Building a Greenhouse ©2001 AgriTeach.com (103001ms) Greenhouse Materials Source: Fact Sheet University of Maryland Co-op Extension Service, David S. Ross, Extension Agric. Engineer, Dept. of Agric. Engineering

Structural Materials A good selection of greenhouse frames and materials is available. A good selection of greenhouse frames and materials is available. The frames are made of wood, galvanized steel, or aluminum. The frames are made of wood, galvanized steel, or aluminum. Build-it-yourself plans are available for wood or metal pipe frames. Build-it-yourself plans are available for wood or metal pipe frames. Plastic piping is generally inadequate for snow and wind loads. Plastic piping is generally inadequate for snow and wind loads. Frames may be covered with: Frames may be covered with: – Glass – Rigid fiberglass – Rigid double-wall plastics – Plastic film All have advantages and disadvantages: All have advantages and disadvantages: – Each of these materials should be considered – It pays to shop around for ideas

Frames Greenhouse frames range from simple to complex: Greenhouse frames range from simple to complex: – The imagination of the designer is a factor. – Engineering requirements must be considered. – There are several common frame types: Quonset Quonset Gothic Gothic Rigid-frame Rigid-frame Post and Rafter Post and Rafter A-frame A-frame

Quonset. The Quonset is a simple and efficient The Quonset is a simple and efficient It is often constructed with: It is often constructed with: – Electrical conduit – Galvanized steel pipe frame. The frame is circular, and usually covered with plastic sheeting. The frame is circular, and usually covered with plastic sheeting. Quonset sidewall height is low: Quonset sidewall height is low: – This restricts storage space and headroom.

Gothic Gothic frame is similar to the Quonset. Gothic frame is similar to the Quonset. The archways have a gothic shape. The archways have a gothic shape. Wooden arches may be used and joined at the ridge. Wooden arches may be used and joined at the ridge. The gothic shape allows more headroom at the sidewall. The gothic shape allows more headroom at the sidewall.

Rigid-frame The rigid-frame structure has: The rigid-frame structure has: – Vertical sidewalls. – Rafters for a clear-span construction. There are no columns or trusses to support the roof: There are no columns or trusses to support the roof: – Plywood gussets connect sidewall supports to the rafters. – This creates one rigid frame. A gable roof and sidewalls allow greater interior space & air circulation. A gable roof and sidewalls allow greater interior space & air circulation. A good foundation is needed to support the load on the sidewalls. A good foundation is needed to support the load on the sidewalls.

Post and rafter and A-frame It requires more wood or metal than some other designs. It requires more wood or metal than some other designs. To withstand rafter forces and wind pressures, strong sidewall posts and deep post embedment are required. To withstand rafter forces and wind pressures, strong sidewall posts and deep post embedment are required. The post and rafter design allows more space along the sidewalls and efficient air circulation. The post and rafter design allows more space along the sidewalls and efficient air circulation. The A-frame is similar to rafter construction, but a collar beam ties the rafters together. The A-frame is similar to rafter construction, but a collar beam ties the rafters together.

Rafter and A-frame Construction

Coverings Greenhouse coverings include: Greenhouse coverings include: – Long-life glass – Fiberglass – Rigid double-wall plastics – Film plastics with 1- to 3-year life spans The type of frame and cover must be matched correctly. The type of frame and cover must be matched correctly.

Glass Glass is the traditional covering. Glass is the traditional covering. – It has a pleasing appearance. – It is inexpensive to maintain. – Is more permanent than other coverings. – Glass is available in many forms. – Glass would be suitable with almost any style or architecture. Tempered glass is frequently used: Tempered glass is frequently used: – It is two or three times stronger than regular glass. The disadvantages of glass are: The disadvantages of glass are: – It is easily broken. – Is initially expensive to build. – Prefabricated glass greenhouses are difficult to construct. – It requires much better frame construction than fiberglass or plastic. – A good foundation is required.

Fiberglass Fiberglass is lightweight, strong, and resists hail. Fiberglass is lightweight, strong, and resists hail. Poor grades of fiberglass discolor and reduce light. Poor grades of fiberglass discolor and reduce light. Use only clear, transparent, or translucent grades. Use only clear, transparent, or translucent grades. Tedlar-coated fiberglass lasts 15 to 20 years. Tedlar-coated fiberglass lasts 15 to 20 years. The resin coating the glass fibers will eventually wear off: The resin coating the glass fibers will eventually wear off: – This allows 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: Light penetration is initially as good as glass: – It can drop off over time with poor grades of fiberglass.

Double-wall plastic Rigid double-layer plastic sheets of acrylic or polycarbonate are available to give long-life, heat-saving covers. 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. Acrylic and polycarbonate can be used on curved surfaces: Acrylic and polycarbonate can be used on curved surfaces: – The polycarbonate material can be curved the most. Each layer reduces light by about 10 percent. Each layer reduces light by about 10 percent. – About 80 percent of the light filters through double-layer plastic. – 90 percent of light filters through glass.

Film plastic Film-plastic coverings are available in several grades of quality and several different materials. Film-plastic coverings are available in several grades of quality and several different materials. – Generally, these are replaced more frequently than other covers. – The frame can be lighter and plastic film is inexpensive. – The films are made of: polyethylene (PE), polyvinyl chloride (PVC), copolymers, and other materials. Light transmission is comparable to glass. Light transmission is comparable to glass. PVC or vinyl film costs two to five times as much as PE but lasts as long as five years. PVC or vinyl film costs two to five times as much as PE but lasts as long as five years. – However, it is available only in sheets four to six feet wide. – It attracts dust from the air, so it must be washed occasionally.

Foundations and Floors Permanent foundations should be provided for: Permanent foundations should be provided for: – Glass, fiberglass, or the double-layer rigid-plastic sheet materials. Most require a poured concrete foundation. Most require a poured concrete foundation. Quonset greenhouses use posts driven into the ground. Quonset greenhouses use posts driven into the ground. Permanent flooring is not recommended: Permanent flooring is not recommended: – It may stay wet and slippery from soil mix media. – A concrete, gravel, or stone walkway 24 to 36”. – The floor should be covered by gravel for drainage. – Water can be sprayed on gravel to increase humidity.

Building a Greenhouse ©2001 AgriTeach.com (103001ms) Greenhouse Materials Source: Fact Sheet University of Maryland Co-op Extension Service, David S. Ross, Extension Agric. Engineer, Dept. of Agric. Engineering