are all to be poured in place, it is possible to make them with a single pour. This can reduce the likelihood of there being cracks or leaks at the joints where the concrete has cured at different times. The foundation of the buildings designed by Vetsch are built conventionally. Walls Several different methods of external (load-bearing) wall construction in earth shelters have been used successfully. These include concrete block (either conventionally mortared or surface-bonded), stone masonry, cordwood masonry, poured concrete, and pressure-treated wood. Earthships classically use rammed earth tire walls, which are labor-intensive but recycle used tires. Oehler prescribed a very low budget method he termed "post, shoring and polyethylene". This involves buried wooden posts to act a frame, shored up with planks, and with a waterproofing barrier of polyethylene sheet between the planks and the backfill, but no foundation and nothing on the floor except a plastic sheet and carpeting. Untreated wood in direct contact with earth rots within five years of use in earth shelter construction, but can be used for structural support construction in a well-insulated and water/moisture-proofed home. Reinforced concrete is the most commonly used structural material in earth shelter construction. It is strong and readily available. Steel can be used but needs to be encased by concrete to keep it from direct contact with the soil which corrodes the metal. Bricks and concrete masonry units are also possible options in earth shelter construction but must be reinforced to keep them from shifting under vertical pressure unless the building is constructed with arches and vaults. Unfortunately, reinforced concrete is not the most environmentally sustainable material. The concrete industry is working to develop products that are more environmentally friendly in response to consumer demands. Products like Grancrete and Hycrete are becoming more readily available. They claim to be environmentally friendly and either reduce or eliminate the need for additional waterproofing. However, these have not been extensively used in earth shelter construction yet. Roof The roof of an earth shelter may not be covered by earth (earth berm only), or the roof may support a green roof with only a minimal thickness of earth. Alternatively a larger mass of earth might cover the roof. Such roofs must deal with significantly greater dead load and live load (e.g. increased weight of water in the earth after rain, or snow). This requires stronger and more substantial roof support structure.