A greenhouse roof is not one size fits all, and effective greenhouse roofing can be achieved through a number of different materials. The most common materials used as greenhouse roofs include glass, polyethylene film, double-layered panels of polycarbonate or acrylic, and fiberglass panels or sheeting. Each of these materials has benefits and drawbacks, and one may be more suited to a particular type of greenhouse than another. Each greenhouse roof material has one very important characteristic in common: It is a clear or semi-clear, light-permeable material that will allow the plants inside the greenhouse to receive the light and heat they need to grow.
When choosing which material is best for a greenhouse roof, the builder will want to consider his budget and the type of greenhouse he would like to build. Different materials may be better depending on whether the greenhouse will be small or large, or if it is a permanent or semi-permanent structure. The type of framing materials can also make a difference in the best materials for a greenhouse roof. Having a well-constructed plan is a good first step in determining which roof is best suited to the greenhouse.
The most traditional and permanent solution for a greenhouse roof is glass. It is the clearest, most light-permeable material available, and can be quite long-lasting when installed correctly. Glass is often the best choice for permanent greenhouses, solariums, or sunrooms. Tempered glass, which can be up to five times as thick as regular glass, poses the least risk of cracking or shattering. A greenhouse roof made of tempered glass may be the only roofing material that can hold very heavy, wet snow for long periods without collapsing.
Glass roofing does have its drawbacks, though. It generally requires the highest up front cost, not only due to the cost of the glass, but also because it requires strong, sturdy framing materials to support it. Care must be taken to carefully construct and seal glass greenhouse roofs, as those not sealed properly have a high probability of leaking. Glass also carries a higher risk of breakability than some other materials, and it is not the best insulator.
Fiberglass sheets or panels are a long-lasting, less expensive alternative to glass and may be used in permanent greenhouse structures for 15 years or more. This material is much lighter than glass, so it does not require heavy-duty framing the same way that glass roofing does. Fiberglass is durable enough to withstand many types of weather and other outside elements. Over time, however, the light penetration of this material will be reduced; it does require maintenance in order to maintain the best possible light permeability. Fiberglass is also not as flexible as some other types of greenhouse roofing.
Polyethylene or plastic film is an inexpensive, flexible, readily available and easily applied greenhouse covering. It may be comparable to glass in transparency, lending to excellent light-permeability. Where small, temporary, portable, or seasonal greenhouses are concerned, polyethylene film can be a quick and easy greenhouse roofing solution. It is also easy to apply to greenhouses with curved or arched roofs, and may be used in open-panel or open roof greenhouses when a retractable material is required.
Some types of polyethylene film may be sturdy enough to last up to five years, and some have been treated with additives that help to reflect and radiate heat into the greenhouse. On the downside, film coverings are not as durable as glass or fiberglass, may be more easily damaged than other materials, and require the most frequent replacement. They are unlikely to be a good choice for permanent greenhouses.
Double-layered polycarbonate or acrylic greenhouse roofing, which is generally constructed with equally spaced "webs" between the two layers, retains heat very well. It may be the best roofing material where energy costs or cold temperatures are concerned. It is stronger and more durable than film, and may be flexible enough for use on curved roofs. It is reasonably long-lasting, sometimes up to 10 years.
On the other hand, the double-layer construction does not allow as much light to pass through as glass or single-layer fiberglass. Polycarbonate materials can be susceptible to yellowing, though acrylic is less likely to yellow. This type of material will also not last as long as glass or fiberglass.