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Foundations, basements and footings


Laying a good foundation is an important part of any endeavor in life, but extra important in house building

If you want your home to remain reasonably true and livable for a long time, a  good foundation is critical.There are many choices a foundation types, and of materials to use, when building your home. They will depend on whether you want a basement for a living space, and/or there are special challenges posed by location, and the soil conditions of your building site.

pouring concrete for foundation

Pouring concrete for foundation or footing is hard labor

An ideal situation, is one where the foundation can rest directly on bedrock, but this is seldom possible. It is more common that a house must be “floated” on soil of various degrees of firmness, sometimes on the same site. A foundation should never rest directly on loam, peat or other topsoil, and you should try to avoid disturbed soil of any type. Undisturbed clay or gravel usually form a good base, but there may be special considerations. Some clay, bentonite as an example, will swell when it becomes wet, and can raise havoc with floors and footings resting on it. Care must be taken to provide a consistent moisture level, or to remove layers of this type of material.

Other soils may have chemical properties that will attack and degrade concrete, or other foundation building material. A soil test may be of value, but a good practice is to see what problems have arisen with the foundations of older homes in the area. Problems may take 30 years or more to show up, but we would expect a lifespan of a house to be 100 years or more.

Building on a concrete slab is a very simple method, and is often used for garages and outbuildings.

A well reinforced slab will float on unstable ground, but can be affected by frost heaving. Some building codes may not allow it on a dwelling, unless there is a perimeter footing below frost level.

Hydronic radiant heating may be placed directly in the floor, and can be a considerable advantage.

Very few stair steps are necessary for entryways.

Walls come in close proximity to the ground with this method. Treated bottom plates should be used  and the outside sheathing should be treated for at least a foot (31cm) above ground level.

Use extra reinforcing under bearing walls ( a bearing wall is any wall that supports more than its own weight such as a portion of a floor or roof).

A crawl space leaves room for mechanical installations and may provide some storage. It may be a conditioned space (heated or cooled) or not. If unconditioned, then insulation and air barriers must be provided in the floor. Crawl spaces are usually conditioned in cold climates, and the foundation should include insulation and air barriers. A vapor barrier should be placed on exposed earth, but should not be sealed, as it should allow for water to drain. One method I have seen used, is to place house wrap exterior side down over the soil. I cannot attest to it’s effectiveness, but it does seem to make sense.

A basement is is only a crawl space extended up or down.

basements are usually between 3 and 8 feet in the ground, and with 8 to 10 foot (2.44 to 3.05 meter) foundation walls. Foundation walls may be of one material or a combination. They may be floored with a concrete slab (most common) or a framed wood floor. Any portion of a wood floor within 1 foot of the earth, or otherwise subject to wet conditions, must be of treated wood. Basements are commonly finished to provide extra living space, and as such should have sufficient headroom. This may require 9 or 10 foot foundation walls to accommodate ducting or sound proofing.

A post and beam foundation might be an inexpensive alternative, especially for cabins or smaller homes. They are also commonly used to keep a building above ground level where flooding may be a problem, or when building on permafrost. Similar building techniques would be employed as with an unconditioned crawl space. Posts may be light tubes filled with reinforced concrete, treated wood (either solid or built up) or steel pipes which may be concrete filled. Posts may continue upward to form part of the framing of the building. Beams may be of many different types and materials.

All types of foundation require a footing of some kind, except in the case of friction pilings.

A friction piling is either driven into the ground until it meets sufficient resistance, is set in concrete, or is a reinforced concrete pile poured directly into a hole, and relying at least partially on the contact with the sides of the hole to provide stability. The friction piling may not be appropriate where freeze thaw cycles could be a problem.

A footing is either a continuous lineal pad under the foundation, or a round or square pad under a piling or post. The purpose is to spread the weight over a wider area of the soil. Think of a snowshoe. A pad of reinforced concrete 8 inches (20 cm) deep and 16 inches (41 cm) wide, or in circumference, is normally sufficient. If soil conditions are bad, or if posts are far apart, then a wider footing can be used. Extra reinforcing and thickness may be needed. Sometimes a footing of treated wood placed on clean gravel may be used.

There are several types of foundation wall, but all should be firmly secured to the footing. Horizontal forces from back fill and hydraulic pressures are the major concern.

A concrete wall without reinforcing 8 inches or more in thickness has been the norm for many years, although there is no reason it cannot be reinforced, if in an earthquake prone area. Reinforcing should be added above all openings. In some old homes, large stones were added, to save on concrete. This was a poor practice, as it could weaken the structure.  Concrete network.com  is a good info source for all kinds of concrete work.

Although considerably weaker, foundations are often built of masonry blocks, bricks or stone. Solid concrete pillars are often added at intervals to increase strength.

PWF (preserved wood foundations) are becoming more common. The lifespan of these are about the same as concrete, in most conditions. They are popular where concrete is difficult to obtain, or experienced concrete workers are unavailable at a reasonable price. The basement and main floor must be done before back filling. Extra care needs to be taken, to provide base drainage and back sloping, in order to reduce hydraulic pressures. This form of construction is easy to insulate and finish, as it is nearly identical to upstairs wood framing. This is not a good choice on steeply sloping sites, as the ability to resist uneven lateral forces is not good.

A recent innovation is the use of Styrofoam blocks filled with reinforced concrete. The blocks are usually two pieces of 2 ft. by 4 ft. Styrofoam 2.5 inches thick. They are held about 6.5 inches apart by plastic tie, and have embedded plastic strips, for nailing and screwing. They can comprise the entire exterior wall system of the house. Although material may be costlier, construction is fast and simple, and does not require skilled labor. No further insulation or air barrier is needed, and finishing material can be attached directly. Sound proofing is excellent. About 20 percent less concrete is used than in a conventional concrete wall, so this method may be more environment friendly. A concrete pumping truck will probably be required, so availability and cost is a factor. The walls will be thicker, adding an element of difficulty in finishing.

I have used all these types of foundation in certain circumstances, and seen them used in many more, and I do not have a favorite. Cost and conditions are the determining factors. I like PWF for remote locations. Concrete or posts work best on steep sites. Styrofoam block is good when labor is at a premium. Posts are convenient for farm buildings, cabins, and some difficult sites. I find block, brick or stone fine for low foundation walls.

A good understanding is a good foundation to build on.

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