Check with the local zoning office to ensure that there are not permits or licenses required to build a backyard structure like this. Some counties are more lenient than others. The last thing you want is to tear down what you have built for an issue as minor as a proper building permit.
Examine the layout of the area in which you want to put this shelter. You don't want the shelter to be in the path of a felled tree or to be rendered a prison if the house should collapse onto it. Look for trees in neighboring yards that may fall on top of the shelter, and avoid those areas.
Measure the space needed for the shelter using tape measures, and drive a small wooden stake into each of the four corners, making sure the area is level by measuring between the stakes on opposite corners. Tie a carpenter's string to each of those stakes and stretch across the length between them. The distance between each of the opposite corners should be the same. The point in the center of the structure where each of the corners meets should be the exact middle of the shelter: Place a small stake in that spot as well. According to FEMA recommendations, you should plan for five square feet of floor space for each person in a tornado shelter and twenty square feet for a hurricane shelter, with added allowances for any occupants who may be wheelchair-bound or bedridden. (see Reference 6)
Contact a professional to frame and build the foundation and pour the concrete. Make it clear, when you do, that the foundation will be for a storm shelter as that will require different lengths and amounts of rebar reinforcement. You'll need several tons of concrete for this project; without access to a large mixer, doing this portion by yourself would take a very long time.
Building the Shelter
Choose one side of the foundation that will serve as the front of the shelter. Measure the width of the door frame you will be placing in the shelter and mark that area on the foundation with a snap line or carpenter's chalk.
Set the door frame into this opening and bolt it to the foundation using concrete anchors or screws. Brace the frame with a door jack system that will keep the frame from moving while the walls are completed. The steel door frame should have a sheet of 3/4-inch thick bracing at the top. Once the door is set into place, begin laying the block.
Spread a generous portion of mortar on the edge of the foundation and lay out a course of blocks onto the mortar. Begin at one corner and lay the first block as close to that corner as possible. Spread some mortar on the ends of each block before placing another block next to it. Continue placing blocks and spreading mortar until the first row, or course, of blocks is complete.
Check the level of the first course of blocks on each wall. If it is level you may continue, if not, make some adjustments in the alignment. On the second course of blocks, begin at the same corner you began with on the first course and overlap the second block at that corner. Each block should have two square holes in their center. On the first course these blocks are laid side by side. On the second course, the holes in the center of the blocks should overlap two different blocks. This will add strength to the structure and prevent a single wall unit from collapsing into the shelter.
Complete the courses of blocks, overlapping each layer up to the point where the height of the door frame is reached. If the frame has the added support of the 3/4-inch steel bracing at the top check the level with the surrounding block. If it doesn't, you will have to cut a piece of galvanized steel to fit over the door frame and the surrounding block.
Building the Roof
Lay out and mortar one more course of blocks above the door frame height. Place blocks over the door frame and mortar into place.
Spread mortar onto the bottom and sides of three inch high by 9-inch long spacer blocks. Place these spacer blocks on the outside rim of the blocks on the wall. This will create an area over the shelter where the inside rim of the blocks will serve as a support for the roof. Let the mortar dry overnight.
Lay a solid piece of 3/4-inch treated plywood over the inner rim of these blocks. If the shelter is large enough, you may want two pieces laid across the top of the shelter. Make sure the plywood lines up against the block spacers well.
Prop up the plywood from inside the shelter using a 6-inch-by-6-inch wooden post that extends from the underside of the plywood to the concrete foundation. Place this post in the center of the shelter. Place another post in each corner and another in the center of the back wall. Make sure each is level.
Pour the 1 1/2 inch layer of concrete into the indented area on top of the shelter. Let the concrete dry for two days. Check for any cracks or weakness in the roof and if you can't find any you may remove the supporting pillars inside the shelter. If your shelter is wider than 5 feet across you may want to consider using rebar and additional supports.
Things You Will Need
- Concrete foundation
- Cinder blocks
- Steel doorframe
- 3/4-inch sheet of galvanized steel
- Spacer blocks
- The number of blocks, cinder and spacer, will depend on the size of shelter you are building.