At the entrance side of the shelter, each roof beam is rested on the inside 4 inches of the block wall. The outside 4 -- inch space is filled by mortaring blocks on edge. The wooden bracing between the roof beams is placed flush with the inside of the wall. Mortar is poured between this bracing and the 4 -- inch blocks on edge to complete the wall thickness for radiation shielding. (For details see inset, fig. 5.)

The first one or two roof boards (marked ``E'' in fig. 6) are slipped into place across the roof beams, from outside the shelter. These boards are nailed to the roof beams by reaching up through the open space between the beams, from inside the shelter. Concrete blocks are passed between the beams and put on the boards. The roof blocks are in two layers and are not mortared together.

Work on the roof continues in this way. The last roof boards are covered with blocks from outside the shelter.

When the roof blocks are all in place, the final rows of wall blocks are mortared into position. The structure is complete. (See fig. 7.) Building plans are on page 21.

Solid concrete blocks, relatively heavy and dense, are used for this shelter. These blocks are sold in various sizes so it seldom is necessary to cut a block to fit.

Solid blocks are recommended because hollow blocks would have to be filled with concrete to give effective protection.

Bricks are an alternative. If they are used, the walls and roof should be 10 inches thick to give the same protection as the 8 -- inch solid concrete blocks.