Pitched roof coverings 2: Concrete tile, slate & rubber

	Concrete tile and slate
	Fibre cement slate
	Reconstituted slate
	Rubber tile / slate

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

Concrete tiles and slates

Concrete roof tiles and slates are perhaps the most economic form of roof covering. Concrete tiles compete directly with 'natural' clay and slate in providing a wide range of formats and colours as well as an ability to closely imitate the colour, form and texture of the original.

Concrete roof tiles and slates are typically made using a mortar mix with a sand grading known as 'sharp sand'. Iron oxide pigments are added to give the desired colours. The mix is extruded under high pressure before being placed in moulds and then cured at temperature before being coated, dried and packaged.

The main environmental impact of concrete tiles and slates is of course as part of its cement component - though it competes very well against fired clay tiles in its more modest embodied energy rating.

Thermal conductivity

Typically 0.84 W/mK ⁽³⁾

Density

Typically 2100 Kg/m^{3 (1)}

Embodied energy

2.0 MJ/kg ⁽¹⁾

Life expectancy

50 - 60 years

Fibre cement slates

Fibre cement slates are normally perceived as the economic alternative to natural slates. The slates are manufactured using Portland cement, silica and fillers together with a blend of cellulose and synthetic fibres. The mix is pigmented during production before being compressed under high pressure and air cured (though some products are autoclave-cured to accelerate the process).

What fibre cement slates lack in natural appearance, they can make up in ease and flexibility of instalment which together with a predictable uniform aesthetic can often make them the preferred design choice.

Manufacturers of fibre cement slates are quoted as saying that their products are more energy-efficient than natural slates, but this claim has yet to be demonstrated convincingly.

Thermal conductivity

(unavailable)

Density

(unavailable)

Embodied energy

8.5 MJ/kg ⁽⁸⁾

Life expectancy

60 years ⁽⁸⁾

Reconstituted slates

Another alternative to the use of natural slates are slates made from reconstituted crushed slate (typically 60 - 80% content). This method has the advantage of being able to make use of waste from quarrying whilst providing a close match to the appearance of pure slates.

Slates manufactured in this manner can be moulded to imitate the simple format of natural slates or to provide an interlocking facility for application on lower-pitched roofs.

Reconstituted slates are made by crushing natural slate and mixing it with other aggregate, pigment, resin and glass-fibre reinforcement. The mix is then applied to a mould where it is heated and compressed.

Thermal conductivity

Typically 1.5 W/mK ⁽⁹⁾

Density

Typically 2100 Kg/m^{3 (9)}

Embodied energy

(not currently available)

Life expectancy

60+ years ⁽⁹⁾

Rubber tiles / slates

Using waste as a raw material is highly desirable - reducing the quantity of material heading for landfill whilst reducing the environmental damage resulting from the extraction and processing of raw materials.

Recent developments led by WRAP have developed techniques whereby tyres are ground down to produce rubber granulate. The granulate is moulded to form tiles or slates which benefit from low weight and durability.

It is worth noting too that BBA Certification is available with some products.

Thermal conductivity

n/a

Density

Typically 1100 Kg/m^{3 (2)}

Embodied energy

n/a

Life expectancy

20+ years ²

Roof tile and slate products on GreenSpec

 

References

1 Inventory of Carbon & Energy (ICE) - Version 1.6a - Hammond & Jones, Univ Bath
2 Dynaslate
3 CIBSE Guide A 1999
4 Sandtoft safety data sheet
6 Source: Lawson Buildings, Materials, Energy and the
Environment (1996)
8 Marley Eternit, from a written response 2010
9 Monier, from a written response 2010
3 CIBSE Guide A 1999

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