There are several issues to take into consideration when choosing roofing materials.
Thatch is one of the oldest types of roof and is found in almost every country in the world. Most 'peasant' and poorer dwellings were thatched in the UK untill the industrial revolution which made tiles much cheaper.
Thatch is a very underestimated roofing material which needs to be at least considered for the sustainable roof. If thatch became more popular it would help to preserve our wetlands which are fast disappearing due to economic non-viability, providing increased wildlife habitat during the growing period. Having lived under thatch, I can think of no better roof, and my heating bills were miniscule.
Although grass, heather and other materials have been used in the past, thatching today involves the use of either water reed, wheat reed or long straw, though plastic materials have been tried - and failed. The reeds are either native or imported from Turkey, as our native wetlands have largely been ignored or drained.
Water Reed is the reed that grows in estuaries and shallow fresh water and is the most popular thatching material. Wheat reed is wheat straw that has been cut with a binder and the ears cut off so that it is straight and stemmy and resembles water reed when thatched. Long straw is wheat that has been wheat straw that has been through a Combine. The ears and stems are mixed up together and give a shaggy, soft appearance when thatched.
Water reed has the longest lifespan, with Turkish reed lasting the longest, alough it is not sustainable due to the transport involved.
Lifespans depend on several factors:-
Turkish reed lasts the longest, up to 60 years, English reed is second at up to 50 years, wheat reed lasts up to 30 years and long straw up to 20 years. These figures assume a pitch of over 45°, though the steeper the pitch the better and the longer the thatch will last. It is important to renew the ridge every 10 - 15 years as this is the waterpoof 'cap' for the thatch.
Thatch roofs provide good thermal insulation can withstand high winds and heavy rains, and have amazing sound reduction properties. Thatch is light and needs only a simple support structure, and is flexible so can be used for any roof shape. On the downside, thatching is labour intensive and a certain level of skill is required. The materials can be expensive as reeds are increasingly imported from Europe though this is not sustainable building.
Shingle Roofs are another great natural alternative to thatch for those who want to use nature's own, renewable products. Shingles, like thatch, have much better insulative properties than most common man-made roofing products.
The use of both thatch and shingles helps to reduce carbon emissions as they both take up carbon dioxide during the growing period.
Unfortunately, most shingles are imported from Canada, so the roofing system can have a high environmental impact. Some UK companies however are now offering English Oak shingles and these offer a far more sustainable solution. Please visit http://www.carpenteroakandwoodland.com/
These are usually cheap, readily available locally and have no embodied energy as they are waste materials. The problem may be that you cannot source enough of one type to cover the roof and is is very difficult to mix tiles.
Tiles may be concrete or clay.
Pantiles have very different profiles - some are shallow, some are deep - and it is important that you obtain tiles which are all the same or the roof may not be watertight.
Avoid tiles which are begining to show signs of weathering, even if only slight, as once the weathering starts, the tile has a very limited lifespan.
Slates come in all different sizes and the battens must be set accordingly. If you dont want to re-batten the roof, then take an old slate with you to ensure that you are buying the same size.