction and refining of oil

Con

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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