A plastic greenhouse can be used for one season or more, depending on the quality of the construction materials. A structure made from building-grade plastic and PVC pipe will last one or two seasons, while one made of high-grade greenhouse plastic and PVC conduit that is treated against UV damage may last for decades. Constructing a plastic greenhouse is relatively simple, using simple tools and materials that can be found at most hardware stores.
Measure and mark the centre of a 2x4 board. Use the circular saw to cut the board in half.
Make a box using the two full boards and the two half boards. Lay the 24-foot boards on end, parallel to each other. Place the 12-foot boards on end between the long boards at either end. Butt the joints together. The four boards should form a rectangle that is 12 feet by 24 feet.
Nail the 12-foot boards to the 24-foot boards.
Divide the 26 pieces of rebar into two bundles of 13, one bundle for each of the long boards in your box. Space the rebar pieces equally along long boards. Stand the rebar on end and use the hammer to pound each piece 1 foot into the ground. Make sure each of these rebar stakes is directly across from one on the opposite side.
Form your arched greenhouse frame by slipping a piece of PVC conduit over the end of a rebar stake on one side of the rectangle. Bend the conduit pipe downward across the box, slipping the other end of the conduit over the piece of rebar on the opposite side. Repeat the process for all the PVC pieces.
Pass a length of wire over the top of the greenhouse. Secure the wire by wrapping it around each pipe where the wire and pipe touch. Tie off the loose ends of the wire by wrapping it around the end pieces of pipe twice. Clip the wire at each end with wire cutters. This will stabilise the greenhouse.
Examine your greenhouse plastic. The plastic will be folded in a W-shaped fold, then rolled up. Unroll the plastic over the top of the greenhouse frame. Then unfold the plastic so that it will cover the greenhouse frame.
Staple the greenhouse plastic to the pressure-treated boards on three sides of the frame. Weigh down the plastic on the fourth side with breeze blocks. When the greenhouse becomes too warm, lift the plastic up over the top of the greenhouse to allow it to vent.