Step 9. Install Plywood Ceiling.
Using a drill and a skill saw, cut a 4 ¼ inch diameter hole in a sheet of plywood for the intake vent pipe. (Detail 9.1) Cut a 4 ¼ inch diameter hole in a sheet of plywood for the exhaust vent pipe. Place the vent pipe/flange assembly with the plywood hole aligned with the pipe hole and mark in pencil the bolt hole locations. Drill holes in the plywood for the bolts. Cut a 2 ½ foot diameter hole in a sheet of plywood for the shelter entrance.
A continuous strip of 2 x 8’s will be used as a center crown board. Using a table saw or radial arm saw, cut a thin 2.5 degree slice from each end of the board. (Detail 9.2) Nail the crown boards to the main support beams from the front to the rear of the shelter. (Detail 9.3) The seams of the crown boards should always terminate in the middle of the support beam. Cut crown boards if necessary.
Cut ¾ inch thick plywood into 2 inch wide 8 foot long strips. Chalk a line on the top crossbeams from the front to the rear of the shelter, 1 ½ feet from the edge of the plywood walls. Nail these strips of plywood to the chalk line. (Detail 9.3) Always terminate the plywood strips to the middle of the crossbeams. Cut the length if necessary.
The shelter is supported vertically along 3 lines. The 6 x 6 columns provide the center support. One foot of granular soil on each side of the shelter provides the other two lines of support. It is critical that the plywood rest upon the soil rather than the plywood shelter sides. The plywood roof should be raised slightly above the sidewalls and the area underneath packed solid with granular soil. (Detail 9.4) Beginning at one corner, place the sheets of plywood in place and nail to the crown board. The plywood should have one-foot of overhang over the sidewalls & one-foot of overhang over the inner edge of the earth walls. (Detail 9.5) Install the plywood from the front and back and work towards the middle.
Cut one sheet of plywood into two 4-foot by 4-foot segments. This will be the center roof boards. (Detail 9.6) These should be nailed to the middle of the structure, between the middle two cross beams. The edges of the plywood should be centered on the crossbeams. The plywood sheets that will be nailed on either side of these center boards will need to be cut to fit properly. Cut approximately one foot off the length of the plywood for these 4 sheets.
After the ceiling is in place, it is advisable to scan the entire interior of the shelter and hammer down any protruding nails. These nails will be a hazard in a dark shelter subjected to significant ground motion.