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close this book Self - Help construction of 1-story buildings
close this folder Detailed planning for construction
close this folder Planning foundations
View the document Footings
View the document Foundation walls
View the document Deciding upon the materials and dimensions for foundations
View the document Drawing final foundation plane

Planning foundations

If the walls of a building are built directly on the ground surface, the weight of the building will soon press them into the earth. This causes the building to sag, crack, or leak.

Foundations are strong platforms built below ground level, where they won't sink. A building with a foundation can "stand" securely.


Planning foundations

 

The foundation of a well-constructed building does several things:

• it provides a level platform for the building to stand on;

• it helps protect the building if earth tremors, strong winds or rains shake the structure;

• it keeps out water and dampness.

Here's a list of things the foundation must NOT do;

• sag;

• dissolve (or rot) by water erosion;

• crack from stress at a certain point;

• slip on uneven ground

• buckle under pressure of water in the ground (this is called "scouring");

• collapse from the weight of the building.

Foundations for 1-story structures generally consist of two parts: a footing, and a foundation wall.

The footing should be a pad of concrete resting in a level trench dug under the position of all walls that will support the roof.

The foundation wall is a solid wall attached to the footing and rising to ground level at the point where the ground is highest under the building. The foundation wall can be made of many different materials. Each material must be planned for in a different way.

Note: There are other foundation designs such as pier and beam, or post and beam that communities with access to heavy wood beams might wish to use. See Appendix 7 for reference materials that discuss these designs.

 

 

Footings

All foundations should have a concrete footing. The concrete can be poured directly into the foundation trench, or into wooden forms in the trench. In either case, the bottom of the trench must be level and the sides of the trench or of the wooden forms must be square: that is, they must be exactly vertical, at 90 to the trench bottom.


Footing

The depth of the trench in which the footing rests depends on:

• what the foundation wall is made of;

• the stability and strength of the soil;

• the frost line (this only applies in areas where temperatures drop below freezing in winter; the frost line is the maximum depth to which soil freezes locally);

• the unevenness and/or slope of the ground.

The width and thickness of the footing depend on what material the foundation wall is made of.

 

 

Foundation walls

The foundation walls can be made of rock, or they can be made of blocks of concrete, sand-cement, or stabilized earth.

All of these materials are strong enough to support the walls and roof of most 1-story buildings. The choice depends on what materials are available, the builders' budget, and whether or not earthquakes or severe weather conditions will require reinforcement in the foundation.


Foundation walls

 

Rock Foundation Walls. A rock foundation wall is built by setting stones that are 20 to 40 cm. long in mortar. The rocks must be cleaned so that no rocks remain on them. All the spaces between the rocks must be filled with mortar (these spaces are called "joints"). In addition, and most important, they must be laid so they overlap. If a straight line can be drawn between the rocks from the top to the bottom of the wall, a crack will develop.

Rock foundations are the least expensive to build. However, they require a large number of rocks, and it is difficult to clean, level, and overlap the rocks properly.

If the building position is on rocky ground, or on dry, well-packed clay soil, the footing for a rock foundation wall should be 4 to 8 cm. thick.

In less stable soils, such as sand, or gravel, the footing should be at least 10 cm. thick. In soft black soil, drained marshland, and made earth, the footing should be reinforced and should be 10 cm. thick.

In rocky or hard-packed clay soil, rock foundation walls need only be 30 cm. deep. In other soils, they should be at least 45 cm. deep.

Rock foundation walls should be at least 30 cm. thick, and they will be much more stable if they are flared at the base to 45 cm.

Block Foundation Walls. Whether built of concrete, sand-cement, or stabilized earth, block foundation walls are made by laying level rows of blocks on concrete footings until the wall reaches the planned height of the floor. Each row of blocks, called a course, is joined by mortar, as are the ends of each block.

Block foundations cost more to build than rock foundation walls (except in areas where the rock must be transported over large distances), but block foundation walls can be put up faster, and they are easier to build well.


Block foundation walls

 

In rock or firm clay soil, a block foundation should be 45 to 60 cm. deep. In less stable soil, a block foundation should NOT be used.

The width and thickness of the footing depend on the size of the blocks being used. In general:

• the footing should be as thick as the blocks are wide;

• the footing should be 3 times as wide as the blocks.


The footing should be 3 times as wide as the blocks.

 

Frost Line. The frost line is the depth to which the ground in any area freezes in the winter. In climates with freezing temperatures, the footing must be entirely below the frost line.

The table below gives suggested depths to be safely below the frost line in different climates:


Frost line

Lowest Temperature in Winter, Degrees Celsius

Safe Minimum Depth for Top of Footing

-1

45cm.

-5

75cm.

-11

90cm.

-18

1.05m.

-22

1.20m.

-28

1.30m.

Sloping Ground. If a building is built at an angle on a slope, it will tend to slide downhill, causing the foundation and walls to slip and/or crack. Thus if the ground under a building slopes or is uneven, the trenches for the foundation footing must be completely levelled.


Sloping ground

 

If the ground slopes sharply, it may be easier, or necessary, to "step" the trenches. When block foundation walls are planned, it's important to make each step the height of one or two courses of blocks.


Important to make each step the height of one or two courses of blocks

 

 

 

Deciding upon the materials and dimensions for foundations

In general, the best way to decide what the foundation walls will be made of is to choose the least expensive and most easily accessible material.

Once the choice has been made between rock and blocks, the depth of the trenches and the dimensions of the footing and foundation wall may be approximately determined by using the figures outlined on pages 85-86.

It is important to remember, however, that these preliminary figures are only approximate guidelines. Any number of local conditions can make a deeper, stronger foundation advisable.

One of the best ways to determine whether preliminary foundation plans are safe is to compare them with the foundation structures of buildings in the local area. The field worker or community member should ask:

• How deep are the foundations of buildings that have cracked walls or other signs of weakness?

• How deep are the foundations of buildings that have lasted well?

• How wide and thick are the foundation footings and walls of weak buildings? Of strong buildings?

• What are the foundations of weak buildings made of? What are the foundations of strong buildings made of?

If preliminary plans for the foundation are similar to those of strong buildings built nearby on the same kind of soil in the local area, they should be safe to use. If the preliminary plans are similar to those of weak buildings they should be strengthened before the start of construction.

 

 

Drawing final foundation plane

After the materials and dimensions of the foundation have been planned, the last step before construction of the foundation can begin is to draw a final foundation plan.


Drawing final foundation plane

 

The plan should include two simple drawings:

- a scale drawing of the footing and foundation wall as they would look in a cross-section;

- a scale drawing of the footing and foundation wall measurements as they would look if seen from above.

Both these drawings are simple to produce with a ruler. Step-by-step instructions will be found in Appendix 2.


A scale drawing