Section 4: Common Greenhouse Structural Components and Materials
Greenhouse frames (support structure) may be constructed of wood, steel, aluminum or concrete. Modern greenhouses are usually constructed of steel and/or aluminum. Aluminum is the material of choice since it is light-weight, strong and rust-resistant. Low-cost small quonset greenhouses with polyethylene covering may use curved aluminum tubes or even bent electrical conduit or pipes for physical support.
Wood is typically only used for hobby or homeowner greenhouses, cold frames and hot beds. Wood is difficult and expensive to maintain as it needs to be treated with a preservative or periodically painted to prevent rotting. When using wood in a greenhouse, cold frame or hot bed, never use creosote or pentachlorophenol-treated wood because they contain phytotoxic volatiles. Chromated copper arsenate (CCA), ammoniacal copper arsenate (ACA), copper naphthanate and zinc naphthanate are typically recommended for use on wood being used in greenhouses.
Floors may be constructed of porous concrete, standard concrete, gravel or compacted clay covered with a strong polypropylene fabric. Porous concrete is usually strong enough to support most loads encountered in greenhouse situations (except for heavy equipment) and allows for drainage through the surface, but it is difficult to clean. Standard concrete is stronger, easy to clean, does not allow drainage through the surface and is more expensive than porous concrete. Often, standard concrete is used in areas where heavy traffic (i.e. equipment) must be supported and porous concrete is used on other areas. Unless being used as part of an irrigation system (i.e. flood floors), concrete floors should have a slight grade to promote drainage (preferably towards a drain where it can be recollected) and prevent puddling of water. Gravel is low cost and allows drainage but can allow the growth of weeds and may not accommodate all types of equipment. Polypropylene fabric is low in cost but the floor can become uneven over time, which results in uneven irrigation, puddling and algae growth.
Various types of translucent (allows light to pass through) coverings, referred to as glazings, may be used to cover the greenhouse structure. Greenhouse glazings are discussed in more detail under the “Lighting” learning unit. The glazing may cover the entire greenhouse structure including the roof and side walls or the greenhouse may have a curtain wall. Curtain walls are non-translucent (not allowing light to pass through) sections of the greenhouse side wall. Curtain walls are located along the lower 2' - 4' of the greenhouse side walls. Curtain walls are typically constructed of concrete block, cement, brick or some other non-transparent and well-insulated materials. Because the curtain wall only extends up to approximately bench height (if elevated benches are used), it does not significantly reduce the light available to the crop. However, because it is constructed of a well-insulated material, it reduces heat loss from the greenhouse. In northern climates, the entire north wall of the greenhouse may be constructed as a curtain wall to reduce heating costs. In northern climates, a relatively small proportion of the light entering the greenhouse does so through the north greenhouse wall. Therefore, the savings in heating costs outweigh the reduction in light levels in the greenhouse. However, where plant material is being grown of the floor, the curtain wall may block light from reaching plants that are placed close to the sidewall (typically within a few feet of the curtain wall). On the northern side wall, this loss of light available to the plants might not be significant. However, along the other side walls, the loss of light due to shading from the curtain wall can be significant and may result in reduced growth of plants placed close to the curtain wall.
