Underground shelters are built to prepare for nuclear attack, tornadoes, civil unrest preparedness, and when not in regular use, for food storage. Regardless of your original intent in building a shelter, you want the strongest possible one when it is done. Cinder block-built structures are typically very durable. An underground shelter is protected from high winds, projectiles that fly during high winds, and in some part, from the effects of radiation. Unfortunately, they are susceptible to flooding and leaking if they are close to the water table.
Planning for the Underground Shelter
Step 1: Look Into Potential Licensing and Permits
Inquire about licensing and permits required to build your shelter. Whether you need a permit or not depends on the specific laws in your area. Some prefer secrecy when building a shelter, so that as few people as possible know about it. Having permits in order, however, will make certain aspects of the building process easier.
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Step 2: Create a Plan for the Shelter
Plan the location and size of the shelter. Ideally the shelter should be far enough away from trees and buildings that may collapse on top of the door, making escape impossible even once the danger is passed. The shelter should also be built close enough that everyone in the family can get to it easily. The Federal Emergency Management Agency, also known as FEMA, recommends that a 10-foot by 10-foot shelter is appropriately sized for a temporary shelter for up to six people. Plan the size of your shelter based on the number of people that may use it.
Step 3: Determine How You'll Build the Shelter
Choose the method of underground building. The most direct method involves scooping out the dirt in a pattern just a little larger than what the structure will be. This method, however, can lead to difficulties in getting materials to the bottom of the hole to complete the project. Another method involves building the structure in a natural valley of the property line above ground and covering the structure over with dirt when the project is finished. This method allows access to each wall and the ability to move materials easily from one end of the shelter to the other.
Begin To Build
Step 1: Dig a Hole Using a Back Hoe
You want the hole to be 3 feet wider and longer than the actual size of the shelter. You also want the hole at least 2 feet deeper than the shelter will be tall. This will allow for a solid roof covering consisting of a layer of concrete and a layer of soil.
Step 2: Frame the Foundation of the Shelter
The foundation should be 8 inches wider and longer than the dimensions of the actual shelter. Lay an 8-mm moisture barrier across the bottom of the hole and build the frame for the foundation on top of this barrier.
Step 3: Install the Posts
Drive one 2-inch square post every 18 inches of foundation length and width. Use 3/4-inch plywood boards and secure the plywood to each post using three 2-inch self-tapping screws.
Step 4: Install Reinforcement Steel in the Foundation
Lay rebar in the foundation frame in a checkerboard pattern. Drive rebar stakes into the ground with a hammer to help secure the rebar in place.
Step 5: Pour Cement Into the Foundation
Call a professional cement mixer to come pour the cement into the foundation. A concrete professional will be able to inspect the frame for strength before pouring what will be the most expensive part of your shelter. Let the cement dry for a week so that natural expanding and contracting of the wet cement can take place before cinder block is laid.
Building the Walls
Step 1: Install the Door Frame
Use concrete bolts driven into the foundation. The cinder blocks will be stacked around the door frame.
Step 2: Lay the Cement Blocks
Use a mason's trowel and spread mortar along the edges of the foundation. Press each block into place in each row. A row of blocks is called a course. Spread mortar along the sides of each block before placing it next to another block in each course. It is optional to stagger the pattern of blocks because you will be filling the spaces in the blocks with cement to give the walls added strength. Repeat laying blocks until the height you wanted is reached. Allow the mortar to dry for three days.
Step 3: Apply a Moisture Barrier Spray
Apply a generous layer of moisture barrier spray to the outside of the walls. Let it dry. Apply two more coats of spray.
Step 4: Reinforce the Walls
Fill the empty spaces in the cinder blocks with poured concrete. This will add strength to the walls.
Step 5: Install a Ventilation System
Bolt a 2-inch PVC pipe into each corner of the building with plumbing brackets and concrete screws. Each pipe should extend 3 feet over the roof of the shelter after it has been covered, and at least 3 feet from the roof into the shelter. These pipes will be the shelter's ventilation system.
Building the Roof
Step 1: Install and Anchor Plywood
Anchor 1-inch plywood to the top of the cinder block walls using 3-inch concrete screws. The plywood should extend over the entire shelter.
Step 2: Install Secure Support Beams
Secure four 4-by-4-inch support beams on the inside walls of the shelter next to the pipe in each corner. Use an "L" bracket at the top of each beam where it meets the ceiling and drill a screw through the bracket at each hole. These support beams should reach from the floor to the plywood ceiling of the inside of the shelter. Place three more 4-by-4-inch support beams in the center of the shelter. Place one against the left wall, one on the right wall and one in the exact center of the shelter.
Step 3: Level the Beams
Seat each beam so that each one is level. Keep them in place using "L" brackets. Once a beam is seated properly between the floor and ceiling, place an "L" bracket at the top of the beam and attach to the ceiling using self-tapping screws. The center beam can be removed when the concrete roof is dry, or kept in place if preferred.
Step 4: Reinforce the Roof with a Moisture-Resistant Barrier
Lay down a moisture barrier, as you did for the foundation, and pour a 1 1/2-inch thick layer of concrete for the roof. You may want to consider calling the concrete professional you used for the foundation. Let the concrete dry for three days. Your shelter is now ready for use.