Underground concrete shelters are the safest and strongest shelters known. They can last for centuries and hold up against great forces, such as strong winds and explosions. The secret to a reliable underground concrete shelter lies in its construction. Failure to follow simple tried-and-true procedures will result in structural failure. The good news is that the average do-it-yourselfer can construct an underground concrete shelter in two to three days, depending on the size and scope of the project.
Dig deep enough that only 6 to 8 inches of a 7-foot-tall shelter will be exposed. Remove enough soil from the shelter site to allow for drainage pipes around the walls of the structure. Try to place the shelter in a hillside if possible.
Pour footers for the concrete shelter. The footers should be 12 inches thick and double the wall thickness. For example, if the shelter wall is 8 inches, then make the footers 16 inches wide. Add 1/2-inch horizontal rebar into the wet concrete footers, as well as 1/2-inch vertical rebar set 4 feet apart. The vertical rebar will help tie the shelter walls to the footers, and the horizontal rebar will help strengthen the footers.
Lay 8-inch concrete blocks onto the footers after they have dried for 24 to 48 hours. Place the blocks so that the vertical rebar that was set into the footers extends through the center of the block cells. Stagger the concrete blocks so the block joints do not align--the end of each previous block should align with the center of the next block that is laid on top. Leave a 32-inch-by-60-inch door opening for a steel door in the wall of your shelter. Once the blocks have dried, install the door by removing the door from the frame and sliding the frame into the block opening. Using a hammer drill, drill through the predrilled holes in the door frame and into the block opening and insert concrete Tapcon screws using a screw gun and Phillips-head screw bit. Once the frame has been attached, reinstall the metal door onto the frame hinges and install a doorknob.
Lay porch metal--also known as corrugated metal--on top of the shelter walls so that it overlaps the blocks by 2 inches but do not allow it to cover the block cells of the walls--you will fill these cells with concrete. Form around the top edges of the block walls with 2-by-6 lumber.from the inside of the shelter, and place temporary 2-by-4 braces under the corrugated metal until the poured concrete has dried. Cut a 2-inch hole in the top of the roof metal before you pour concrete and insert a 2-inch PVC pipe for ventilation, allowing it to extend 4 inches above the finished concrete line (or to the top of the 2-by-6 form boards). Add a vent cap on the outside of the pipe to prevent rainwater from entering the pipe. Add 1/2-inch horizontal rebar spaced no more than 12 inches apart, and pour the top of the shelter solid, making sure the concrete fills the block cells solid, as well.
Add 6-inch perforated drain pipe to the outside of the shelter walls and extend it beyond the shelter so that any accumulated water will drain away. Add roll-on tar to the outside of the shelter to prevent moisture form entering the walls. Add gravel over the drain pipes, and cover the concrete shelter with soil.