«The Cob Builders Handbook You Can Hand-Sculpt Your Own Home Table Of Contents: Acknowledgements and Warning INTRODUCTION What is cob? Why build ...»
A vital part of designing the roof is to decide where the water from the roof will end up. Think about this very carefully!
Components of a roof rystem Beams Beams are the big pieces of roof (wood or steel) that support the rafters, the rest of the roof and their own weight.
Beams can span the building or lay along the walls at the top and bottom of the roof slope.
I-beams can be bought commercially or made yourself.
Rafters Rafters slope the same way as the roof. The roof sheathing is often attached to them.
Rafters sit either directly on the walls or on beams. One of their jobs is to support their own weight and the weight of whatever is on top of them. The rafters need to be close enough together to prevent the sheathing from sagging between them.
Rafters often form the sides of the space for insulation, establishing the height of that space. It's a good idea to have lots of insulation, so the rafters should be as deep as possible if they are to house the insulation.
Sometimes the ceiling is screwed to the rafters. If the ceiling is attached from below, when you are on the roof, be sure to keep your weight on the rafters, not on the ceiling. It could pull away from the rafters, and you may end up on the floor unexpectedly!
If you use unmilled poles for rafters and you're using a fairly rigid rectangular sheathing and/or ceiling (plywood or sheet rock), do your best to choose straight poles for rafters and try to line them up so the sheathing pieces can be nailed to the poles.
Boards can be attached to the ends of the rafters (called fascia boards). These boards
can be used for the following purposes:
These are short pieces (usually wood of the same dimension as the rafters), placed between the rafters to make sure they stay an even distance from each other and to help prevent them from twisting. The nogs are positioned so they can also provide a nailing surface and support for the edges of the sheathing.
If the nogs are used in the insulation space they must have large holes drilled in them to allow for ventilation of the insulation. Drill the holes before you put them up.
Bracing Cross bracing makes any angle many, many times stronger. If you plan to extend a beam or rafter out past the wall and you want extra support for the overhang, you can think ahead and bury a diagonal brace into the cob.
If you are using posts to support your roof before you build your walls, it is important to design them so they will be outside (or inside) the cob walls, but not surrounded with cob. If an upright post is buried in a wall, it will create a weak point where cracks will form as the cob settles and dries. (See illustration page 134.) You can bury posts into a low wall (2 or 3 feet) and the cob will be OK. It is very important to have good drainage under the partially cobbed posts. If you live where termites are a problem, it might be best to keep buried wood to a minimum.
Take care to protect the bottom of the posts from moisture and support them up off the ground. If you plan to bury your posts in the ground, you can make them less inviting to rot and bugs by charring the end in a fire.
If you set the post back from the very end of the roof beam it will stay drier, and will also make it a little trickier for critters to get up onto your roof. A diagonal brace from the post to the beam will help secure the connection between the two and encourage the post to stay plumb.
If you are supporting the beams on posts, it is strongest to set the beams directly on top of a post - rather than attaching them to the side of the post and depending on bolts or big nails to keep them there.
Roof sheathing Roof sheathing refers to whatever you use on top of the rafters to support the roof surfacing. Plywood, chip board, and wooden boards are the most common choices of sheathing materials. Tongue and groove boards are often used. (See page 136 for more on roof sheathing.)
The sheathing can be placed flush with the ends of the rafters and the fascia can butt up to the bottom edges of the sheathing.
Gutters Purposes of gutters: To stop the water from pouring off the roof and splashing up onto the walls.
• This is a common cause of erosion at the outside base of the walls in old cob homes.
To keep the runoff from the roof from pouring onto the ground next to the • house and seeping under the foundation, possibly causing uneven settling.
To collect water that falls on the roof and direct it into the down pipes and • then to somewhere useful.
Put the gutters on sooner rather than later. If you procrastinate on this job, you will put the health of your home at risk!
You would be surprised how much water is collected on the roof. Make the gutters big enough to handle a large amount of water and think carefully about what happens to that water. Water is a precious resource if it's where you want it, but it can also cause a lot of erosion as it flows out the down pipes.
It is very important to extend the roof surfacing and/or sheathing out over the gutter at least 3/4 of an inch.
The water collected by the roof should fall into the gutter - not down between the gutter and the building.
If you have an organic shaped roof, it is very tricky to get the gutters to follow the edges of the roof. Remember to design your roof with the gutters in mind. Check out what your local gutter people have available and ask them for any suggestions.
You might want to try making your own gutters out of wood and covering them with painted cloth. (See page 140 for more on this idea.) You could also experiment with gutters made of bamboo, large diameter pipe cut in half, tile, etc.
There will be more on roof surfaces, insulation and ceilings later in this chapter.
Some common roofs You can use any of the following types of roofs or use more than one kind in combination with each other.
Domes and Vaults In many countries, buildings are roofed with domes and vaults made out of unbaked earth. I have never built a domed or vaulted cob roof on a full sized building and I don't know anyone who has. I imagine that, cobbing carefully, it could be done. The weight of the domed or vaulted cob roof puts a large amount of outward pressure on the walls below them. The bottoms of the walls may need to be built extra wide or be reinforced with buttresses.
Earthen roofs are commonly built in climates with very little rainfall. In wetter areas, a water-repellent coating would be needed to protect the earthen roof from eroding or from becoming saturated with water and possibly collapsing. The earthen roof would also need to 'breathe' out interior moisture through this roof coating. The natural building world will be delighted when somebody perfects such a material. Making
Any of the following roof type structures can be made with peeled poles, milled lumber, bamboo, or steel weight bearing members.
Cone shaped roof This shape looks right on a rounded building. A cone roof shares its weight evenly around the tops of the walls. The junction of the rafters in the middle of the roof may seem a little daunting...
Bolt the rafters to something rigid like steel or plywood.
Bolt or screw the rafters to each other at the top of the roof.
A tension ring, further down the rafter, takes a lot of the stress away from where the rafters meet at the top and minimizes any outward pressure on the walls. A simple tension ring can be made by running a cable through holes drilled through the rafters, or by attaching a cable to notches in the ends of the rafters.
The rafter ends will run out over the walls, creating the eaves of the roof. The sheathing and the ceiling pieces need to be cut at angles to cover the spaces between the rafters. If you put the rafters the same distance apart from each other, the angles should end up the same. If you are thatching this roof, you can weave a basket-like frame to tie the thatch to.
A shed-type roof is one plane, the easiest kind of conventional roof to make. If you've never built a roof before and you don't have a dedicated carpenter friend, a shed roof might be a good roof to choose.
Double shed with clearstory windows This simply means two shed roofs at different heights. This can be a very practical design for passive solar and natural light. The glass between the two roofs can be placed to let the sun shine deep into the cool, dark side of the house.
Gable Roof A gable roof is a common design with two roof planes meeting at a peak or ridge.
This style of roof gives you the option to have an attic. Attics can be used as spare rooms, for storage, or for bulky insulation like whole straw bales.
A gable roof with a shed roof on one or more sides looks great and is a good design for a home that is built in stages Gambrel roof For a loft or two-story home you may want to put another angle in the gable roof design. This is called a gambrel roof. With the added steepness on the sides of this roof design, the walls can be shorter and you'll still have enough headroom for an upper story.
This type of roof is a little bit trickier because of the angles that need to be figured out and cut. Books on roofing will make it easier for you. These roofs are especially popular for straw-bale buildings because they distribute the roof's weight to all the walls.
Organic shaped roof The organic shapes of many cob homes lend themselves to flowing, organic roof shapes. These are more challenging to most folks than roofs that are on one or two straight planes. If you love the organic look and have a flexible mind and the time, go for it!
Nature will provide you with a supply of gracefully twisting and curving logs and branches to inspire adventurous roofs. You will need some sort of sheathing and roof surfacing material that will flex and bend to the organic shape. The small branches that thatch is attached to are perfect for making an organic shape, as is thatch itself.
You could also sheath the roof with small sections of plywood or boards cut to fit from rafter to rafter.
For a sheathing that will flex to an organic shape, have a tree or two milled into long thin 1/2 inch boards. The boards will flex easier if they are put on while the wood is still green. The edges of these boards can be milled or left in the natural shape of the tree trunk.
Starting at the bottom, bend the boards to the shape of the rafters and screw them down to the rafters. The boards can be butted up to each other or laid with each board overlapping the lower one an inch and a half or more.
Continue until you have finished the sheathing.
The roof design is a big part of the external character of the home. Notice which roofs appeal to you.
When designing your roof structure, consider the weight of the roof components.
The heavier they are, the stronger the supporting roof structure needs to be.
A common cause of failure in old cob buildings is the roof's weight pushing sideways on the walls and causing them to bow or crack. The steeper the unbraced roof, the more it pushes on the walls.
Supporting the roof laterally to itself will solve this potential problem. With bracing, the roof will become one unit with its weight going straight down onto the walls.
130 The smaller the building, the less you will have to concern yourself with bracing.
There will be a lot less weight involved and a small building with thick cob walls is naturally sturdier. With a small building and a shed roof, if the supporting roof rafters have to span less than about 15 feet, it's OK to rest the rafters on the cob without bracing.
The distance a rafter or beam can safely span depends on the dimensions of the wood and the weight of the roof. You can check builders' charts to see how big a piece of wood you'll need to span the distance between your walls.
The steeper the roof, the faster the water will run off. The required steepness, or pitch, of the roof depends on the surface material. Some are better than others at getting the water to run off. Conventional roofing will come with a minimum pitch recommendation: this is often 3 in 12 or 4 in 12. These numbers mean that for every 12 feet the roof spans horizontally, it must rise 3 or 4 feet.
Make sure that every part of the roof is angled enough for the water to run off. The flatter the roof, the more likely it is to have leaking problems.
Dips or valleys in the roof are potential trouble spots. A valley is where the bottoms of two roof slopes meet each other on an angle. Water flows into the valleys from two directions, meets there, then a large amount of water flows down the valley.
In snowy climates the snow gathers in the valleys, sometimes freezing, thawing, and refreezing there. This is a common place for leaks to develop, so take extra care when building the valleys in your roof. Debris also tends to gather there and will need to be swept off once in a while.
When designing your roof structure, plan where the insulation will go. The insulation can lay within the roof structure or on top of it, as with a thatch roof. In a design with an attic, the insulation usually sits on top of the ceiling, separate from the actual roof. An attic or large insulation space is necessary for bulky insulation.
The roof structure will be the part of your cob home most susceptible to fire.
Some roofing and insulating materials are more flammable than others, so consider the fire danger in your area when choosing your roof materials.
If you plan to collect the sun's heat with solar panels, the roof is a good place to mount them. Check in a book about solar energy to find the optimum angle for collecting the sunlight at your latitude. You may want to make the roof, or part of the roof, at that angle to set the panels on.