All lumber for construction should be treated
for ground contact with an environmentally safe preservative such
as CCA. Avoid wood preserved with chemicals such as pentachlorophenol
or creosote. Lumber treated for ground contact has a higher concentration
of preservative and therefore will last much longer than untreated
or lightly treated wood. Except for the covering, the structure
should last 10 to 12 years.
Unless the site is very flat, you will need
to level the foundation boards. This is best done by digging a
trench on the high side instead of raising the low side, as the
foundation boards should contact the ground at all points to ensure
a good seal. To keep rainwater out, however, the top of the foundation
board should not be below ground level, particularly around the
door. the purpose of the 4-inch-by-4-inch post at each corner
(Figure 4) is to anchor the greenhouse and prevent it from moving
in high winds.
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 degree 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.
3. Construction Details
(See also Bill
of Materials )
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.