Strawbale walls require careful consideration before you build. You need to allow for the width of the bales and, most importantly, for the weight that the render adds. So we always allow for the width of the footings to carry the weight of the render. We make the footings 50 to 70 mm wider than the bales. Without well-engineered footings you will have trouble with the walls either sagging – in the case of piers, bearers and joists – or sinking – in the case of footings in the ground. We strongly advise that you consult a structural engineer once you have decided which type of footing suits your strawbale building. We are developing suitable foundations for strawbale walls that minimise cement. Cement consumes an inordinate amount of energy and resources in its manufacture and in our opinion is an unsustainable building material. In this article we will look at three of the foundations we have used in our own strawbale buildings in Ganmain. We do not have any preference as long as the method is sustainable, cheap and easy to accomplish.
Have you ever wondered how the combination of blue metal and ironbark sleepers can carry the weight of a locomotive and various carriages that have been running along our railway tracks all these years? Why would not the same method carry the weight of a house? Well this method can and does. Frank Lloyd Wright has used this method extensively in many of his houses that have stood the test of time. Frank began building homes in 1893 and one of his most innovative buildings was the Imperial Hotel in Tokyo, which was one of the few structures to survive the disastrous earthquake that struck Tokyo in 1923. The Imperial Hotel used a system of caissons that were held in place by the friction of the soil. The soil itself was composed of approximately 18 metres of mud covered by 2.4 metres of filled soil. The caissons supported cantilevered concrete slabs that were anchored at their ends.
We use a similar principle in that the footings are dug to a depth of 700 mm and you can see by the drawing that we use agricultural pipe to drain any water that might get into the footings. It is important to slope your footings so that if any water gets in it will easily drain off.
We also anchor the concrete bond beam every 1.8 metres by using a cement pier that is tied into the bond beam with re-bar. The bottom plate assembly is then fixed to the concrete bond beam by using dynabolts or a two-part chemical bolting system. This method not only uses one-tenth of the cement but also reduces the cost to a quarter of a conventional cement and steel footing.
We have conservatively estimated that Australians consume some 18 million-rubber tyres per year (four million cars using 4.5 tyres per year). What happens to them? We are all aware that the disposal of tyres is a major environmental problem in the world. Tyres have an estimated half-life of 30,000 years so they are around for a long time.
Once they are filled with dirt and compacted, tyres make the most earth-friendly and strongest foundations that I have seen. They will outlive any cement foundations by thousands of years. They will not outgas once they are buried because tyres only outgas when they are exposed to direct sunlight. They are termite proof and, once buried, are fireproof as well.
This method uses a fair bit of labour but we are working on methods of premixing the earth fill and pouring this into the tyres in the ground. In most cases your local tyre supplier has to pay a levy for their disposal and this adds to the cost of your new tyres. We have used 55 tyres in a small round building (four metres diameter) in Ganmain – approximately ten years consumption of tyres, in our case. This is also the cheapest method of foundations that we have ever used.
The system is working out very well for this building and I can see no reason why it would not work in the footings of a much larger building. In fact we know that there are several large buildings in the USA that have used this method not only for the footings but also for the walls themselves. This method of building is know as earthship building. We visited one such earthship built by Jack Ehrhardt in Kingman, Arizona. Jack is the co-founder of CERBAT, the Center for Environmentally Responsible Building Alternatives in Arizona.
This is probably our favourite method of foundations for strawbale buildings because it is easy for owner builders to use, suits our climate well, and can be used on sloping blocks without soil excavation.
It is, however, the most expensive of the three methods and takes the longest to do. You also need to be able to use a chainsaw with some degree of accuracy. Once you have had your soil analysed for its stability you can have a structural engineer design the size of the timber members for the piers, bearers and joists. Your engineer will also be able to recommend the span that you can carry with your timbers.
We prefer to use recycled cypress pine for our piers west of The Divide and prefer recycled turpentine on the East Coast of Australia. These two timbers can be found in many different types of structures such as timber bridges, wharves and shearing sheds. They both are resistant to termites and are to be found in many of our timber buildings that have stood the test of time. There are several suppliers of recycled timbers in Australia with the most noted being Nullarbor Timbers of Moama in New South Wales.