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Like many gardeners, I tend to start several seedlings for my garden. Tired of setting them up in my house, I decided to build a greenhouse and wondered why I never saw a greenhouse with a flat roof.
A greenhouse can have a flat roof, but curved or arched designs are typically more common. These structural differences help the building stand up to the elements, such as rain, which may pool water on top of the structure and damage the roof.
When you design a greenhouse, you want a structure that will protect your seedlings or plants and provide them with the necessary warmth. You don’t want the system to have its roof ripped off during a spring storm, which could happen with a flat roof.
Different Styles of Greenhouses
There are many different designs for greenhouses, and you can buy a pre-designed and pre-made greenhouse or design and build your own. Regardless of your route, you will not likely see a greenhouse with a flat roof.
When sunlight shines on a greenhouse, the shorter wavelengths of light are transmitted through the glass, polycarbonate, or plastic film covering the roof, and the longer wavelengths are blocked.
How light is transmitted through greenhouses with flat roofs
The short wavelengths are converted into infrared radiation, increasing the temperature inside the greenhouse. The design of the roof helps influence how much light is transmitted and, therefore, the temperature inside the greenhouse.
The simplest type of greenhouse is a cold frame, which helps keep your soil warm and protects your plants from variations in temperature and sunlight, particularly when the seasons change. More detailed designs include a Gothic arch.
Lean-to greenhouses with flat roofs
You can also attach your greenhouse to another structure, such as a pre-existing shed or house.
The disadvantage is that the structure may shade your greenhouse significantly, limiting how much light will reach your plants and affecting how warm the building gets.
The two main types of attached greenhouses are lean-to and even-span greenhouses.
Freestanding greenhouses come in a wider variety of shapes and sizes. Many elect to build a Gothic arch or hoop-house style greenhouse because the rounded edges help protect the building from outside forces. An A-frame is another standard style and is often cited as one of the easiest to build.
Of the different styles of greenhouses, you don’t often find a greenhouse with a flat roof. While this would allow plenty of light into the greenhouse, snow and rain could pool on the top rather than run off.
The squared edges could also be easily buffeted by wind, potentially tearing the roof of your greenhouse right off.
Why the Design of Your Greenhouse Roof Matters
When designing your greenhouse, the best angle for your roof is determined through a simple rule of thumb. Take your latitude in degrees and add 20 degrees to it.
The angle of the glazing material your roof is made of affects how much light is transmitted through it. Now that rule is an oversimplification, and you usually don’t need a roof nearly as steeply designed as that would suggest.
The maximum light is transmitted when the beam hits the surface straight on or at a perpendicular angle.
Conversely, if the ray of light hits a pane of glass at an angle, less light is transmitted: severe angles mean that even less light will enter the greenhouse, instead ending up reflected off the surface of your roof.
On average, across all of the different materials you might make your roof out of, such as glass or polycarbonate, there is little difference in the transmission of light between 45 and 50 degrees from the perpendicular.
That suggests the maximum amount of light transmitted would occur on a flat roof with the sun beaming directly overhead.
However, the sun moves across the sky, affecting the degree that the ray of light hits the surface of your roof. As such, having an angle to the roof allows the maximum amount of solar energy to pass through the roof.
Another advantage is that you could utilize some of the roof space for Solar to run the equipment within the greenhouse. Of course, there are pros and cons to solar energy.
There are disadvantages to having a flat roof design, as previously mentioned. Precipitation is typical in most areas of the world, and a greenhouse with a flat roof will accumulate rainwater or snowfall before running off it.
This is especially true with snow, which can quickly pile up on the greenhouse’s roof and cause it to cave in. Instead, even having a mild slope to your greenhouse roof will allow precipitation to run off and potentially increase the amount of light transmitted across the panes.
Other Considerations for Your Greenhouse Roof
As you do a little research into greenhouses, you will find many considerations for your greenhouse. How big you want it is just the tip of the iceberg!
Measure the area where you’re looking at placing the greenhouse and see how much sun it gets. The amount of sunlight may affect what type of roof or covering style you design for your greenhouse.
The height of a greenhouse is a critical consideration. Taller greenhouses are better at maintaining a better overall temperature.
The optimal height for a greenhouse appears to be 13 feet, allowing for better airflow and temperature regulation. Most plants are exposed to mild but consistent temperatures around the bottom of the greenhouse.
The extra space when you design the height of your greenhouse roof can also make it easier to grow larger plants, including vining plants.
Plan on floor space to grow many plants and areas to hang baskets from which your flowers and even indeterminate tomatoes can hang.
At a bare minimum, consider the height of the people entering the greenhouse, including yourself — you don’t want to hit a beam every time you walk into your gardening space.
Also, consider the plants you want to grow there: trees need significantly more room, even dwarf varieties, than plants like Swiss chard and tomatoes.
As odd as it might sound, you might also consider an open roof for your greenhouse structure. This allows gardeners to use the elements better when growing their plants. These greenhouses can open or close roof panels depending on the weather.
For example, if there is rainy weather or snow, the panels can be primarily closed to allow plants to have airflow still while being significantly protected from the elements.
In many cases, gardeners will slowly increase the angle of the panels being opened throughout the day until about midday, when they begin to close them again to help trap in solar heat to keep the plants warm with regulated temperatures during the evening.
An open roof structure for the greenhouse also helps harden plants off more successfully because the plants are exposed to the elements rather than simulate external conditions. The plants acclimate more readily to their new growing locations when transplanted.
Preparations if You Have a Flat Roofed Greenhouse
If you have a greenhouse with a flat roof, you need to ensure that the roof has sturdy and secure construction to minimize the possibility of the roof blowing off.
While some temporary greenhouses can be secured with pressure clips to keep the panels and roof on, that is not enough to prevent the wind from buffeting the top off during severe weather conditions.
You also need a way to remove precipitation. Even a slight angle to your roof will aid rain and snow sluice off the rooftop.
Otherwise, you need to be able to drain or brush off rain, snow, sleet, hail, and other versions of water, or the weight may be too great on your greenhouse roof.
Home Made Greenhouses
You can buy a greenhouse in many places, and the prices can go from a few hundred dollars to thousands. If you can afford those prices, maybe consider building your greenhouse, as I did in the video below.
This has been with me for the past eight years and has stood the test of time and 100 MPH winds. Watch the rebuild below when I moved it to a new location.
FAQs on Greenhouse Roofing Materials and Designs, You Need to Know
Can a greenhouse have a solid roof?
No, greenhouses need to be able to let light transmit to the plants inside, so they cannot have a solid roof. Like greenhouses, shade houses may have covered roofs, although they typically let light transmit across a barrier. They are helpful for plants that need to be shaded from the sun.
Does a greenhouse need to have an airtight roof?
Greenhouses do not need an airtight roof, although controlling the areas that air can enter and exit helps maintain the structure’s ability to act as a heat sink and keep the temperature up. Many greenhouses with open roofs or wall panels are designed to allow airflow.
What is the best material for a greenhouse roof?
The roofs of greenhouses used to be constructed entirely of glass, but modern technology allows for lighter-weight materials. These include poly film plastic, twin-walled polycarbonate, and single-walled polycarbonate. The greenhouse sides may be made of the same material.
A greenhouse can be made with flat roofs, usually lean-to style. But consider your local weather; curved roofs and arches release snow much better.