Adam Weismann and Katy Bryce look at roofing options for cob buildings
A roof is an extremely important element to get right. The moment the roof materialises on a new structure, it’s transformed into a building, and can fulfill its requirements as a shelter and a home. The roof system comprises two main components: the structure, which gives the roof its shape and strength (the timber frame), and the protective skin, which is attached to the structure and provides waterproofing and protection from the elements (thatch, slate etc).In modern, conventional buildings, it’s commonplace to install a synthetic waterproof membrane between the structure and the outer skin. With roofs made of natural materials, this isn’t always necessary, and indeed, much of the time, their very success and health rely on their ability to breathe naturally, which depends on the free movement of air. As with all vernacular building practices, traditionally, the construction of the roof and the materials used were directly connected to the immediate natural surroundings. This ensured that the roofing system was suited to the local climate, and blended seamlessly into the natural landscape. For example, the steep pitched timber roofs of northern Europe were constructed from the abundant pine forests to prevent snow building up and becoming too heavy. In the desert, where there are no trees, vaulted roofs are made out of earth blocks, designed to maximise cool airflow around the building to relieve the heat of the day. Traditionally, most cob buildings in the UK were covered with a roof of thatch attached to a timber frame. This was because most cob cottages were constructed and lived in by the rural workforce, who had at hand the waste straw or wheat reed that remained after the harvesting of the grain. A traditional timber frame is the perfect accompaniment to a cob building. The chunky timbers and traditional methods of construction complement the thick, rounded cob walls. A new cob building doesn’t need a special roof design. However, special consideration must be given to the overhang of the eaves and the distribution of the weight of the roof on the walls. In thatch or turf roofs with no guttering, the overhang must be at least 450mm/18”, to ensure that rainwater has adequate clearance from the walls as it falls to the ground. A wooden wall plate must be inserted onto the top of the cob wall to ensure even distribution of the weight of the roof. The wall plate also acts as an attachment point for the roof structure, and anchors it securely to the walls below. With the freedom of design that cob allows, there’s potential for some unique roof designs. However, unless you’re fully experienced in roof design and construction, always have your ideas approved by a professional engineer, timber framer, or specialist builder. You can’t afford to make any mistakes in this area.Thatching A thatched, cob building involves the four most simple, readily available natural building resources on the planet – grass, mud, stone, and wood. From the cob cottages of rural Cornwall to the mud dwellings of Burkina Faso, these materials have been used together since the beginning of shelter building to create structures of comfort and beauty. In the UK, it was applied to nearly all smaller houses up until the end of the Middle Ages, and the mid-19th century in rural areas.As we mentioned earlier, the most common material – straw or wheat reed – was generally a by-product of wheat grown and harvested for grain. It was very cheap (if not free) and demonstrated an efficient utilisation of available resources that, out of necessity, we were so good at. Thatching is now regaining popularity, not just because it’s aesthetically pleasing, but also because it has many practical advantages.Thatch is an excellent natural insulator. The hollow stems of the straw efficiently trap air, thus preventing any unnecessary loss or gain of heat and cold. A 400mm thick covering of thatch has an excellent U-value rating of O.20W/m2K, which is half that of an uninsulated slate or tile roof. Thatch, like cob, is a great regulator of temperature, helping to keep the internal space cool in summer and warm in winter. So, thatch is an extremely energy efficient roofing system. When installing a thatch roof on a cob building, an extra layer of insulation, such as sheep’s wool, is needed to compensate for the high U-value of the cob walls. This will improve the overall thermal efficiency of the building. The reed and straw used to make the thatch is a 100% renewable resource – it can be planted year after year. Moreover, thatch made from wheat straw is the by-product of the grain threshing process (itself a relatively non-polluting activity) requiring little energy or sophisticated tools. This is especially true when grown and harvested using more traditional methods. It also has the potential to be a completely locally produced resource.Thatch is 100% biodegradable. The rotten straw from a repaired or re-thatched roof can be recycled – traditionally, especially in Scotland and Ireland, roofs that were soot laden from interior open fires were completely stripped every two to three years, composted on dung heaps or spread directly onto the fields and gardens as a rich manure. Compare this to a roof made of asbestos or cement slates…A simple thatch roof, especially in a rural setting, blends into the surrounding landscape. Like cob and wood, a thatch roof will age gracefully, changing from a golden colour into a mellow shade of silver-brown. The challenges of thatchAs with all things, there are some less than positive aspects of a new thatch roof:
High labour costsThatching is a highly skilled craft that requires many years’ training and practice, so it’s not for the self-builder. Also, it’s extremely labour intensive, making it a relatively expensive option. It’s ironic, as is the case with most traditional skills practiced in the modern world, that thatching was at one time the roofing method of the poor, rural labourer. However, if your design is simple and building of modest size, we feel it’s worth the extra cost, all the more so because most contemporary thatchers really love their work and take pride in their craft. This can really be seen and felt in the final outcome.
Fire riskThere’s an assumption that thatched roofs are at greater risk of catching fire, but English Heritage, in their booklet Thatch and Thatching, a guidance note (2000) claim that insurance records don’t back this up. With certain precautions, any risk can be greatly reduced. Current building regulations stipulate that all new thatch should have a fire retardant membrane between the thatch and the ceiling. In one respect, this is a shame because it interferes with the breathability of the thatch, and means that it can’t be appreciated in its natural state from underneath. If fitting a fire retardant membrane, it’s essential to have a significant air gap between the inner face of the thatch, to provide adequate ventilation for the thatch layer. There are also precautions that can be taken with chimneys, wiring, and general household activities. In areas where thatching is common, a local thatcher or the local fire department will be able to advise you.Types of thatch and lifespanThe lifespan of a thatched roof will depend on the type and quality of the thatching material used, how well and how thickly it’s laid (400mm is the suggested thickness), how well it’s maintained, and the local weather patterns. In the UK, traditionally, all types of grass material, including heather, were used, but today, three main types of cereal grass are used: long straw (uncombed wheat reed), combed wheat reed, and water reed Water reed has the longest life expectancy, 50 to 80 years, then combed wheat reed at 30 to 40 years, and finally long straw, 10 to 15 years. The last two are grown abundantly throughout the UK, but as mentioned earlier, modern harvesting methods and the use of nitrogen fertilisers that weaken the grass have rendered them virtually unusable for thatching. The wheat is best grown organically and harvested in the traditional way to produce a strong material. Water reed, used mainly in East Anglia and Scotland, needs to be grown in marshy or wet conditions. Its production in the UK has greatly decreased because of lower demand and the drainage of much natural marshland. This means, unfortunately, that the best material has to be imported from Europe. The best water reed available in the UK is Norfolk Reed from East Anglia. Thatch roofs will reach or exceed their maximum life expectancy if they’re well maintained. This includes repairing damaged patches from birds or winter storms as soon as they appear, and keeping plant growth over-hanging the roof to a minimum, to avoid debris and leaf drippings. It’s best to avoid covering the thatch with wire meshing (to deter birds and vermin) because this hampers the natural water shedding ability of thatch: water normally runs unrestricted from straw to straw down the steep pitch of the roof (minimum 45º, ideal 50º), and falls off the eaves before it can penetrate into the thatch. If wire mesh is used, the rainwater is deflected across the mesh surface, and has more chance to soak into the thatch.Life expectancy can also be affected by geography – in the wetter, western counties, it will obviously be lower than in the drier, colder eastern counties. On a site specific level, orientation is important, with a potentially increased life expectancy given to buildings with a gable end faced towards the prevailing wind and rain, as opposed to the whole side of the building. Wooden shingles or shakesWooden shakes or shingles (shakes are split, shingles are sawn) have been used since the Bronze Age as an effective roofing material. In the UK, oak shingles were used in the Middle Ages as an alternative to thatch. Some can still be seen today on church spires around the country, especially in the south east of England. Indeed, Salisbury Cathedral was originally roofed with shingles from the New Forest. Their use diminished mainly due to the arrival of new roofing materials such as clay tiles, from the 16th century on. Today, they’re gaining in popularity again as a roofing system with good environmental credentials, as long as they’re purchased from a certified sustainable source, and aren’t fire-proofed and preserved using carcinogenic substances. They’re from a potentially renewable source and are 100% biodegradable. There’s little energy needed in their production. They have a relatively high insulation value, and don’t require a waterproof membrane underneath them, as they need to breathe and dry out between rainstorms. Wood shingles or shakes shouldn’t present a major fire hazard in our wet climate. They do need a bit of extra maintenance to ensure long life, mainly to prevent the build up of dirt between the shingles, which may cause rot if allowed to remain. Aesthetically, a shingle or shake roof complements a cob building and it can bend around curves – great for circular or contoured cob structures. They’re also extremely light and so require a less sturdy roof structure than the heavier loads of thatch and turf roofs. As with all the other natural roofing materials, they age well, softening and mellowing in colour over time, and blend quietly into the surrounding environment.The UK has a number of sources for sustainably produced wood shingles and shakes, which are listed in our book.Natural slate roofsNatural slate roofs became the common roofing material throughout the UK from the mid-19th century, when canal transport enabled them to be distributed economically, mostly from Wales and Westmoreland. They quickly superceded thatch.There are three main sources of naturally quarried slate in the UK – north Wales, Cornwall, and the Lake District. Today, slates are sourced from all across the globe and cost, sadly, much less because they’re produced with much cheaper labour. It doesn’t seem right to be shipping slates from halfway across the world when we have our own quarries in the UK, even though it makes financial sense. The wide variety of hues and tones found in British slate, the greens and purples of north Wales, the greens and reds of Cornwall, and the greens and blues of the Lake District, all complement the earthy colours of a cob building.A slate roof needs at least 300mm (12”) of insulation to bring it to a U-value of 0.13. It’s best to go above the standards required, reaching a thickness of up to 450mm (18”). Turf roofsTurf roofs have always been produced locally by people building for themselves. The methods are simple, the resources are infinite and can be used direct from their source. Turf roofs are currently gaining in popularity within the green building movement as a sustainable roofing system. Contemporary turf roofs have been inspired by the ancient sod roofs of northern and central Europe. Like thatch, a turf roof utilises grass as a roof covering, but differs in that it’s a living system, which can support a wide range of plants, birds, and insects.A turf roof consists of a covering of vegetative matter, growing out of a layer of soil or organic matter on top of a waterproof membrane. At their simplest, they consist of a covering of grass, but can be created to support anything from herbs, vegetables, and wildflowers, to even bushes and small trees.Turf roofs are beneficial to the environment and the people who live under and around them. They’re an excellent roofing system for self-builders because they’re relatively simple and inexpensive to construct. For natural builders, they can fulfil a desire to use predominantly local, naturally sourced and recycled material.
Efficient use of materialsTurf roofs can be created to become an extension of the ground below. You can use the topsoil and turf that have been removed from the building site during clearing and preparation. During excavation, separate out the topsoil and carefully store the turf in a shaded area and water regularly until used. This provides a very satisfying utilisation of ‘waste’ excavation material, it ensures that no habitats are lost and that the building will genuinely be a part of the environment.
Aesthetic sensibilitiesNatural living roofs, again, work within the magic synergy of the four basic natural resources – stone, earth, wood, and grass. Turf roofs will complement the soft, rounded nature of cob walls and add to the natural essence of the overall building.
Thermal insulationTurf roofs provide excellent thermal insulation because of the thick blanket of earth, and the ability of the plants to trap pockets of insulating air between them. In the winter, they help keep the building warm, and in the summer, keep it cool by absorbing heat from the sun. This can be beneficial in urban areas, where concentration of concrete buildings can create a ‘heat island’ effect, causing a rise in temperature of up to 5ºC.
Rainwater Turf roofs have the ability to absorb up to 85% of storm rain, reducing the amount of water entering your drainage systems.
Air qualityTurf roofs can improve local air quality, which is especially pertinent in an urban environment. The plants, obviously, can absorb carbon dioxide in the atmosphere and release oxygen.
Animal habitats and green spaceIn an urban environment, turf will provide habitats for insects, encouraging insect feeding birds, and attractive and much needed green spaces and areas to grow food.
Perfect for the self-builderUnlike thatching, it’s a fairly straightforward roofing system that can be constructed using simple hand tools, with little previous experience. It can generally be created out of materials sourced from the site or the local vicinity.
Long lifespanWell maintained turf roofs can last indefinitely if they’re well constructed, and a good quality waterproof membrane is used (an EPDM liner is best)Things to consider First, turf roofs are very heavy (two or three times heavier than a slate roof), especially when wet or covered in snow. The design of the whole structure will need to take into account this extra weight. Turf roofs require a synthetic rubber liner, which uses a very high amount of embodied energy to produce, and is expensive. Traditionally, in Scandinavia, 16 layers of birch bark were used until the mid-20th century. This isn’t a viable option in modern Britain, but there are other, more eco-friendly options on the market: EPDM liners (ethylene propylene diene monomer) cause minimum environmental damage as they’re extracted from monomers. They’re extremely durable, effective, and will last indefinitely if protected from the sun. They can also be re-used if they’re not glued or nailed in place.It can be difficult to locate the source of a leak in a finished roof, to carry out a repair. However, there are some simple precautions:l
Never walk on the membrane in shoes during the construction processObsessively check the roof structure for any protruding nails, staples, stones, sharp slivers of wood, and any other sharp objects that may puncture the liner laying.Cushion the liner from below and above with carpet underlay, matting, cardboard, or any other soft material to help protect it (see laying steps below).
The roof structure for a turf roof needs to be at a pitch of 25º or less (it can be flat), so that the turf isn’t constantly trying to defy gravity. If a pitch is too steep and the turf slips to expose the waterproof membrane beneath, premature disintegration of the membrane can occur from exposure to the UV radiation of the sun.Layer 1 – base timber layerOn top of the roof rafters, lay a solid timber base from which the turf roof covering will be supported. This can be plywood sheets, floorboards, or any other solid timber sheathing. At the lowest points of the roof, it’s necessary to insert a drainage release point to move excess water away from the roof. A standard shower tray drainage piece fitted snugly into a hole created in the timber sheathing works well.Fascia board It’s vital to attach a fascia or edge board around the whole perimeter of the roof. This contains the turf so it doesn’t slip down off the roof, and also provides a lip for the waterproof membrane to be draped over. This can be constructed of any durable wood – roundwood, bendable plywood, or thick milled wood. The design is flexible as long as there’s at least a 6” lip above the roofing base (Layer 1).Layer 2 – cushioning for the waterproof membraneThe whole of the solid timber base layer should be covered with a soft cushioning to prevent the waterproof membrane being punctured by any sharp object. Soft carpet underlay works well and is easy to lay. Try to find someone who has just taken up an old carpet, or ask for scraps at a carpet store.Layer 3 – waterproofing membraneThis is the most essential part of the turf roof. For a long-lasting, watertight roof, it’s worth investing money in an EPDM membrane. Its longevity and effectiveness makes it a worthwhile sacrifice for your pocket and the environment.Make sure that you: l
purchase more membrane than the total roof area, so that you can cut it down to size, and the edge will overlap the fascia – it can be cut back to fit once the turf is in place. It’s also important to leave some give in the membrane as you lay it, especially if laying in warm weather, as the membrane will contract during cold temperatures.l don’t wear shoes while laying the membrane, and avoid walking on it at all once it’s laid. This is the easiest time to damage the membrane, and it will cost you in the future. l use a whole swathe of the same piece of material to avoid vulnerable seams.l take care fitting the membrane around openings in the roof, such as around chimneys and flue pipes – ensure that the membrane fits very tightly, and insert the appropriate flashing material. l create the appropriate holes in the membrane to line up with the drainage pieces.
Layer 4 – second cushioning layer The function of this layer is to protect the membrane from damage from the soil, and provide a rooting medium for the vegetation. This can comprise the same material as in Layer 2. Take care when laying this over the waterproof membrane, and take the precautions mentioned above.Layer 5 – gravel drainageA small layer of gravel, 6” wide, needs to be laid around the perimeter of the roof, up against the fascia board. This is to prevent the accumulation of soil in the drainage piece that would block the free exit of water. It will also help to prevent water pooling on the roof.Layer 6 – layer of organic matterTo help the turf to get established, it helps to lay a 1 or 2” layer of topsoil on top of the cushioned layer.Layer 7 – turf layerFor an instant green roof, locally cut turf (best from the land surrounding the building, or saved from the excavations from the ground works) can be used. It can be cut in 1’ x 1’ squares, and needs to be 3 to 4” deep. Alternatively, a local turf company may have second grade turfs, not good enough for lawn laying, which they sell at a discount rate. Simply arrange them on the roof so they’re butted tightly up against each other. The best time to lay the turf is late autumn, so it can benefit from the rainy season. Turf laid in the summer or during a dry spell will need to be well watered so that it doesn’t die.An alternative to laying cut turf is to seed the compost/top-soil layer with grasses or meadow flowers of your choice. If you take this option, the compost/top-soil layer will need to be at least 4” deep. This option will give you more flexibility in the vegetation you have on your green roof, but it makes the soil more vulnerable to slippage in the early days before the roof system gets established.Optional insulation layerFor extra insulation, cork or foamed glass can be installed on top of layer 1, the base timber layer. The material used must be able to withstand contact with moisture.MaintenanceA turf roof requires very little maintenance, especially in the UK where long periods without rain are rare. If in the summer the roof begins to look very dry, a simple watering will revive any flagging grass and encourage new growth. You can, however, allow the grass to die back and wait for rain to regenerate the roof with new growth. We allow our turf roof to die back completely so that it looks sad and lifeless in the summer, but watch with glee when the first autumn rains bring it bouncing back to green life again. Obviously, if you choose to grow herbs or vegetables, a more intensive watering regime will need to be carried out.Check at least twice a year that no patches in the soil have emerged, leaving the membrane exposed and vulnerable to degradation by the sun. Cover any patches with more topsoil or a piece of turf.
This article is from the December 2006 issue of Country Smallholding magazine. << To order back issues click the link to the left.