Chapter 12: Roofs for earth houses
Any type of roof used on ordinary houses can also be used on earth houses. Thatch or sheet metal roofs are commonly used. But often it will be cheaper to use an earth roof on your house.
Three types of earthen roofs are used:
1. Bunker fill roofs
These are things an earth roof must do:
1. It must be strong enough to remain in place;
BUNKER FILL ROOF - A bunker fill roof is a flat roof in which large timbers, (sometimes called "vigas" in parts of the U.S.) support a tamped or compacted earthen blanket. See figure 85. These supports can be sawed timbers or the trunks of small trees, six to eight inches in diameter, spaced approximately 30 inches apart. For large buildings, such as the school shown in Figure 86, large trunks, ten to twelve inches in diameter, can be used. A nice appearance is produced by alternating the large and small ends of the vigas. Also, putting the small ends all in the same direction would cause that end of the roof to be weak. A tilt or pitch is given to the vigas so that the finished roof will slope about ½ inch per foot of length.
On top of the vigas, place some type of covering to support the earth fill. This can be 1" lumber, some type of reed such as bamboo, or even small saplings. Run the sheathing diagonally from center to center of the vigas. This will compensate for the unevenness of the vigas. Over the sheathing, use heavy building paper or a thin layer of straw to keep the soil above from seeping through.
A compacted earth fill is placed on top of all this. The thickness, when tamped, should be at least four inches. Deeper fills up to eight inches are better and will increase the life of the roof, reduce the amount of heat going through the roof and lessen the chances of seepage.
The roof should be built up in lifts and packed like rammed earth. The thickness of the loose lift placed at one time depends on the size and weight of rammer used.
The soil type for bunker fill roofs should be similar to that used for rammed earth. Clayey soils will shrink and crack and allow water to seep through. Often, it will pay you to use a stabilizer in your roof material. Lime, cement, or any waterproofers will do. Another way is to add oil or asphalt to the last layer of earth to be compacted. After allowing a few minutes for the oil to penetrate, tamp it into a dense water-tight layer.
A protective surfacing may be needed in climates that get a lot of rainfall. Gravel spread on the top of the surface lessens erosion caused by rain. A coat of asphalt will help hold the loose gravel in place.
Water from the roof must not be allowed to run down the walls of the house. A spout or canal projecting out away from the wall will solve the problem. A half section of split bamboo would do nicely, or, a trough made of wood or sheet metal would do as well. The spout should extend far enough away from the walls to prevent erosion - at least two feet.
ARCH OF VAULT ROOFS - (See Figure 88.) These types of roofs are used in areas where timber is scarce. They require some skill to build although in some areas such roofs have been used for centuries and local masons know how to build them.
Arched, vaulted or domed roofs are usually built of burnt brick although there is good reason to believe that well stabilized earth blocks can be used.
The outside walls of the house must be strong enough to resist the outward force caused by a vault or dome. In most cases walls are made very thick - 2 to 3 feet - depending on the distance between walls. In many cases rooms are placed side by side to help absorb the forces but the outside walls will still have to be very thick. A reinforced concrete bond beam around the top of the outside walls will add greatly to the strength and may make it possible to reduce the wall thickness. However, a qualified structural engineer should design any such walls and beams.
The mortar used in laying brick or stabilizing earth block in an arch, vault or dome is usually of a quick-setting type. In many middle Eastern countries a mortar made with gypsum is common. If cement or lime mortar is used, it is necessary to have a form to support the brick until the mortar sets. This is usually too expensive and is rarely used in small house construction.
If you should use this type of roof, get methods and design from someone who has done it before.
EARTH TILE ROOFS - Earth tile have also been used for roofs. They can be pressed in a block making machine by using fillers. They can also be of sun-dried adobe but in either case it is best to stabilize the earth. The tiles are placed on a wooden frame just as shingles are. The tiles should be 1 1½" to 2" thick and about 1' long. Good sun-dried tiles are made with a thatch (or grass) "tail." See Figure 89. The thatch tail helps prevent rain from eroding the block, and provides insulation for the inside of the house.
The best earth tiles are made with stabilized soil. Lime, cement and asphalt work well. Since they are so thin, tiles should have a very high resistance to the spray test.
The roof frame must be built strong enough to support the weight. Also, wooden strips (called stringers) must be placed in the roof at close enough intervals so that each tile rests on two stringers, either directly or indirectly. Tiles are often made with a lip or groove near the upper edge so that they will lie securely on the stringers.
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