Living roofs on houses fall into two basic categories: turf roofs with about 15 cm. of soil and Sedum roofs with about 4 cm.
Sedum roofs differ from turf roofs in that they are much lighter and can usually be built with the same type of roof structure that a normal roof would require whereas the weight of 15 cm. of sodden turf requires a heavier, and specially designed structure. Sedums are suculant plants which can withstand long periods of drought on a thin layer of soil. There are many types of sedum with flowers varying in colour from red to white to yellow. There are also several other types of drought resilient plant such as the House Leek family. Living roofs tend to work best between flat and 30° slope although the Scandinavians often go up to 45° using Platon and incorporating reinforcement into the root matrix.
The ecological benefits of living roofs are
- They provide habitat for birds plants and insects
- They help with SUDSSustainable urban drainage systems. Various ways of holding back rain water and allowing it to percolate into the ground instead of taking it to a drain and sewer. This helps prevent flash flooding. See Surface rainwater and SUDS
- They can use local materials as a covering (except for the membrane)
- They could potentially have a longer life than most roofs as the waterproof membrane is covered by soil.
- A long term study in Berlin showed that living roofsA roof with a covering of soil or growing medium and plants. They tend to be divided into turf roofs with a 150mm layer of soil and sedum roofs with a thinner layer (about 40mm). see Living RoofsA roof with a covering of soil or growing medium and plants. They tend to be divided into turf roofs with a 150mm layer of soil and sedum roofs with a thinner layer (about 40mm). see Living Roofs improved the performance of roof mounted PVPhoto Voltaic. referring to the generation of electricity from sunlight collectors by keeping the air cooler around them. They are less efficient when hot.
- They can provide extra garden space
As mentioned above they are not particularly well suited for rain water harvesting as they tend to absorb quite a lot of water themselves.
The idea that they provide thermal insulation is very seldom correct in the UK. Earth is not a good insulator and anyway there will usually be a vented space below the roof decking which will be cold. However they are quite good at providing acoustic insulation because of the muffling effect of the soil. They also give good fire protection which may be required by the building regulationsThese are the legal regulations which govern how a house is constructed. (not to be confused with Planning Permission which is about whether you are allowed to build the house at all or what it might look like) see Building Regulations), depending on proximity to your neighbours.
Turf roofs have a history going back thousands of years but took on new life when historically important buildings in Norway and other Nordic countries needed major improvements over the last few decades. To preserve their character they developed a special indented plastic sheet to place beneath the turf to help drain it and to hold the turf in place on steep slopes. Then modern house designers took up the idea.
While there are several elaborate, well designed and expensive green roofing systems on the market, these are usually best suited to large commercial buildings. Self builders can often do all the work themselves with quite basic materials provided the design is right in the first place. (It is important to not confuse ‘extensive’ roofs with ‘intensive’ ones. Extensive are the ones that most self builders are looking at and are relatively simple while intensive ones are the sort of thing you might get on large buildings and they will have a much greater depth of soil to accommodate bushes and small trees.
The green roof industry has taken off in the last few years and some suppliers seem hell bent on selling you as complicated and unecological a bit of kit as possible, usually the sort of thing that is more appropriate for intensive roofs. For most self build situations a much simpler approach is better.
For small, simple roofs such as garages and sheds (and assuming they are sealed with a membrane such as butyl rubber or a heavy grade of black polythene or even felt) then you can lay a few centimeters of soil and gravel, contained round the edges with brick sized stones and scatter bits of sedum and gently rake in. (Sedum is readily available from gutters on farm outbuildings).For nearly flat house roofs (10º or less) which are sealed with a membrane such as TPOThermoplastic polyolefin. Often used as a roofing membrane then the roof should first be covered with a geotextile to protect the membrane from any sharp objects in the soil and then covered with two layers of turf or with c. 4 cm. of local soil and then sedums and other plants such as sempervivums, saxifrages and small dianthus. (Note that the 150 mm. of turf should only be used if the roof has been designed to take the extra weight).
For roofs which have a slope of more than 10º and less than 45º then sedum is difficult to hold in place until it is established (but not impossible). Turf on steep roofs can be anchored by a combination of oak cross members or rope reinforcement and Platon membrane to provide stability and drainage.
The following assumes that a roof decking such as EPDMethylene-propylene-diene monomer. often used as a roofing membrane has been completed with a waterproof finish.
There are two approaches to laying a sedum roof
- Purchase a proprietary system which comes as a roll out matting complete with sedum. After edge fixing and laying the matting you simply water it until it is well-established. Done!
- Do it yourself by laying the necessary layers and planting the various species.
Which is best? Well the first method will save some time and usually give a good result but is considerably more expensive and in terms of transport is less ecological. You may get quotes of between £50 and £100 per sq. m. for the living part of the roof. Materials may have to come half way across the country.
By far a better way is to purchase protective textile by the roll (if necessary), plastic drainage layer (if necessary) and then use local soil as a growing medium. Sedum plants are what clog up gutters on farmers’ sheds and are extremely easy to propagate by breaking up clumps of the plants and planting the roots. People with sheds and outbuildings are often very happy for you to clear away build-ups of sedum.
For flat roofs and those with a slope up to 10º (which already have a waterproof membrane) it is only necessary to place 4cm. of growing medium on a geo-textile (to protect the roof membrane beneath from any broken glass etc). Sedums and other plants can then be sown. Rain water simply drains off the surface as it would normally.
It is necessary to incorporate a drainage layer such as Platon. This is a tough, dense plastic formed into a shape a bit like cardboard egg trays but not as thick.
It has four functions.
- It helps prevent the soil and root structure from sliding off the roof, especially as the planting is getting established.
- It has small holes at the top of each indentation which allow rain water to escape through and out into the space beneath and then run down the roof membrane.
- It provides small pockets which trap rainwater.
- It gives protection to the main roof membrane against any broken glass etc. which may be in the growing medium.
There is a whole range of plants that will live on quite shallow soil. You can broaden the range if you increase the soil depth slightly here and there and place stones about the size of a thick paperback in the soil. This tends to conserve water available during droughts and helps anchor the roots of small bulbs such as Scilla. For species try allium, phloxes, saxifrages, aubrietia, sempervivum Scilla and miniature daffodil.
It is not advisable to go above 45° slope with sedum
Flat turf roofs (and up to 10°)
These can be laid with a geo-textile membrane directly onto the roof membrane, without a drainage layer. Two layers of 75 mm. thick turfs are laid, the first layer up side down and the latter the right way up, with their joints offset. They should be watered till they are well established and knitted together.
In areas of low rainfall it will help to add a layer of Platon as a way of catching rainwater.
Sloping turf roofs up to 30º
require the incorporation of a layer of Platon or similar to help drain water and prevent the turf sliding off the roof.
Steep turf roofs
There are plenty of traditional examples of steeply sloping turf roofs. They need some method of holding the turf in place till the roots become established and they also require Platon or similar. There are several ways of anchoring the turf
- By adding reinforcement such as chicken wire or old nylon fishing net between the turf layers.
- By adding timber racks into the lower layer of turf. (low grade oak is often used)
- By using the gutters to hold the turf in place.
This method removes the need for any type of attached gutter with all that that entails in terms of initial cost, maintenance, cleaning and repair. The steel supports need to be fabricated and heavily galvanized and very firmly attached to the ends of the roof joists as they will be taking a considerable part of the weight of the turf, especially initially before it gets a grip on the Platon. A decorative profile can be given to them. Of course fascia boards and soffits can cover the ends of the joists in the normal way.
Care needs taking to seal the point at which the land drain cuts through the roofing membrane to enter the fall pipe.
In the North of England where timber gutters are still commonly available it is possible to extend the roof membrane down into the gutter and slightly out over the edge. This fully protects the gutter so that it does not need treating. The land drain can then be laid in the gutter and covered with pea gravel.
Oldroyd do a range of green roof membranes, fixings etc
The Building Regulations part A covers the structure of a building. The Approved DocumentsThese are a part of the Building RegulationsThese are the legal regulations which govern how a house is constructed. (not to be confused with Planning Permission which is about whether you are allowed to build the house at all or what it might look like) see Building Regulations) which ensure, if you follow them, that your plans will be automatically approved. The full set of the documents is available here part Apart A of the Building Regulations Approved Documents relates to Structure. See an abridged version which covers house building go into a lot of detail for traditional masonry buildings but almost none for timber frame, steel frame, earth building SIPsStructural Insulated Panels - prefabricated (usually in a factory) timber panels often forming part of an integrated building system and aimed at fast site erection. see more on SIPs etc. For these you will need to consult a structural engineer (while SIPs structures are usually handled by the manufacturer)
With most forms of construction there will be implications concerning fire safety. These are covered in the Building Regulations and you can see examples of how to conform with these in the Approved Documents Part B (Fire Safety)
The regulations include a section called “Part C – Site preparation and resistance to contaminants and moisture” which covers site remediation along with protection from nasties which might affect the construction and occupants such as damp, rain, radon etc. There is an abridged version of the Approved DocumentThese are a part of the Building Regulations which ensure, if you follow them, that your plans will be automatically approved. The full set of the documents is available here specially for houses.