«The Cob Builders Handbook You Can Hand-Sculpt Your Own Home Table Of Contents: Acknowledgements and Warning INTRODUCTION What is cob? Why build ...»
tire tiles Tire tiles are made out of car tires (with the side walls removed) that are cut into six tiles. I'll tell you how I've seen it done: A jig saw with a razor-type blade was used to cut the side walls off, then the tire was clamped down securely and sawed into tiles with a chop saw. There was lots of yucky black smoke. Once cut, the tire tiles are laid the same way as ceramic tiles and screwed onto wooden sheathing with non-rusting screws.
Pros: Recycling old tires is next to godliness. They last a long time. They are reasonably lightweight compared to many roofing materials. They provide some insulation.
Cons: Making the tiles is a nasty, dirty, potentially dangerous job. There are a few folks who manufacture them. There is some concern about how hard it would be to put out a tire fire. They might emit various kinds of toxic gas.
sod The dictionary defines sod as a section of grass-covered surface soil held together by matted roots. It doesn't mention anything about sod roofs. I guess these roofs are too old fashioned for the dictionary.
The basic idea is to put dirt on your roof and have plants growing up there.
There are also fancy plastic, rubber, and bitumen products that can be used for the liner that last longer than plain old plastic. These are probably the safest membranes to use if the roof is not very steep, because you can order it in one piece. A cloth and paint roof may work under sod. (See next page.) Another product that can be used for the water barrier is called 'torchdown'. It is a rubberlike asphalt sheeting that melts onto your roof sheathing when you torch it with a flame-thrower. But it is expensive, costing about $50 a square foot, and requires a certain love of burning.
The roof has to be steep enough for the water to drain off of it, and shallow enough for the soil to stay in place until the roots grow together into a mat. A board is used at the bottom of the roof to keep the sod from falling off at the edges. You can run the liner up the board and make a drain hole through it and through the roof sheathing into the gutters. You may want to slant the roof slightly so it drains to a corner of each roof plane.
One way to make a sod roof is to cut pieces of turf out of the ground and set them up on the roof. You can make your own sod, in place, by shoveling top soil up onto the roof and throwing seeds on it. The soil can be as little as 3 inches deep.
Another way to make your own sod is to set straw bales on the waterproof membrane.
Tie the bales together around the edge of the roof. Then you can cut the baling twine or wire that holds each bale together. With time, the straw will decompose and the seeds that are in the straw and windblown seeds will sprout themselves. Or you can add some manure to hurry the composting process and seed it yourself.
People sometimes fantasize about flowers and vegetables on the roof, but it usually doesn't turn out that way. Gardening on a roof is kind of awkward. The plants dry out quickly because it is such a thin layer of soil. Water is very heavy and not very insulating. It's best to sow drought-resistant plants so you don't have to water the roof to keep it alive.
Pros: A sod roof is very charming and looks right on a cob home. It helps replace the biological area that the house displaced, and is great camouflage from the air. It is easy to make and maintain. This type of roof is cheap if you use a plastic liner. It will do a great job protecting you from the heat of the summer sun, and has some insulation value in the winter too. A sod roof can have a very low pitch if you have an adequate liner.
Cons: That yucky waterproof membrane. A high quality liner can be expensive. Sod, especially wet sod, is heavy so it needs a lot of roof structure to support it. When the sod is wet, the insulative value decreases considerably. If you live in a cold climate you may want to add some extra insulation, as well as a reflective material on the ceiling to keep in your internal heat.
Pros: If you make them yourself, they'll cost very little money. There's a special shingle making tool you can get that makes the job easy. They are beautiful and natural and look great with cob. They can be used on an organic shaped roof.
Cons: If you buy wood shingles, they are expensive and you don't know what kind of destruction was done when the wood was harvested. They are very burnable! They need to be on a fairly steep roof so the water will drain, or the shakes will start to rot.
cloth or canvas and paint for the roof surface This is a simple roof idea I read about and plan to try next year. It sounds so easy and cheap I thought I'd pass the idea along. I learned about this roof surface in an article written by Rev. J. D. Hooker in Backwoods Home Magazine.
You start at the bottom of the roof, laying a strip of cloth or canvas (the wider the better) along the roof, and tacking only at the top of the cloth. Pull the edge of the cloth around the edge of the roof. Then nail it on to the underside of the overhang with a small strip of wood. You may want to extend this cloth to line homemade wooden gutters. Paint the cloth with an exterior paint (store bought or homemade).
(See the section on page 165 for recipes). When applying the first layer of paint, put plenty on so it goes through the cloth and sticks the cloth to the roof sheathing.
Lay the next piece of cloth onto the roof, overlapping the last piece by about eight inches, tacking it only along the top edge of the cloth. Wrap the sides around the edges of the roof and nail the cloth to the underside again. Paint this cloth. Continue until the roof is covered. The ridge piece is not tacked down at all, except where it wraps around the edges of the roof at the ends of the cloth.
When you get the roof covered with painted cloth, go over the whole roof with another two coats of exterior paint, letting each coat dry before applying the next.
When the roof starts looking a bit faded, you can put on a new coat of paint. If you use better quality paint it will require less maintenance. A cheap paint may need a new coat every year or two; a good quality paint may need repainting every ten years or so.
Pros: Cloth and paint roofs are cheap, if you scrounge the cloth and the paint (or make the paint). They are lightweight, durable and easy to maintain. They are easy to put on and will mold well to the shape of an organic or cone roof.
Cons: The paint or the linseed oil in homemade paint could be toxic. It would also be expensive if you bought new, good quality paint and canvas.
metal, tin Some cautions about metal roofing: the edges of the metal are very sharp and can cut you. Wear gloves and don't let pieces of roofing fall off the roof onto someone or
Pros: Metal roofing is fast and easy to put up. It's strong and helps brace the rafters to each other. This gives you the option of eliminating the sheathing. Sometimes you can get off-cuts from big construction jobs for free or very cheaply. (The cut edges will be extra sharp!) Metal roofing doesn't burn, and is less toxic for water collection than asphalt roofs. You can recycle the steel when you replace the roof. Snow slides off it easily. It bends and twists a little into organic shapes.
Cons: The production of steel is hard on the earth, and metal is more expensive than asphalt shingles. It rusts quickly if it's used near the sea.
asphalt shingles Roofing felt-paper is usually stapled onto the roof sheathing under the shingles to prevent leakage. The asphalt shingles are made of fiberglass, asphalt and other questionable 'man'-made materials.
Pros: They are easy to put on and comparatively cheap. They flex (if they're not too cold) to organic shapes fairly well.
Cons: The shingles are probably pretty toxic to produce and may let off toxic gas.
They're very poisonous when they burn. Different grades of shingles are guaranteed to last for 25 to 40 years - then they make a yucky mess when they're taken off the roof, because they aren't recyclable or biodegradable.
rolled roofing and felt paper This is basically the same stuff as the asphalt shingles in big rolls. It needs the roofing paper under it too.
Pros: It's very cheap and easy to apply. It can be used on a roof with a low pitch.
(That means not very steep.) Cons: It falls apart faster than the shingles do, but still isn't biodegradable or recyclable. Like asphalt shingles, it's toxic to produce and probably lets toxins into the air. It comes in rolls that are so heavy they are hard to get up onto the roof.
asphalt This is a standard roofing surface on some commercial buildings. You can hire someone and their tar-heating machine to roof your house.
Pros: Hot tar and gravel are very good for flatter roofs.
Cons: It's toxic and it stinks. It burns. You'll probably have to hire someone to do it.
I came across this idea in an article by E. Crocker, about a historic hogan roof on an old Native American dwelling in the Southwestern United States. It is a totally different concept than we are used to. The gently sloped roof was made up of layers of different types of clay (some absorptive and some not) laid on a log and stick framework. The strategy seemed to be to absorb the water in the absorptive clay layers, and to contain it in the roof with the non absorptive clay layers, letting it dry out between rains. I have never tried this type of roof but I think it's an idea worth exploring further.
Insulation Insulation can be anything that creates still air spaces. Lots of roof insulation makes a house comfy. In most climates, insulating the roof is extremely important.
The temperature inside your house will always try to equalize with the outside temperature. A generous layer of roof insulation will help a lot in keeping heat inside when it's cold out, as well as keeping the hot summer sun from making your house too warm. So insulate your roof!
How much insulation? Check with local builders to find out how much is suggested for your area. They will probably tell you something like, 'an R-value of 35'?! R-values are simply units of measurement used to describe how much insulation-value a particular material will provide. A higher number means a greater 'resistance to heat-flow'. The building supply store can tell you what the R-values are for the insulation materials they sell.
You'll have to do more research to estimate the R-values of less conventional insulation materials. When in doubt, more is better.
Making a space for the insulation and its ventilation (See page 132 for more on designing the space for the insulation.) Where there is a moisture barrier and a temperature differential on either side of it, you will get condensation! If you are using a roof sheathing that is a moisture barrier, regardless of which type of insulation you choose, it's a good idea to create ventilation for it. Design the insulation space to be 2 or 3 inches deeper than the amount of insulation you plan to use. This extra space above the insulation is for air to flow through, taking any condensation with it. The top and bottom of this space need to be vented out of the roof through the cob walls. To make vents in the cob walls, bury a three-inch pipe running through the cob. Cover the pipe with screen mesh to keep critters out of the roof. The edges of the screen can be temporarily attached with a rubber band, then cobbed over along with the pipe.
145 These vents need to be between each rafter so that every insulation space is vented.
For ventilation on the tops of gable roofs, there are all sorts of fancy spinning metal vents available at building supply stores. It's easy to make your own venting, though.
Leave a space at the ridge of the roof between the top pieces of sheathing and the tippy-top of the roof skeleton. This creates a vent opening along the entire top of the roof. Staple heavy-duty mesh along the opening to keep the critters out. Build a little roof (8 to 10 inch overhang) along the top of the roof over the opening, or buy the special flashing that's made to keep rain out.
If you will be making a flat ceiling to create an attic space, make large vents from the attic space to the outside.
Mice and rats love to live in the attics or insulation spaces if they can get in. These places are warm, safe from predators, and have a wonderful supply of nesting materials. Your first line of defence is simply not to build or leave any entrances! Cob is so thick and hard the critters will have a hard time drilling through your walls. Use extra heavy duty mesh to cover all roof vent openings, and don't leave any gaps where wood meets cob or where wood meets wood.
Thatch The air in and between the tubes of grasses or reeds that make up the thatch are great insulation in themselves. The thatch is usually about a foot thick. In cold climates you may want to add extra insulation under the thatch.
Sod The earth, the plants, and their matted roots provide some insulation on a sod roof. It's probably a good idea to supplement it with some other type of insulation and/or reflective material, because when the sod gets wet the insulative properties decrease significantly.
Straw If you use straw, make sure it's straw and not hay. Hay is the nutritious part of the grass and is much more inviting to critters and mold than straw is. Straw bales placed close together in the insulation space have fantastic insulative properties. They are fairly heavy and require a fairly substantial roof structure of wood or steel to support their weight. They're cheap. If the straw was grown organically and if it was kept dry and mold-free, it should be nontoxic. Straw bales will smolder if exposed to enough heat, but won't really catch on fire if they can't get enough oxygen.
Loose straw has fairly good insulative properties, but one of its major drawbacks is that it burns well.