Framing for a greenhouse takes second billing to the material used as siding, but an unsuitable frame material can mean an unsteady frame that will never really be square. It is very hard to complete a greenhouse successfully unless the initial frame is properly built, and if you do get it completed, you may later find that it has cracks and openings that are hard to seal against drafts and invasion from insects and small animals.
Some greenhouses are sold in kit form that won't give you any options for the framing materials, but if you are building a custom greenhouse, you have the option of choosing the material you use for the frame structure. Here are things to consider with the four most common options for greenhouse framing: wood, aluminum, galvanized steel, and PVC plastic pipe.
Wood is a beautiful material that makes for a classic greenhouse structure. But wood is somewhat impractical as the frame for a greenhouse unless the structure you are building is more like a sunroom or garden shed. Wood has good insulation value and is fairly easy to fabricate and assemble into a frame. But greenhouses are wet, damp spaces, and most wood will eventually warp and rot under the ongoing presence of moisture found in a greenhouse. If you do use wood, opt for a species with known resistance to moisture and rot, such as cedar or redwood. Or, use chemically treated wood designed for outdoor use. Whatever wood you use, applying a sealer every few years will extend its life.
Wood is a suitable material for a greenhouse that will use rigid glass or polycarbonate for its solar panels.
Aluminum is a very low-maintenance material—it doesn’t rust or break down from exposure to the elements. But aluminum is not very strong and when used for a greenhouse frame, the support members must either be made from heavy gauge pieces or doubled up for strength. Aluminum does, however, provide a good rigid form for glass or polycarbonate panels. Aluminum can be painted or anodized in any color you choose.
Galvanized steel provides durability at a low cost. Because steel is very strong, your greenhouse will require fewer framing members, which means that fewer shadows will be cast into the greenhouse. However, most steel frames are designed to be used with polyethylene film rather than solid glass or polycarbonate panels. Greenhouses with galvanized steel frames and polyethylene film covering are popular with commercial growers, but they are not particularly attractive in a residential setting. And a big negative for galvanized steel is that the galvanizing will eventually wear and the steel will rust.
PVC Plastic Pipe
PVC plastic pipes are low cost, lightweight (portable), and very easy to assemble. A frame made from a plastic pipe is not as rigid as metal or wood, but the industry is addressing that designs that incorporate metal supports along with the PVC framing. PVC also has the advantage of allowing less heat loss than metal framing. Greenhouses with PVC frames almost always use polyethylene film for walls. These are usually smaller, hobby-type backyard greenhouses; plastic pipe does not work very well for a large greenhouse. Most hobby greenhouses sold in kits now come with PVC frames.
The biggest negative for PVC is that sunlight can eventually deteriorate it. But a PVC greenhouse that has pipes that are UV-protected should last at least 20 years; this is something to check when you purchase a PVC kit greenhouse. Another drawback is that the frame pipes must be large diameter in order to compensate for PVC's relative lack of strength, and the frame, therefore, casts more shadows than a metal frame.