Chapter 1

Greenhouse Structures and Design

Major Components of a Greenhouse

Greenhouse Framing Materials

A greenhouse frame is an important part of any greenhouse because it serves to provide both strength and insulation. Greenhouse frames are made of many different materials including galvanized steel, aluminum, PVC, and wood.

Galvanized Steel Greenhouse Frame

Most commercial greenhouses have galvanized steel frames because they are long lasting, low cost, and require less framework (thus less shadowing) than any other framing material thanks to steel's natural strength.

Aluminum Greenhouse Frame

Aluminum is a popular material for greenhouse framework because it is relatively lightweight, rustproof, affordable and easy to assemble. Aluminum frames are generally less expensive, have longer life-span, and require little maintenance.

PVC Greenhouse Frame

PVC pipe can also be used as support for small greenhouses that are to be covered with polyethylene, but PVC requires additional support to withstand strong winds. Being flexible, PVC can be used particularly well in hoop houses.

Wood Greenhouse Frame

Wood is an incredibly popular greenhouse framing material for smaller greenhouses. It is easy to work with to build many different sizes and designs. Wood is a good insulating material so it will not allow heat loss in cold weather. Reinforced wood has superior strength and has an excellent load bearing capacity. Being a natural material, it is susceptible to water and moisture damage. So, when you choose wood to build the framework of your greenhouse, be sure that you pick a high quality hardy wood such as redwood, cypress or cedar.

Greenhouse Structural Components

There are number of structural components some which include bar caps, gutters, purlins, rafters, ridge cap, sash bar, and sideposts. Bar caps are attached to the outside of the greenhouse sash bars to hold the glazing material in place. The bar caps also hold the glazing compound used to seal around the glass (or other glazing material) to prevent leaks. The bar caps shield the glazing compound from the ultraviolet rays. A greenhouse fitted with bar caps is almost maintenance free. The eave plate rests on the sideposts and columns. It forms the support for the roof members. In ridge and furrow greenhouses, a gutter is used instead of an eave plate. Gutters are used to collect the runoff water and channel it away from the structure. Gutter connect greenhouses are composed of a number of “bays” or compartments running side by side along the length of the greenhouse. Purlins are horizontal supports of a greenhouse. They run parallel from rafter to rafter and are usually spaced 4 to 8 feet apart depending on the size of the greenhouse. They are used to support the sash bars on a wide house. Rafters are the fundamental vertical support structure of a greenhouse (See Figure 1.18). They provide strength to the basic structure and a hence placed at a distance of 2, 3, 4 feet or as required. There are two types of rafters—truss types (gabled) and curved arch.

Greenhouse Foundations, Flooring, and Curtain Walls

Foundations

When setting up a greenhouse, one of the most important aspects of the construction process is the foundation. There are a few different types of foundations that can be used for greenhouses. The type of foundation used is determined by the style of greenhouse and building codes. Essentially, the foundation is the complete system on which the greenhouse structure sits. One of the key components of a foundation is the footing. The footing refers to the point at which the structure meets the soil. Footers are typically poured concrete and their exact depth is determined by local building codes and the location’s frost line to avoid structural damage, which can occur with the freezing of surface-level soil.

Greenhouse Floors

Growers may choose from a range of floor surfaces for container grown crops: concrete to gravel/dirt floors. The actual floor design will be dependent upon the type of production being planned and the available capital. Entire concrete floors will help minimize weed growth under benches, reduce insect and disease host habitats, and allow for capture of run-off into holding ponds. Gravel or dirt floors are inexpensive, but often not worth the initial savings.

Curtain Wall

In some greenhouses, usually of older construction or institutional, a sidewall, known as a curtain wall or knee wall, is attached between the support posts and extends around the perimeter of the greenhouse. Curtain walls are made of poured concrete, cinder or cement block, corrugated fiberglass-reinforced plastic, wood, or any other solid building materials.

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