The PVC pipe bows are attached to the side boards of the foundation with electric metallic tubing (EMT) clamps (Figure 4). It is best to attach the clamps loosely to the side boards with wood screws before you secure the side boards to the end boards and corner blocks. They may be tightened after the PVC pipe has been slipped into place.
PVC pipe may be purchased in 12- or 20-foot lengths in either schedule 40 or schedule 80 weights. Schedule 80 PVC pipe has a thicker wall and is stronger than schedule 40 pipe. It is therefore recommended for greenhouse construction. The 12-foot width of the greenhouse was chosen so that each half of the rib section of pipe is 10 feet long. PVC pipe may be easily cut with a hand say, although a neater cut can be made with a pipe-cutting tool specially designed for that purpose. Care must be taken during assembly because the cement used to fasten PVC is fast-acting and permanent. It should not be used when the temperature is below 50 degrees Farenheight. Check the directions on the can.
It is best to lay all parts out on a flat surface for assembly. For the ribs to be 2 feet apart, the mid-rib pipe must be cut into pieces about 22 1/2 inches long (Figures 3 and 5). After all joints have been cemented and allowed to harden for a few minutes, position the entire assembly over the foundation boards, bend the ribs, and secure them in place with the EMT clamps. Assembly is best done by two or more people to prevent over stressing the cemented joints. Ten feet of galvanized EMT positioned inside the mid-rib will give added strength to the structure.
Figure 3 - Construction Details
After all the PVC ribs have been secured, the end frames and door may be constructed. Although there is a door on only one end, both frames are essentially the same. The two diagonals on each end give strength to the structure and make it easy to secure the cover. The top of the end frame may be fastened to the end ribs with either a wood screw of a small metal strip bent into a U shape. If wood screw are used, the PVC pipe must be predrilled to prevent splitting.
Select the type of cover material according the intended use of the greenhouse. Clear 4- or 6-mil plastic greenhouse film is generally best if the house will be used for growing plants. If it will be used exclusively for propagating or overwintering plants, consider using 4- or 6-mil milky or white copolymer film. White copolymer film reduces the amount of heat and light within the house and therefore limits the fluctuations in these factors, keeping conditions more nearly constant during propagation or over the winter. However, shading clear plastic with a 30 to 50 percent polypropylene shade cloth or greenhouse whitewash available from horticultural supply companies will produce nearly the same effect.
Foundation side and end boards are attached to 4-by-4 posts to anchor the structure. Note the EMT clamps on the side boards hold the PVC pipe ribs.
The cover may be attached with small tacks, but staples are much faster and are more secure. Allow the cover to extend several inches past the bottom of the foundation boards and cover the extra material with packed soil to prevent the entrance of moles and rainwater. An inch of coarse sand or fine gravel on the floor will reduce problems with mud and weeds.
The mid-rib pipe is cut into sections about 22 1/2 inches long and rejoined with PVC crosses to which the ribs are attached. Ten feet of EMT is placed inside the mid-rib for strength.
