How to Build a Cinder Block Safe Room

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The Federal Emergency Management Agency (FEMA) recommends construction of a "safe room" in every house in areas subject to extreme winds, such as tornadoes and hurricanes. A safe room has walls and ceilings strong enough to withstand very high winds and falling or flying debris. Cinder blocks, concrete blocks made with coal ash rather than gravel for aggregate, may be used to build a safe room. Safe room cinder block construction is a major undertaking, and requires both masonry and carpentry skill. It also may require professional assistance.

Things You'll Need

  • Tape measure
  • Chalk line
  • Moisture barrier
  • Tape
  • 2-by-4-inch framing lumber
  • Concrete
  • Wheelbarrow
  • Trowel
  • Cinder blocks
  • Mortar
  • Level
  • 4-by-4-inch beams
  • Bolts and nuts
  • Drill
  • Wrench
  • Sheet steel
  • 2-by-6-inch framing lumber
  • Framing nails
  • Hammer
  • Concrete nails or anchors
  • Steel or similar pre-built door
  • Reinforcing bars
  • 3/4-inch plywood

Location

  • Find an appropriate spot to build a cinder block safe room. Put one in a basement, preferably in a southwest corner, with floor joists high enough to allow a strong roof. Convert a large closet, little-used room or section of an attached garage in a house built on a concrete slab foundation. Add a separate addition if there is no other suitable space.

  • Identify a space large enough to accommodate all the residents of the house comfortably. Provide easy access from the house and find or build door frames strong enough to support thick and heavy doors. Plan the construction so there is ample access to the work area for cinder blocks, mortar, steel bars and plates, and other components.

  • Determine if the underlying structure or foundation is strong enough to support the weight of cinder block walls and a strong ceiling. Look at door bottoms and floor edges to see if you can determine the thickness of the concrete slab. Consult an architect or builder if necessary, as putting too much weight on a floor can cause the concrete to crack.

  • Reinforce concrete floors if necessary by adding another layer of concrete. Build wooden forms around the chosen space.Set 2-by-4-inch boards on a 2-inch edge and brace them with boards nailed to the outside with framing nails and a hammer. Lay reinforcing bars inside it in a grid pattern, fill the form with concrete mixed in a wheelbarrow and level it with a trowel. Add cinder block piers under the wall locations in a house with a pier and beam foundation, and crawl space access.

Basement

  • Mark a square location in a basement corner, preferably the southwest one. Measure the outline with a tape measure to the size you have chosen. Mark inside corner locations with chalk and snap chalk lines on the floor between marks to outline wall locations. Leave a slight gap between the concrete basement wall and the safe room cinder blocks. Tape a plastic or similar moisture barrier to the outside wall.

  • Lay cinder blocks to form walls on all four sides. Mix mortar in a large container from a prepared mix and spread it on the sides and ends of a block using a trowel. Set that block in place along the marked line and level it with a level. Add other blocks along and on top to make a wall. Offset the blocks so each block above the first layer rests on top of two blocks below. Use a level to keep layers plumb and level.

  • Leave a door opening on one wall, the width for a rough frame for a pre-selected door. Build walls to within about a foot of the joists supporting the house and floor. Put bolts upright in concrete in holes on the top blocks to secure roof beams. Drill holes with a power drill in 4-by-4-inch pressure-treated wood beams to match the bolts. Set those beams across two walls to make a roof frame.

  • Cover the beam ceiling with sheet steel, with holes drilled in the plate to go over the bolts. Fasten the beams and steel with nuts on the bolts and tighten the nuts with a wrench. Buy steel plate from a supplier who will drill holes at the specified locations.

  • Frame the door with 2-by-6-inch framing lumber, two boards nailed together with framing nails and a hammer. Put vertical 2-by-6s on each side of the doorway and a horizontal header on top of the two side frames. Set this frame plumb using a level. Fasten the frame to the cinder blocks with concrete nails or anchors.

  • Install a pre-built door. Level and plumb its frame with tapered wooden shims on the edges. Fasten the door frame to the rough frame with screws, driven with a screw gun through the door frame and shims into the rough frame. Follow manufacturer's directions for specifics of door installation, as these can vary.

Interior

  • Make a safe room in a closet, room or garage, using a chalk line to mark wall locations. Leave a gap between wood-framed walls and the cinder block, so wind-driven framing will not put pressure on cinder blocks. Put vertical steel reinforcing bars in the holes in the cinder blocks, starting with the first block. Secure bars with mortar or concrete at the base.

  • Lay layers of blocks, putting mortar on the bottoms and edges of the blocks to fasten them. Stagger the joints between blocks so each block after the first layer rests on two blocks. Place blocks over rebar where it is installed. Leave space for a door; put rebar in blocks on both sides of that opening. Build walls to within about a foot of the ceiling or floor joists at the top of the house wall. Mix concrete in a wheelbarrow and fill cavities in the blocks around rebar with concrete.

  • Top the walls with a ceiling of sheet steel or built-up wood framing. Fasten 4-by-4-inch beams across the walls with bolts set vertically into mortar in the holes of the last layer of cinder block or cover the beams with multiple sheets of 3/4-inch plywood screwed to the beams. Use at least two sheets of plywood; FEMA estimates it takes 5 layers of plywood alone to withstand hurricane debris damage. Buy sheet steel to size, with holes drilled to go over the bolts and be secured with nuts fastened with a wrench.

  • Frame the door opening with two 2-by-6-inch boards nailed together. Set side frames from the floor to the opening top, with a doubled 2-by-6 header board across the opening. Fasten 2-by-6s to blocks on the sides and top of the space with concrete nails or anchors through the boards into the blocks. Plumb the rough frame using a level.

  • Install a pre-built door in the rough frame. Use tapered wooden shims to get it plumb and level, and fasten it to the rough frame with screws. Follow directions from the door manufacturer for specifics of how to fasten the door; most will be secured through the sides of the finished door frame, but some have lips on one side to hold screws.

Addition

  • Build a safe room as an addition by marking the location at an entry point from the house. Square that location and dig footing trenches around the outline, at least 18 inches deep or below the soil freeze line. Mix concrete or have it delivered and pour it into the trench to make concrete footings. Set steel reinforcing bars in the footings vertically at corners and 4-foot intervals inside.

  • Make a form of 2-by-4-inch boards, set on a 2-inch edge and braced securely with wood stakes on the outside edges. Square the form by measuring diagonally and adjusting framing until the diagonals are equal. Mix concrete in a wheelbarrow or have it delivered and pour a slab inside the form. Lay steel reinforcing bars in a grid pattern inside the form when it is half-filled. Smooth the concrete surface with a trowel.

  • Leave a gap between the house wall and the cinder blocks. Lay blocks over the rebar through holes in the blocks. Spread mortar on the bottoms and sides of the blocks to secure them and set them in place one at a time. Use a level to keep blocks level and walls plumb. Build walls to a desired height, usually about 8 feet. Make space for a pre-built entry door, with rebar in blocks on both sides. Make this space to the specifications of the pre-built door. Fill cavities around all rebar with concrete.

  • Outline the roof with 4-by-4-inch beams on top of the walls, fastened with bolts set vertically into concrete in the top layer of blocks and secured with nuts. There should be a 4-inch gap on the inside of the blocks. Put a base of sheet steel inside the beams. Lay rebar horizontally in a grid pattern between the beams and pour concrete over it to form a ceiling.

Tips & Warnings

  • Consult an architect for any questions about support for block walls or alternatives on safe room ceiling construction.
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