Untreated wood rots within five years of use in earth shelter construction. 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.