Each roof material has benefits and drawbacks, designed to fit different growers' needs.
How Long They Last
Durability levels separate greenhouse roof materials. Glass, especially tempered glass, is a long-lasting material commonly used on greenhouse roofs. It can last decades. Even the strongest glass, however, breaks under a sudden impact, such as from a rock accidentally thrown at the glass or from hail. Fiberglass doesn't shatter as easily, but it breaks down and requires replacement or new resin after 10 to 15 years, according to the West Virginia University Extension Service. Double-wall plastic provides a lifespan of about 10 years while a utility-grade polyethylene film plastic may need to be replaced every year.
Let There Be Light
The amount and type of light entering the greenhouse influences how well the structure's plants grow. Glass refracts sunlight slightly, but clear glass doesn't typically block much light. Fiberglass tends to filter more light as it degrades, making it less effective as a greenhouse roofing material after several years. A transparent plastic roof offers lighting benefits similar to those of a glass roof.
How Well They Maintain Temperature
How well a greenhouse holds in heat and moisture affects the quality of plant growth. A glass roof can help create a weather-tight structure that keeps most heat and humidity from escaping; often vents are necessary to release excess heat in warm weather. Fiberglass doesn't offer as much insulation as glass. Many film plastics contain additives that reflect heat into a greenhouse, helping to provide adequate insulation most of the year.
Other Matters to Consider
Several other factors help growers decide which greenhouse roofing option is best for them. Those factors include price and ease of installation. Glass, for example, is often the most expensive option. It's also the most difficult to install in most cases because the greenhouse frame must be a sturdy material designed to hold glass panels in place on the roof. Film plastic, however, is flexible enough to cover a greenhouse of nearly any shape and lightweight enough to work with different frame materials. It's also the least expensive roofing option.