Greenhouses come in all shapes and sizes. While most have pitched or sloping roofs, others have hooped or domed roofs. Flat-roofed greenhouses are not the norm, partly because slanted roofs increase the transmission of light and solar energy, which benefits plants. Also, an angled or sloping roof greenhouse is more practical for the run-off of rain and snow.
A greenhouse can have a flat roof, it just isn’t the most popular roof design for small greenhouse structures. At the same time, a flat retractable roof has become increasingly popular for greenhouse crop farming in many parts of the world. But these structures are large.
Can I grow plants on a flat roof?
You can grow plants on a flat roof, providing the roof is properly sealed and waterproofed. But you cannot grow plants on a flat greenhouse roof.
Traditionally, greenhouses were built of glass that was attached to a frame. Flat glass roofs are completely impractical, and even if you tried one, you certainly couldn’t use it for growing plants.
First of all, the plants would block out the sun, and second, the weight of your plants would create a risk of the glass breaking.
But if it’s a good solid flat roof, a small greenhouse structure should work. You could even use synthetic turf on the surface. It will look good and won’t have the weight of a grassed roof made with soil sods.
Another thought is that you can establish a “green roof” on a flat roof. But you do need to know what you are doing.
Green flat roofs are designed to be planted. But the key to their success is a proper installation process.
For example, green roofs need to be established above a waterproof membrane that will protect the roof and support the soil without damage to the membrane.
Typically there will be a series of layers before you plant your green roof. These will include the deck itself, a vapor control layer, insulation, a waterproof membrane, a protection mat, drainage, and then a filter layer.
Properly constructed, you can create an amazing rooftop garden. But it will, in most situations, require professional input.
How do you build a flat roof on a greenhouse?
When you build any roof, be it for a house or a greenhouse, the basic design concepts are the same. So, your first step will be to get a professional to design the structure.
The truth is that flat roofs are not very easy to design. So, unless you have architectural experience, you will need plans to work from.
The major issue is to prevent water from seeping in through the roof. This is why it’s a lot easier to build a slanted roof.
But, here’s the secret. Most flat roofs are actually very slightly pitched – not more than 10 degrees.
Then, all you will need to do is follow the plans – to the letter.
Benefits of a flat roof greenhouse design
Even though flat greenhouse roof designs are not that common, a flat greenhouse roof does have benefits.
Cost is the major benefit of a flat greenhouse roof. Because you don’t have trusses, structural, material, and installation costs are reduced.
Where outdoor weather conditions are suitable, retractable and folding roof designs can be very effective. According to the University of Massachusetts Amherst, being able to open the roof increases the intensity of light and helps to control the growth habit and flowering of plants.
These designs also reduce electricity costs by reducing the need for expensive cooling with fans.
A research report from the Arizona Agricultural Experiment Station also found that flat retractable greenhouse roofs had major advantages. Plant scientist Ursula Schuch believes they are “the next best thing to being outdoors if you’re a plant.”
This is because they enable agricultural greenhouse growers to control the wind, sun, and cold temperatures. At the same time, they get the benefit of ventilation and additional light when they need it.
But, of course, the greenhouses and flat roofs that these people are talking about are huge. Also, the retractable designs are a lot more expensive than the kind of flat roof greenhouse design that you or I might consider.
Other greenhouse designs
Remember, a greenhouse is simply a structure that is clad with some sort of transparent material. This was traditionally glass, but today plastic materials that are both rigid and flexible are commonly used.
Greenhouses were very popular in Victorian England, and they usually featured high-pitched roofs and were made from wood and glass. Sometimes brick walls were incorporated into the bottom section of the design.
Today, greenhouses are loosely defined as structures that we use to grow plants. Just about anything goes.
Greenhouses can, and sometimes do have flat roofs, but the most common designs are pitched or rounded (curved). This is primarily because flat roofs don’t cope as well with rain, snow, and hail which results in water pooling on the surface.
When you opt for a DIY structure, another quite common choice is a lean-to that is attached to your home or garage. The roof will typically have a slight slope to allow for the run-off of water. It is also simpler for any type of roof repair.
As a matter of interest, roof angles in the U.S. usually specify a pitch ratio rather than the best angle for the greenhouse roof.
Greenhouses can have flat roofs. But if you are serious about propagating and growing plants in a greenhouse, it’s probably better to opt for more traditional designs.
A flat greenhouse roof is more likely to leak than one that has even a very slight pitch.
At the same time, if you need an agricultural greenhouse, a design with a flat, retractable roof could be the answer. But it will only likely be sustainable if you are going to grow plants in your greenhouse for sale.
All flat-roofed structures have potential drainage issues, and greenhouses are no exception. With professional help, a flat-roofed greenhouse can work.