As with most building materials, historically people didn’t have much choice as to what kept their houses dry, except that which was natural and locally available. Nowadays of course what’s naturally and/or locally available is largely irrelevant.
Hand-made or otherwise, clay tiles are the Mediterranean roof covering of choice – probably because they are inexpensive, have a good thermal mass and are hygroscopic thus helping to keep buildings cool. They won’t last too long in damp places susceptible to frost though – which is probably why they are less used in Northern Europe. They are reusable and fully recyclable.
One of the most economic and durable forms of roof covering, concrete tiles are also available in a wide range of shapes, colours, and textures.
Since they are impervious to moisture, they are unlikely to suffer from frost damage. However, due to the cement content they have a large embodied energy. They are reusable and recyclable as aggregate.
Fibre Cement Slates
Created as an economic and durable alternative to natural slates, these contain cement, silica, and a blend of both synthetic and natural fibres, which are compressed under high pressure and generally air cured.
They lack the organic appearance of natural slate, however their uniformity and light weight allows for ease of installation and re-use. Some of the synthetic ingredients are oil based, and highly persistent in the environment, and since they contain cement their embodied energy is high.
Not just corrugated iron panels, but also zinc, copper, stainless steel, and aluminium, which although potentially have a very high embodied energy, are all likely to include a high percentage of recycled material (up to 100%), thus bringing the embodied energy right down.
Interestingly, corrugated iron roofs are the roofing material of choice in much of the developing world, because they can be easily bought and sold as and when required.
Advantages are largely aesthetic, although they are of course 100% recyclable. Disadvantages are that they have no thermal mass but high heat transfer, and are the target of thieves.
Formed by mixing crushed slate waste with other aggregate, resin, and glass-fibres, reconstituted slates have more of a natural look than fibre cement. The mix is compressed into moulds and heated, adding to its embodied energy, as well as containing oil-based resins.
However, reconstituted slates make use of post-industrial waste, are durable and reusable.
Have a strange uniform look, but are made from 100% recycled car tyres, are reusable, very durable, and lightweight.
Stone and slate sit somewhere between clay and concrete, with sandstone being more susceptible to frost damage than slate. Stone tiles are particularly heavy and therefore require more roof structure to keep them up.
The decision to have stone or slate tiles is largely one linked to the local vernacular. They are reusable and downcyclable.
In environmental terms thatch roofs are very sustainable, with the lowest embodied energy of all, and very pretty. In practical terms, thatch roofs are a fire hazard and thus expensive to insure, and expensive to replace/maintain. However if maintained well, which could mean yearly repairs, and an expensive complete replacement every 20 years or so, the roof should last as long as any other.
These are second only to thatch as a sustainable roofing material, largely because of the extra embodied energy involved in cutting the timber and transporting it - most shingles are imported from North America, which is generally drier than the UK, and where they are used extensively. Reusable to a degree, compostable, from a renewable source, and sequester CO2, shingles are however treated with preservatives, need a lot of maintenance, and have an inherent fire risk.
Flat Roof Coverings
Single-ply membranes such as EPDM (Elastomeric membranes), TPO (Thermoplastic Polyolefin), and PVC are all oil-derived products, which are not particularly easily reused or recycled, and are all prone to puncture. TPO has the least environmental impact, PVC the worst.
Mastic Asphalt and Roofing Felt are both bitumen based which is derived from crude oil, are unable to be reused or recycled, and thus environmentally about as bad as it gets.
If a building is to have a flat roof, one cannot do much better than have a green roof, especially in an urban situation. They have proven social, environmental, and economic benefits.
As well as the obvious bio-diversity improvement and attractive appearance, green roofs attenuate rainwater run-off, so alleviating pressure on sewers. They absorb greenhouse gases, cool and humidify the surrounding air, absorb pollutants, protect the roof thus extending its life, and add extra thermal and sound insulation.
Green roofs consist of the structural support and roof deck, insulation, roofing membrane, membrane protection, root barrier, drainage/water storage layer, filter fleece, growing medium (soil), and finally the vegetation.
There are many companies who have developed their own systems, but all components are available separately, and as long as care is taken in weatherproofing, constructing your own green roof is a relatively simple procedure.
Types of Green Roof
‘Extensive’ roofs have a relatively thin layer of soil (50-100mm) principally for growing species of sedum or moss. They are therefore lightweight, simple, and need very little maintenance.
‘Intensive’ roofs could have soil up to a metre or more in thickness, for growing a wide range of plants, vegetable, or even trees. They will naturally attract much more animal and insect life than an Extensive roof. They require regular maintenance and watering.
‘Brown’ roofs can be of any soil thickness and therefore Extensive or Intensive – local soil is put onto the roof which is left to be self-seeded by the local flora and fauna.
To read more about green roofs, have a look at our blog.