Dome Magazine: Fall 1990, Vol. 3 | No 1
It would seem that the logical place to start with this subject would be at the bottom, so this is where we will begin.
One of the more perplexing problems of dome construction today is trying to dislodge ancient, inadequate and antiquated foundation systems that have been around for years.Great pressure is exerted on the building industry, in areas that host concrete plants, to outlaw the use of anything but poured concrete. That, coupled with the general efforts of the Portland Cement Association, has caused many areas to enact restrictive building codes that require the use of concrete, or cement blocks, which drives up the price of housing, already too high for too many Americans.
The dome is unquestionably the strongest of all residential housing units available to the home buying public today. Yet, very little information is generally to be had regarding the types of foundations that work the best under domes, and the reasons why they do work the best. Since economy of construction, as well as strength, also favors the dome, we will discuss these factors as they apply to dome foundations.
Options
1. Cement block. As the first option available, this foundation can be used either as full basement or a crawl space. It is used extensively where the Portland Cement Association has had an influence on the local building codes. This option requires a poured concrete footing to frost grade.
2. Poured concrete. This can take the form of a concrete slab, with poured concrete footings to frost grade. It will work for either a slab on grade or full basements. In some instances, where no other buildings or extensions are attached to the dome, and frost footings are not required, the thickened edge slab will be acceptable.
3. Permanent wood foundations (PWF), also called all-weather wood foundations (AWWF). This is the use of treated lumber (foundation grade) designed specifically for below grade construction.
Permanent Wood Foundation Systems (Basement)
Permanent wood foundation systems have been rigorously tested by the U.S. forestry Department, the University of Wisconsin Department of Forestry and the American Plywood Association, as well as the Canadian Division of Forestry. These tests have shown conclusively that the lumber will last in excess of 100 years in a wet, underground situation. These tests have also brought about approval from the U.S. Department of HUD and the VA, as well as the big daddy of all codes, the UBC. This system excels as a foundation for domes in both strength and economy. It matches perfectly two of the inherent features of the geodesic dome. It works equally well with full basements or crawl spaces.
Basic Requirements for Permanent Wood Foundation
Let’s take a look at the basic requirements of dome foundations that are different from standard building designs. The dome has a unique requirement not generally recognized by cement contractors in that it is “point loaded.” This simply means that the structural load comes down on the foundation, specifically at the points.
This is different from rectilinear structures, where the total building load comes down on the full length of the foundation. “What difference does that make?” you ask. It simply means that the cement foundation systems, when used for a dome, be they poured concrete or cement block, must have particular emphasis paid to them at the points and added attention must be given at those points as to strength. The areas between the foundation points carry none of the building framework load.
Objects to Concrete
Concrete adherents would like to have you believe that there is economy in a poured concrete basement. The fact remains that two walls must be made to make the form. Adding the cost of reinforcing rod and concrete, plus the cost of poured footings, the labor cost of making the forms and, finally, the labor cost of removing the forms, usually adds up to a total of $85 to as much as $100 per lineal foot of an eight-foot-high basement wall.
Nothing is said about the cost of time delay between the pouring of the footing, the erection of the forms, the pouring of the concrete and the removal of the forms and, most important, the need to have good warm, dry weather while this process is going on.
Cement blocks and poured concrete have somewhat similar problems in terms of cost. A poured concrete footing is necessary for both systems. The block layer has three specifications that one must adhere to. They are cord lengths (the exterior length of each foundation wall), the angle at the points of contact, and most important, the radius at these points. These dimensions must be absolute to effect an accurate bond between the dome and the foundation.
Some dome designs have walls of several different lengths at the foundation level, further confounding the block layer. If any of these dimensions is wrong, it is very difficult to get a dome framework to bond accurately with the foundation, since all the dome dimensions and angles are cut to close tolerances with jigs at the factory.
The average cost of a cement block foundation is usually over $60 per lineal foot for an eight-foot-high basement.There are some serious flaws of strength in the use of cement block foundations when compared to permanent wood foundations. For example, consider this:The permanent wood foundation consists of a series of trusses. The stud walls are generally 2 x 8 lumber, treated for below grade use, with 2 x 8 top and bottom plates. If the basement is back filled to a maximum of seven feet, the studs will be on sixteen-inch centers. If back fill is over seven feet, they will be on twelve-inch centers. The exterior is covered with below grade treated plywood. This is covered with a mastic sealer applied with a trowel, and the wall is covered with a six-mil poly, below grade. The walls are secured with double hot-dipped galvanized nails or stainless steel fasteners. Either is acceptable. These trusses are bolted together at the points which gives a total of a 4 x 8 post support under each dome point.Now comes one of the most controversial, but cost- and time-saving, features of the permanent wood foundation. The foundation walls rest on 2 x 10 treated foot plates, and the whole system rests on 3/4 inches minus crushed rock footing. A drain tile is laid around the foundation perimeter and drained to day-light or to a sump. This assures that the basement will always be dry.
No concrete is necessary, and none is recommended. The rock is normally placed in a trench that is twenty inches wide and eight inches deep. The whole foundation can be erected in one day or less, regardless of weather. Try that with any of the other types of foundations that we have discussed here.
Once the basement is back filled, the unit takes on a monolithic characteristic that no other foundation can equal for strength. Poured, reinforced concrete may come the closest in structural strength but will probably cost more than twice as much to install. Since each wall is bolted to the other at the angled points, it is impossible to move them once they are back-filled, even if they were hit with a bulldozer.
This doesn’t mean that the average home builder is going to try and hit his wall with a dozer. However, many cement block walls have collapsed when being back-filled, or, in some cases, from hydro-static pressure. Because of the inherent design, it would be impossible to push in a permanent wood dome foundation once it is back-filled. This strength characteristic is important if one is building in tornado or hurricane areas, but most important is the strength of these basement trusses in the face of massive quakes. Consider what happens to cement blocks when the world starts to shudder. They break up, and the house falls into the hole where the basement was located. And yet, ironically, many architects, builders and building inspectors in earthquake zones are still allowing cement block foundations to be built, both under conventional structures and dome homes, without a thought to the ultimate consequences. Cement block foundations, when subject to serious earthquake tremors, have very little strength.
Ultimate Advantages
Now let’s consider other ultimate advantages. The permanent wood foundation wall lends itself to insulation and finishing with standard building materials. The permanent wood foundation interior wall can be finished as easily as any other part of the house, and at a minimum cost. The exterior wall will be easily covered with conventional standard building materials.
Added to all of these features is the fact that termites do not like treated lumber foundations. No need to spray with poisons to keep your home from being chewed up by insects. All of the above features of the permanent wood foundation can be purchased for an average price of $30 to $40 per lineal foot of an eight-foot-high basement.
Beyond the two basic features of the permanent wood foundation basement, i.e. strength and economy, it is possible to live in an environment that is warm and dry. One of the first things you will notice if you live in a permanent wood foundation basement is the fact that it is dry and comfortable. No more damp and cold areas in your home. If you have made the wise decision to install a geodesic dome on top of this foundation, you are in for a double delight and an exciting experience in living.
Don’t be fooled by concrete propaganda. Talk to folks who have had the ultimate experience, that of having built, and of living in a structure that is being supported by a permanent wood foundation.