«The Cob Builders Handbook You Can Hand-Sculpt Your Own Home Table Of Contents: Acknowledgements and Warning INTRODUCTION What is cob? Why build ...»
If you are building in a place where it snows a lot, see what you can learn about designing in snow country from the library and from local practice. Observe different roofs in heavy snow. Think very carefully about where the collected snow will end up when it slides off the roof. It would be very depressing if it piled up in front of your sunny windows or in front of your door.
Snow on the roof is added insulation but it can also add a lot of weight to your roof structure. If you want the snow to stay on the roof for insulation, build a sturdy enough structure to support it. If you want the snow to slide off the roof quickly, use a more steeply pitched roof and a smooth metal roof surface.
Porches and sheds add wonderful living and storage space to your dwelling.
Every house needs at least one. For shed or porch roofs that run off the main building, you can put the rafters right into the cob walls as you build. Support the low ends of the rafters on a post and beam structure.
If you live in a wet climate, it's a good idea to put gutters on these little roofs too.
The place where the chimney, stovepipe, or skylight comes through the roof is a favorite spot for naughty leaks. If these potential trouble spots are placed high up on the roofs slope, they will have less flowing water to contend with. The flatter the roof, the more likely it will be that you'll have leaking problems. Be extra meticulous about sealing these areas. A good carpentry book will give you some helpful tips.
If you choose to do a wooden roof structure, it will require basic carpentry skills. This is a good time to call on your carpenter friends.
If you plan to add onto your home in the future, it is important to design the roofs so they complement each other well. Design and build the first roof with the future addition(s) in mind. Make sure the water will run off every roof and not onto another roof. You may choose to build the whole roof for the finished house first and build the cob additions up to the roof as you find time. (See the next section for more on this idea.) It's easier and more fun to roof your home with a group of people: some folks handing stuff up, three or four people on the roof aligning the beams or rafters and attaching the roofing. Watch out that the people on the roof don't drop things on the people below. Be careful when you're on the roof. Steep roofs are harder and more dangerous to work on. Gravity wants to bring you back to earth. Get up and down carefully. It's a good idea to stop and get off the roof before you get too tired. If you're working on a really steep roof, you can tie yourself up there with a safety rope.
Building the roof before the walls You can build the roof before the walls and even before the foundation. There are
some advantages to this, as follows:
133 You and the cob will have great protection from the sun and rain while you are making the rest of the house.
Putting up a pole-supported roof can be done with fairly standard carpentry skills. If you are pressed for time or energy and want to hire someone, it's much easier to find someone who knows how to put up a pole structure than to find someone who knows how to put a roof on a cob building.
Because wood is rigid and cob is malleable, it's easier to mold the cob to the roof than to make the roof fit onto the organic shape of the cob walls. Cob can fill all the odd little gaps between the walls and the roof. It's also easier to get the ceiling up first and then cob to it rather than trying to cut the ceiling into the organic shape of the walls.
Many counties will be willing to give you a permit for a pole structure, especially for an 'agricultural' building. They're often unconcerned about what kind of material you want to use to fill in between the posts. Currently, most building officials seem to know very little about cob.
You can support the roof on permanent posts or poles, or with temporary posts that can be knocked out after the cob has been built to hold the roof up. Cob walls are load-bearing and will hold the roof's weight. Permanent posts can be used outside the walls as porch supports, or inside as part of interior walls, or for beam supports. Plan the posts so they won't be completely buried in the cob walls - that would create a weak point in the cob. Posts can be partially buried in the cob. I have buried up to 2 and 1/2 feet of the base of a post, making the cob extra thick at that point, and it hasn't made the cob crack. The cob wall can wrap around the posts, half covering them vertically. Make sure the cob is approximately as thick as the rest of the wall.
If you haven't built your roof first, don't finish the walls and then try to put the roof on them. You'll need to finish the walls and put the roof structure up at the same time. It is much easier to do these two things together.
The weight-bearing roof beams and/or rafters can be buried into the tops of the cob walls. Like the window frames, you can make a keying system to attach them to the cob. Another way to secure the roof to the building is to bury a piece of wood part way between the foundation and the roof and connect the wood to the roof structure with cable or wire.
When you add the pieces of wood that connect the beams and/or rafters to the cob, think about the forces working on the junction of the wall and the roof. Each part of the roof must be securely attached to the next, especially in high wind areas. The roof is like a wing - wind will try to lift it off the building and up into the air. Gravity will try to slide it off the structure.
Embedding rafters You can embed the rafters straight into the cob walls. If all the rafters are aligned with each other, it makes it easier for the sheathing to sit on the rafters and line-up for
If your house is an irregular, organic shape and you're putting a flat planed roof on it, making sure the rafters are in the same plane can be very tricky. You can align them with guide strings or set them on top of guide poles.
Build the walls to the approximate height of where the rafters will sit. Stretch two or more strings or poles across the building, perpendicular to the rafters. Check that each string or pole is level. You can support the strings or guide poles on cob columns built on the wall, (these will later become part of the walls) or on temporary posts. Hold each rafter up where it will go using the strings or poles as a guide. This will show you how high you'll need to build up the supporting walls to rest the rafters in the same plane as each other.
strings or beams If you use guide poles, they can be left in to support the rafters like beams. If it's such a short span that the rafters don't need the support of these pole beams, they can be left in for looks or removed later by sawing them off and plastering over the stubs.
If you are using unmilled poles for the rafters, choose fairly uniform ones. This will make it easier to attach sheathing to the tops of the rafters and the ceiling to the bottoms of them.
To make an organic shaped roof, you can build the walls up to the shape you want, or use guide poles that are curved to rest the rafters on.
The rafters will need to line up with the edges of your sheathing and ceiling, so take the time to measure them and place them accordingly.
Embedding beams Permanent beams can be buried in along the top of the walls to support the rafters.
These can be used instead of guide strings to establish where the rafters go.
It's ok if the walls are curved, and the beams straight, as long as the wall supports the beam well. Where the wall doesn't line up with the beam, you can simply cob up to the rafters.
If the rafters need more support, bury another beam or two across the house for the rafters to sit on. If you want a parallel look, make sure these beams are parallel to each other. If you want a flat plane on the roof, check that the beams are set level on the wall before cobbing them in.
Your beams can extend out past the walls to help support the rafters that sit on the outside and protect the walls of the house.
The beams need to be big and strong enough to do the job. Diagonal bracing may be necessary.
How to get your big heavy beams up onto the tops of the walls When you're about to move a big beam, prepare the spot where it's to be placed before you pick it up! If you will be bracing it, have the necessary bracing materials and tools handy.
Probably the easiest way to get a heavy beam up there is to get a bunch of friends together and lift it up. You may want to do this in two steps: lift it first onto the scaffolding or loft, then up onto the top of the wall. If you don't have enough peoplepower around, you can set up a ramp by leaning poles on the walls and pull the heavy beam up onto the building with ropes. Be careful!
137 Roof surfacing The roof surface is the top layer of water-repelling or waterproofing material.
Many modern roofing surfaces - like plastic, tin, or asphalt - are moisture barriers.
They protect against moisture from the outside but tend to trap the moisture created inside the house. Moisture barriers are often a place where vapor in the air condenses and problems arise. This potential problem can be reduced by supplying lots of fresh air to the area between the sheathing and the insulation. There's more about how to vent this area in the section on insulation. (See page 132.) When attaching the roof surfacing to the roof, always start at the bottom of the roof and work your way up.
At the bottom edge of the roofing surface the water sometimes sticks to the roofing material. It then flows around the edge and starts to move back up along the bottom of the roofing, wetting the roof structure underneath.
There are various ways to prevent this from happening. You can use a drip edge or flashing to encourage the water to fall off.
water sneaking around the bottom of the roof surfacing and off the drip edge The steeper the roof, the more likely it is that the drips will fall right off.
If you want to, (or have to) collect drinking water or water for your garden from your roof, choose a roofing surface that is least likely to add toxins to the water. You may want to put the water through a filtering system.
Some roof surface options thatching Thatched roofs are made of reeds, grain or grass stalks attached to a wood frame beneath with string, or with a wire screwed into a roof sheathing. Plants that grow in
Only the bottom ends of the pieces of thatching material are exposed. The water drips from one end to another from the top of the roof downwards until it drips off the edge of the roof. Except for the ends, most of the stalks never get wet. A thatch roof needs to be at least a 4 in 12 pitch to ensure that the water will drip from one stalk end to another. The top part of the thatch will require extra care and attention and will need to be replaced more often than the rest.
In parts of the Orient, Africa, and Latin America thatch is still commonly used, but in the western world thatching is not a widespread art anymore. There are few people in Europe who still practice it, and only a handful in America. Like any art, the best way to learn it is to watch someone who knows how to do it, and then try it yourself. I suspect that thatching will enjoy a renaissance and it will become easier to find someone to teach you how to do it.
A common fear about thatch is that it will burn. By putting a 'solid' wooden or plaster ceiling under the thatch, the fire danger will be greatly decreased because the air needed to fuel a fire will be mostly blocked off. With a 'solid' layer under the thatch, you must secure the thatch with a wire that's attached with a screw to the wood underneath.
Pros: Thatching is a wonderful roofing material. It provides insulation as well as a water-repellant surface. It is lightweight, so it requires a minimum of wood to support it. Thatch looks perfect with cob walls. And it breathes! A good thatch roof can last up to 60 years with proper maintenance. Surprisingly, thatch roofs have been used successfully by people living in very wet climates.
Cons: Hardly anybody knows how to make thatch roofs, and many of those who do want to keep the information to themselves. It is very expensive to hire someone to do it for you. It takes time. It takes a lot of materials which are sometimes hard to find. If you leave seeds in the thatching materials, birds and mice might move in and damage the thatch. It is difficult to catch the water running off the roof, so you need really wide gutters. If the gutters can't catch all water as it pours off the roof, lay down some gravel and/or plant foliage to help cut down the amount of water that will splash up and hit the walls where the water hits the ground. Thatch is flammable so it may be hard to get code approval and insurance for a thatch roof.
Some of the old buildings in Europe were roofed with large thin pieces of stone laid as shingles. In parts of England 'man'-made slate stones are available.
Pros: Slate roofs are very charming. This roofing material will last forever (in humantime) as long as the structure and roof support hold up. The stones won't burn. The spaces between the stones allow the roof to breathe.
Cons: This kind of roof requires lots of support because it's obviously very heavy. It would be next to impossible to find someone to teach you how to build a slate roof, so you would probably have to rediscover it on your own. Finding the stones for the job could be very challenging.
ceramic tiles If you were very ambitious, had a good supply of clay, a kiln, and lots of fuel, you could make the tiles yourself.
Cons: Ceramic tiles are expensive to buy. They require a lot of embodied energy to fire them. A tile roof is heavy and needs a sturdy roof structure to support it.
composite tiles There are various commercial tiles made of different composites of plastic, polymers, cement, and fiberglass. These are expensive and have different life-spans and toxicity levels. Do some research and find out what's available in your area.