There are three main types of greenhouse frames: the simple tunnel or “Quonset” shape; the taller gothic shape with arched sidewalls and a ridge beam; or the vertical side walls with a gable roof. Your choice of construction depends largely on building costs, long-term gardening goals, and building expertise. The Quonset and gothic shaped greenhouses are suitable for amateur gardeners with do-it-yourself experience. The third type is aimed mainly at professional horticultural nurseries and gardening enthusiasts willing to spend a considerable amount of money on a professionally built greenhouse.
The Quonset greenhouse is an inexpensive, tunnel-shaped structure with the frame made of various grades of plastic tubing — from electrical conduit to 1/2-inch Schedule 40 PVC pipe, depending on width of the greenhouse. This type of structure is suitable for the amateur gardener wishing to extend the growing season, and/or to cultivate early spring seedlings.
The base is an oblong, two-by-four frame placed directly on the ground. The roof consists of several lengths of plastic piping bent into half-hoops, with each end attached to the insides of the longitudinal base at three to five-foot intervals. Three to five longitudinal strips of one-by-two lumber are secured to the undersides of each half-hoop to provide structural integrity. The roof covering consists of heavy UV-resistant plastic film stretched over the tubular half-hoops, and secured with one-by-two lumber strips nailed or screwed to the base.
The end walls have door frames screwed to the base with the upper ends tied securely to the outer tubular arches and covered in plastic film. Adjustable sliding doors on both ends provide access and ventilation. This is type of structure is considered to be a temporary as the cover must be removed before heavy winter snowfalls; therefore, approved plans and building permits are not required.
Gothic-shaped greenhouses with arched sides and a central ridge are taller than the Quonset shape; therefore, they provide better storage and headroom along the sidewalls. This type can sit directly on the ground or on a concrete slab if raised beds or growing tables are used. The base also consists of an oblong, two-by-four frame. Evenly spaced, half-elliptical, wooden arches are attached to the inside edges of the longitudinal sides of the base frame.
The ridge consists of two-by-four or two-by-six lumber, depending on the size of the greenhouse. Structural integrity consists of two or three, one-by-two lumber strips glued and screwed lengthwise to the underside of each of the wooden side arches. The roof covering consists of flexible polycarbonate or transparent fiberglass panels screwed to the wooden frame. The ends are covered and have frames and adjustable sliding doors to provide access and ventilation.
If proper heating, cooling and ventilation is provided, these greenhouses are suitable for serious gardeners requiring an extended growing season or four-seasonal cultivation. This is a permanent structure requiring approved building plans and permits. However, if you have the necessary construction experience, you can build this type of greenhouse yourself.
Rigid-framed greenhouses are contractor-built, clear-span structures without central supporting pillars. Professional-style greenhouses are built along lines similar to a lumber-framed house, and therefore require approved plans and building permits. These structures come complete with concrete foundation, vertical sidewalls and rafters. They also have electric and plumbing installations to provide humidity control, heating, lighting and ventilation. The outside cover consists of glass, fiberglass sheeting, double-walled plastic or polycarbonate panels. Rigid-framed greenhouses are suitable for all-season cultivation and are usually used by wealthy homeowners and professional horticultural nurseries.