Greenhouse Frames

Greenhouse Specifics

The efficiency and productivity of a greenhouse is greatly influenced by its design. Initial and long term costs of the structure are also affected by the type of materials that are used in the construction. Ridge and furrow style greenhouses provide the highest efficiency in crop production. Aluminum is the most common and most durable type of framing material used for commercial greenhouse structures. Double-wall polyethylene film is the most economical covering material available. Review all available options before making your final decision.

- Greenhouse Frame Materials -

Aluminum - Aluminum is the longest lasting of all of the framing materials because it will never rust, rot, or break down from UV rays. Although more costly, aluminum frames can be anodized in a variety of colors and have low maintenance requirements. Since aluminum is not as strong as steel it will require frames larger in size or more of them. Some shapes are designed to increase the frame's strength. This will also reduce frame shadowing without sacrificing the integrity of the hothouse structure.

Galvanized Steel - Steel's greatest value in greenhouse construction is its natural strength. Galvanized steel is long-lasting, lower in cost, and requires less framework than other framing materials. Steel frames can be thinner than other types, thus creating less shadow and allowing more light to enter the plant nursery. Any steel tube greenhouse you choose should be made with heavy-duty galvanized or stainless steel tubing. These types are more suited for outdoor construction and will protect it from a greenhouse's humid and corrosive climate.

Plastic - The main advantages of a plastic frame greenhouse is its low cost, portability, and ease of installation. Plastic is often used to construct smaller structures because it is not as rigid as metal or wood. Metal wire supports compensate for what plastic frames lack in strength. Because plastic is a poor heat conductor like wood, it causes a small amount of heat loss compared to metal frames. The main disadvantage of a plastic frame portable greenhouse is that ultraviolet rays from the sun will eventually cause the plastic to deteriorate, even if it is a UV protected material. Plastic frames are also typically larger than similar metal frames, increasing the amount of shadows in the greenhouse.

Wood - Wood frames are commonly used for a hobby greenhouse or sunroom. Wood is popular because of it's attractiveness as well as the small amount of heat loss produced when compared to metal framing material. One drawback to using wood is it's limited lifetime in a greenhouse's damp climate before it starts to deteriorate. Redwood or cedar frames are recommended because of a natural resistance to the elements as well as insects. To further protect and increase the life of redwood or cedar you can apply a chemical sealant or stain to the wood periodically. Wood frames are usually larger and heavier than metal frames and can cause increased shadow in the greenhouse.

Greenhouse Frames

- Greenhouse Frame Styles -

A-frame - Classic A-frame greenhouses have slanted sides. They are strong and can easily handle snow however, some space is lost due to their slanting walls. A classic A-frame is more expensive to heat because of its high ceiling.

Modified A-frame greenhouses are built with gable roofs. The shape of the roof is similar to that of a house, except that it does not have eaves. The straight walls of a modified A-frame structure allow you to make efficient use of space and since the roof slope is not as steep it will be less expensive to heat.

Quonset - A Quonset frame greenhouse nursery is circular and rounded on the top. The low sidewalls and ceiling restricts storage space and headroom but is relatively inexpensive and easy to heat.

Gothic - The Gothic frame style is similar to the Quonset, except that the sides are usually straighter and the curved roof is pointed on top. This shape allows more headroom at the sidewall, better use of space along the walls and is inexpensive and easy to heat.

Post and Rafter - The post and rafter frame style is a simple construction but usually requires more wood or metal than some other plant nursery designs. Strong and deeply buried sidewall posts are necessary to support the rafters and withstand wind pressures. The post and rafter frame style allows for more efficient use of space along the sidewalls and good air circulation.

Rigid-frame - A rigid-frame greenhouse has vertical sidewalls and rafters for a clear-span construction. Nailed or glued plywood gussets connect the sidewall supports to the rafters to make one rigid frame rather than using columns or trusses to support the roof. A good foundation is required to support the lateral load on the sidewalls. The conventional gable roof and sidewalls allow for more space and good air circulation.

- Greenhouse Coverings -

Glass - As a traditional hobby greenhouse covering, glass is preferred by many for its permanence, beauty and easy maintenance. Standard glass transmits heat and cold quickly and has very little insulating value. Tempered glass is frequently used in greenhouse construction to increase heating efficiency and decrease potential breakage. Glass is much heavier than other covering materials, requiring a good foundation and more substantial frames such as aluminum, steel, or laminated wood. Other considerations are air leakage due to the number of panes required with a glass covering and the potential risk of burning plants as glass does not diffuse light.

Fiberglass - A fiberglass covering is one of several practical replacements for glass. Fiberglass is lightweight, strong and usually comes in rolls or corrugated sheets that are translucent rather than transparent. You will not be able to see through the fiberglass but the light transmission is relatively equal to glass. With a fiberglass covering light is diffused as it passes through, creating a virtually shadowless greenhouse. It retains heat more efficiently than glass while transmitting less heat into the greenhouse, a benefit in both winter and summer. Using a good grade of fiberglass will eliminate any discoloration and reduction in light penetration that is caused over time with a poor grade material.

Polyethylene Film - Plastic coverings are available in several grades of quality and several different materials. One such material is Polyethylene film. Construction costs are usually lower because the frame can be of a lighter material and plastic film is less expensive than other types of coverings. Light transmission with polyethylene is comparable to that of glass. A utility grade of polyethylene that will last about a year is available at local hardware stores. Commercial greenhouse grade polyethylene film has ultraviolet inhibitors to protect against ultraviolet rays and will last 12 to 18 months before needing to be replaced.

Double-Wall Polycarbonate or Acrylic - Another type of plastic covering is double-wall polycarbonate or acrylic sheets. Rigid double-layer plastic sheets of acrylic or polycarbonate are available and will provide a long lasting, heat-saving cover for your hothouse. These types of greenhouse nursery covers have two layers of rigid plastic separated by webs. The double-layer of material retains more heat so you can save up to 30% on energy costs. The acrylic is a long lasting, non-yellowing material. The polycarbonate will yellow faster, but it is usually protected by a UV-inhibitor coating on the exposed surface. Both materials typically carry warranties on their light transmission qualities. While both types of plastic can be used on curved surfaces, the polycarbonate works better for such wall types. About 80% of light filters through double-wall plastic materials compared to 90% for glass.

- Floors and Foundations-

Permanent Floors - Permanent flooring is not recommended in a hobby or commercial greenhouse because it may become wet and slippery, causing hazardous working conditions. A better choice would be a concrete, gravel, or stone walkway 24 to 36 inches wide for easy access to your plants. Several inches of gravel used on the rest of the floor will provide drainage of excess water. You can later spray water on the gravel to provide additional humidity to the greenhouse.

Permanent Foundations - Permanent foundations should be included in your construction plans when you are going to use glass, fiberglass, or double-wall plastic covering materials. If you purchase a greenhouse kit, the manufacturer should provide plans for the foundation construction. There are three common types of greenhouse foundation: 1) treated 4x4 wood posts, 2) concrete wall with a treated wood sill, and 3) reinforced concrete slab. In most cases, a poured concrete slab foundation to frost depth is your best option. For smaller structures, such as a hobby greenhouse, a wood foundation is a good choice due to ease of installation and low cost.

Greenhouse Frames
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There really is no "best" material for a greenhouse nursery. There are many different materials available for the frames, covering and foundation and each have their uses in greenhouse gardening. Your particular gardening plans and available budget will determine which material is the best choice for you. Do some research and determine exactly what you will need immediately as well as in the future before purchasing.

At Greenhouse Center, our goal is to provide useful information on choosing between the many different types of greenhouses available, buying a greenhouse kit, purchasing plants for your greenhouse nursery and how to maintain your greenhouse for indoor gardening year-round. Visit our Products page to see a sample of the large variety of greenhouse styles that are available.

The Greenhouse Garden | Types of Greenhouses | Greenhouse Specifics | Greenhouse Plants
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