Foundation Primer

Foundation Primer

Copyright (c) 2011, Deanna and Jim's GOLD Team

     Ignoring tents, RVs, and houseboats, houses typically have 4 broad classes of foundation: slab, basement, perimeter-piers-and-crawl-space, and tall piers.

SLAB

The foundation for the house consists of a concrete slab on the surface of the ground, with steel mesh or bar reinforcing inside the concrete, and with deeper extensions into the soil at the perimeter.  Interior structural columns if any will have deeper footings too, often made by boxing holes in the slab form then subsequently digging each hold deeper to pour a small column footing.

The advantages of a house build on a surface slab are low cost (no need to excavate a basement hole), and are no issues of water penetrating the basement, nor of critters getting into the crawl space since there is neither a basement nor a crawl space present.

This disadvantages of a slab are: no basement space, no basement nor crawl space through which to run underfloor ducts for the main level.

BASEMENT

A house with a full basement is like a house on a slab, except that a one-story deep hole the side of the house is excavated perimeter footings are poured, basement walls are either built from blocks, poured in forms, or separately poured and tilted into place (Pulte Homes), and then the baement slab is poured.  Ideally the basement footings are surrounded by a gravel filled trench with a French drain (perforated drain pipe buried in gravel to capture and route away any water accumulating around the foundation.  Then the exterior of the basement walls are sealed with an asphalt-based tar substance to waterproof them. Then the hole is backfilled around the walls. The result is a house with a basement. The cost is substantially higher, of course.  Basement homes built into the side of a sloping lot may have a walkout to level ground on one side and the other side basement buried in the ground.  

If a substantial portion of the basement is buried most builders will install a sump pump from a sump pit located below the basement to collect and pump out any foundation and sub-foundation water before water penetration can occur.

Basements ease installation of plumbing, wiring and ductwork for the main level which is easily routed below the floor in the ceiling of the basement.

The advantages of basements are: lots more space in an area that is inexpensive to heat and cool. The disadvantages are construction cost and potential water penetration.

CRAWL-SPACE-AND-PIERS

A house built on a crawl space and piers has a space less than one story between the ground and the floor joists for the main level. The load bearing perimeter of this space may be concrete blocks, poured concrete, or piers covered by wood, metal, or vinyl siding or lattice-work.  This kind of foundation is common for a mobile home set on a foundation.  The crawl space usually has an earth floor. The crawl space provides room for installation of ductwork, furnaces, water pumps, plumbing, and or electrical lines. The main level is usually of wood frame construction, although steel beams may be used at the perimeter or for key spans.

Concrete-footed piers of either concrete, metal or wood may be located at various interior locatios to provide intermediate support for long spans.

The advantages of crawl-space-and-piers construction are: lower cost than digging and building a full basement, ease of installation of ducting, plumbing and wiring for the main level.  The disadvantages of this construction include: crawl space is an attractive location for rodents and other creatures to find shelter, if sparingly constructed the house can feel shaky (think how an old mobile home shakes as you walk around).

TALL PIERS

A house built on tall piers has one or more stories of fairly clear air space between the house and ground level. These houses are usually built in areas where flooding is a real possibility or already a fact. They include houses build over bodies of water (true lake houses), some houses built on the sides of very steep grades or mountains, beach houses, or houses built by a levee in a flood plain.  The advantages of this kind of construction are: (1) you get to build a house where otherwise you could not, (2) possibility of docking your boat under your house if the house if over water, (3) underhouse carport space if  the house is not over water, no issues with water penetration of a basement.  The disadvantages are (1) no secure basement in which to store valuables, (2) inconvenience of climbing up and down stairs everytime you come, go, or take out the trash, and (3) somewhat increased construction cost.