Foundation Basics

All houses have some sort of foundation. The foundation is the portion of the house that is below ground level, and which anchors the house to the earth, spreads the load of the weight of the house, and prevents the house from settling unevenly. Foundations can be minimal and simple in some locales, or can be complex and large where seismic activity is anticipated, where the ground typically freezes several feet below the surface, where the home site is on a hillside, or for other reasons.

Other than supporting the house, the most common problem a foundation is asked to solve is frost heaving due to the ground seasonal freezing and thawing. Foundations in such climates must extend below the frostline--the subsurface level to which frost typically can reach in the locale. In areas where the ground can freeze to a depth of six feet or greater, the foundation often becomes a room below ground level: a basement. Where the frost line is more shallow, shorter "stub" concrete walls are used, creating a "crawlspace" beneath the house that is too short a space in which to stand upright, and which usually remains unused and "unfinished". The flooring of living spaces above basements or crawl spaces is framed of wood joists or TJI engineered wood trusses, and covered with plywood or OSB (Oriented Strand Board).

The bottommost part of the foundation is the A footing which is a pad of concrete several times as wide as concrete stub or basement walls, upon which the basement or stub walls rest. The footing, typically 18" wide and 12" thick, is poured at the bottom of a trench which is dug in the shape of the house floorplan and which extends below the frostline. Footings may be poured first, and the basement/stub walls later, or (depending on the height of the walls) both footings and walls may be poured at the same time.

Slab Foundations

Slab foundations are often used where frost levels remain shallow (and sometimes when they are deep), and where the floor is to be concrete rather than wood (as in the case of most garages). Slab foundations cap the footings with a flat 4" thick slab upon which the house is built. Slab foundations are often said to "float" on the underlying earth. While simple, slab footings are notoriously cold for living spaces, are unfriendly to the feet and knees, and use what some feel to be an excessive amount of concrete. Slab floors can be made more user-friendly by insulating with 2" or 4" of rigid styrofoam under the poured concrete slab. Slab floors are also sometimes used as heat sinks when radiant floor heating is employed.

Foundations for Strawbale Houses

Strawbale walls require support for the bales that is at least as wide as the bales, which is about 18" if bales are stacked on edge, and about 24" if they are stacked flat. If strawbale walls are used atop 6" thick foundation walls, extra width obviously needs to be supplied to support the bales' width. Most strawbale houses that have been built in the US have not been built over a basement or crawlspace, but instead have used a simple wall footing (with adobe or sand floors) or, in fewer cases, a simple slab foundation. The reasons for this appear to have to do with the local climate, and the builder's valuation of simplicity.

Most strawbale house footing/foundations (where stub or basement walls are used) are thus about 24" wide, and extend from about 10" above finish grade to below frostline. Rigid styrofoam insulation is placed against the outer face of the foundation over its full height to prevent the intrusion of cold into the ground beneath the house. Steel bar called "re-rod" (short for reinforcing rod) is inserted into the concrete during the pour to provide tension members inside the concrete and to help prevent cracking of the concrete. Other metal pieces such as eye bolts (appropriate to the specific roof tie-down system being employed) are also inserted in the concrete when the foundation is poured. Both the top and exterior of the foundation is liberally covered with tar to prevent the migration of moisture into the concrete from the surrounding earth, and also to prevent moisture within the concrete from wicking into the bales.

Foundation/footings as described above contain a lot of concrete because they are thick, deep and wide. One alternative method which decreases the amount of concrete was developed and used by Architect Frank Lloyd Wright. A foundation/footing trench is dug to below frostline, as would be done with a normal foundation. But rather than concrete, rubble (6" rocks) is packed into the trench up to the grade level. The rubble is packed and leveled, and a grade beam (which is simply a concrete beam of rectangular cross section) is poured on top of the rubble. The rubble supports the grade beam, and the grade beam supports the house and provides a level building surface. While this method is functional and uses less concrete, it may not be acceptable to local building officials, and may be a questionable choice in areas of seismic activity.

Foundation Design to Keep Bales Dry

The foundation described above provides a flat surface upon which the staw bale walls rest. Concern about a buildup of moisture in the bales at this location has led to a recent series of conversations about how the foundation upper surface can be modified to help discourage moisture buildup. One of the most intriguing and promising ideas (suggested most succinctly by Buzz Burrell is as follows: Two pressure-treated wood 4 by 4's are laid side by side on the foundation so that they support and inner and outer edges of the bale. The 4 by 4's are aligned so that the outer surface of the 4 by 4 is flush with the outer surface of the bale, and similarly for the inner surface of the bale and the other 4 by 4. The bale thus bridges a 16" gap between the two 4 by 4's (in the case of a 24" wide bale wall). The gap between the two 4 by 4's is filled with a porous rock such as pea gravel. The bale is thus supported on its edges by the 4 by 4's, and at its center by the gravel contained by the 4 by 4's. The 4 by 4's are run around the entire perimeter of the house providing a porous "pedestal" upon which the bale wall rests. The 4 by 4's and the porous gravel prevent any wicking of moisture into the bales. Furthermore, any moisture leaking down through the bales from leaks above has an opportunity to escape, and the bales have an opportunity to dry.

for even more active ventilation, one or more 4" plastic drain pipes can be placed within the gravel and run around the entire perimeter of the walls. The pipe interior can be actively or passively vented to outside air so that a continual flow of air can carry away any moisture which might leak in or condense. Active ventilation can be provided by an electric fan, which could be used when moisture testing indicates a buildup of wall moisture. Alternatively, passive draft inducing chimney caps and dorade boxes can provide a constant (or at least regular) air flow through the piping without need of electricity.

Foundation and Gutter Drains

Water problems should always be expected. Perforated drain pipe should be buried at the bottom of the foundation exterior, and covered (backfilled) with porous sand or pea gravel. The drain pipe should be carefully installed to provide a downhill grade so that any water that might accumulate against the foundation of the house can drain away from the house and eventually exit downhill at ground level (preferably at a small pond or garden).

Drain pipes leading from roof gutters are usually routed into the area outside the foundation, above and near to, but separate from the foundation drains. They similarly drain away from the house to an area where the water can be used or saved.

If a drain to ground surface is not possible, a hole called a dry well, 4 to 6' in diameter and 6' deep, can be dug and lined with porous drain cloth. A section of 12" sewer pipe, with its length roughly equal to the depth of the hole is then placed vertically in the center of the hole. The hole is filled with 6" rock (captivating the sewer pipe), a sump pump is lowered into the sewer pipe, the pipe is capped, drain cloth is placed over the rock, and the area is covered with topsoil. Water draining from the foundation and gutter drains collect in the spaces between the rocks in the dry well and is periodically evacuated by a sump pump located in the vertical section of sewer pipe. Dry wells are a last resort however, being costly and requiring electricity, and can be avoided by proper placement of the house with respect to surrounding terrain.

General Site Drainage

If the house is to be located on a hillside, a possibility exists that underground water flowing along subsurface soil strata into the flat area excavated for the house might cause future water problems under the house. Prudence suggests the drilling of a small test hole before construction at least as deep as the anticipated foundation depth on the house site during the Spring ( or wettest time of the year) to ascertain whether this possibility exists. If it does, prudence also strongly suggests that a French drain be dug across the hill, uphill from the house, to at least the depth of the foundation bottom.. A French drain is a trench, lined with drain cloth, containing a perforated drain tile and filled with pea gravel to grade level. A French drain intercepts both surface and subsurface water before it reaches the house and diverts it away from the structure. Adequate soil drainage undertaken before construction is initiated is a step that is often ignored, usually to the detriment of the house structure, the construction timetable, and the peace of mind and pocketbook of the owner.

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James Lux, January 12, 1996