Concrete has long been recognized as a suitable barrier against radiation. In fact, considering its cost and convenience, concrete surpasses all the materials for general purpose shielding. The concrete shelter is easy to build and is virtually free of maintenance problems. It therefore is not surprising that may shelters are made of concrete. A typical six person shelter has an inside measure of 12 by 9 feet. Structural walls are eight inches thick with half inch reinforcing rods running vertically and horizontally through the walls. Box outs and sleeves for ventilation pipes are inserted between the forms before the concrete is placed. The intake pipe is covered with a hood and a screen. Only one opening is used for the exhaust pipe. It is essential the shelter be water tight. It is always a good idea to cover the ground level with a vapor barrier material. Malleable asphalt hardboard and rigid plastic foam are effective materials for this purpose. The floor itself should be 6 inches thick and be particularly careful when curing. Waterproofing should be done not only to the floor but also to the walls.