Unformatted text preview: ral load on the sidewalls.
Post and rafter and A-frame. The post and rafter is
a simple construction of an embedded post and rafter,
but it requires more wood or metal than some other
designs. Strong sidewall posts and deep post embedment are required to withstand outward rafter forces
and wind pressures. Like the rigid frame, the post and
rafter design allows more space along the sidewalls and
efficient air circulation. The A-frame is similar to the
post and rafter construction except that a collar beam
ties the upper parts of the rafters together. manage because temperatures in small greenhouses fluctuate more rapidly. Small greenhouses have a large
exposed area through which heat is lost or gained, and
the air volume inside is relatively small; therefore, the
air temperature changes quickly in a small greenhouse.
Suggested minimum sizes are 6 feet wide by 10 feet
long for a lean-to and 8 or 10 feet wide by 12 feet long
for an even-span or freestanding greenhouse. Structural Materials
A good selection of commercial greenhouse frames
and framing materials is available. The frames are made
of wood, galvanized steel, or aluminum. Build-it-yourself greenhouse plans are usually for structures with
wood or metal pipe frames. Plastic pipe materials generally are inadequate to meet snow and wind load requirements. Frames can be covered with glass, rigid fiberglass, rigid double-wall plastics, or plastic film. All have
advantages and disadvantages. Each of these materials
should be considered––it pays to shop around for ideas. Coverings
Greenhouse coverings include long-life glass, fiberglass, rigid double-wall plastics, and film plastics with
1- to 3-year lifespans. The type of frame and cover must
be matched correctly.
Glass. Glass is the traditional covering. It has a
pleasing appearance, is inexpensive to maintain, and has
a high degree of permanency. An aluminum frame with
a glass covering provides a maintenance-free, weathertight structure that minimizes heat costs and retains
humidity. Glass is available in many forms that would
be suitable with almost any style or architecture.
Tempered glass is frequently used because it is two or
three times stronger than regular glass. Small prefabricated glass greenhouses are available for do-it-yourself
installation, but most should be built by the manufacturer because they can be difficult to construct.
The disadvantages of glass are that it is easily broken,
is initially expensive to build, and requires much better
frame construction than fiberglass or plastic. A good
foundation is required, and the frames must be strong
and must fit well together to support heavy, rigid glass.
Fiberglass. Fiberglass is lightweight, strong, and
practically hailproof. A good grade of fiberglass should
be used because poor grades discolor and reduce light
penetration. Use only clear, transparent, or translucent
grades for greenhouse construction. Tedlar-coated fiberglass lasts 15 to 20 years. The resin covering the glass
fibers will eventually wear off, allowing dirt to be
retained by exposed fibers. A new coat of resin is needed after 10 to 15 years. Light penetration is initially as
good as glass but can drop off considerably over time
with poor grades of fiberglass. Frames
Greenhouse frames range from simple to complex,
depending on the imagination of the designer and engineering requirements. The following are several common frames (Figure 3).
Quonset. The quonset is a simple and efficient construction with an electrical conduit or galvanized steel
pipe frame. The frame is circular and usually covered
with plastic sheeting. Quonset sidewall height is low,
which restricts storage space and headroom
Gothic. The gothic frame construction is similar to
that of the quonset but it has a gothic shape (Figure 3).
Wooden arches may be used and joined at the ridge.
The gothic shape allows more headroom at the sidewall
than does the quonset. Rigid-frame Quonset A-frame Gothic Post and rafter Figure 3. Greenhouses can have a variety of
different structural frames. 4 Double-wall plastic. Rigid double-layer plastic
sheets of acrylic or polycarbonate are available to give
long-life, heat-saving covers. These covers have two layers of rigid plastic separated by webs. The double-layer
material retains more heat, so energy savings of 30 percent are common. The acrylic is a long-life, nonyellowing material; the polycarbonate normally yellows faster,
but usually is protected by a UV-inhibitor coating on
the exposed surface. Both materials carry warranties for
10 years on their light transmission qualities. Both can
be used on curved surfaces; the polycarbonate material
can be curved the most. As a general rule, each layer
reduces light by about 10 percent. About 80 percent of
the light filters through double-layer plastic, compared
with 90 percent for glass.
Film plastic. Film-plastic coverings are available in
several grades of quality and several different materials.
Generally, these are replaced more frequentl...
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- Spring '08