Measure the space for the greenhouse. Add the dimensions of the concrete blocks to the garden space. For example, if your greenhouse plan calls for an 8-by-12-foot greenhouse, add the width of the concrete blocks to the measure. The standard measure of a concrete block is 8 inches tall, 8 inches wide and 16 inches long. Therefore, you would add 16 inches to the foundation's measurements, making it 9 feet 4 inches wide by 13 feet 4 inches long.
Remove the grass and vegetation in the garden space. Move it to the compost pile with a wheelbarrow. Remove the soil to a depth of 24 inches.
Tamp the soil in the greenhouse excavation firmly with a plate compacter. Sprinkle with water and tamp again.
Add 6 inches of crushed gravel. Rake smooth and tamp firmly. Sprinkle with water and tamp again to make a firm base.
Mix the mortar thoroughly according to the manufacturer’s directions. Apply mortar to the sides and tops of the concrete blocks with a trowel.
Lay the first row of cement blocks around the outer perimeter of the greenhouse base, beginning in a corner. Use a spirit level to ensure that the blocks are horizontal. Continue laying the second and third row of blocks, staggering each row.
Place the 2-by-6 boards on their sides, centered on the top of the concrete block foundation. Butt the boards' ends together. Predrill and screw together using the 3-inch screws.
Pound the 30-inch rebar through the hollow inner cores of every other block. Place them on the outside of the 2-by-6 boards and flush with the tops of the boards. Then add the rebar for the hoop supports, pounding a 36-inch piece of rebar into each corner of the greenhouse and one every 3 feet along the long sides of the greenhouse on the inside of the 2-by-6 boards. Allow the rebar to extend a foot above the concrete blocks.
Fill the inner cores of the concrete blocks with crushed gravel. tamping firmly. Backfill with the excess soil around the exterior of the foundation. Sprinkle with water and tamp firmly.
Fit one end of a 12-foot PVC pipe onto the end of the rebar and push all the way into the gravel. Bend the PVC pipe carefully and insert the other end into the rebar, on the opposite side of the greenhouse, and push all the way into the gravel. Repeat with the other PVC pipes. These are the hoop house supports.
Tie the hoops together with wire. Wrap the wire around the pipe at the far end of the greenhouse. Pull the wire gently to the next pipe, wrap, and continue to the third pipe. Repeat until you reach the end of the greenhouse. Wrap the wire around the last pipe and cut it. Twist to secure. Repeat until the pipes are tied together with two rows of wire on each side and one down the center of the greenhouse.
Unfold the plastic sheeting. Draw it up and over the greenhouse. Staple the plastic to the 2-by-6 board along the long side of the greenhouse. Pull the plastic over the PVC hoops snugly and staple to the opposite board.
Fold the end neatly and staple to the board. Repeat on the downwind side; however, leave one corner open for access into the greenhouse.
Place a plastic step inside the opening for easy access to the greenhouse. Add raised beds, a soaker hose and a fan to the inside of the greenhouse. Leave a center aisle open for easy access to both sides of your greenhouse garden.