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Like many gardeners, I tend to start a number of seedlings for my garden. Tired of setting them up in my house, I decided it was time to build a greenhouse and wondered why I never saw a greenhouse with a flat roof.
A greenhouse can have a flat roof, but typically curved or arched designs are more common. These structural differences help the building better 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 warmth they need. You don’t want the structure having 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 which route you pick, you aren’t likely to 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, which increases the temperature of the inside of 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 the 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 choose to have your greenhouse attached to another structure, such as a pre-existing shed or even your 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 people 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 common style and is often cited as being one of the easiest styles 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 running off of it. 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 you are 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 that your roof is made out of affects how much light is transmitted through it. Now that rule is an oversimplification, and you usually don’t need to have a roof nearly as steeply designed as that would suggest.
The maximum amount of light is transmitted when the beam of light 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 being reflected off of 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.
Now that goes to suggest that the maximum amount of light transmitted would occur on a flat roof with the sun beaming down from 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.
There are disadvantages to having a flat roof design, as previously mentioned. Precipitation is common in most areas of the world, and a greenhouse with a flat roof will accumulate the rainwater or snowfall before running off of it.
This is especially true with snow, which can easily pile up on the roof of the greenhouse and cause it to cave in. Instead, even having a mild slope to your greenhouse roof will allow precipitation to run off and has the potential to increase the amount of light transmitted across the panes.
Other Considerations for Your Greenhouse Roof
As you do a little research into greenhouse, you will find that there are many different 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 an extremely important consideration. Taller greenhouses are better at maintaining a better overall temperature. In fact, the optimal height for a greenhouse appears to be 13 feet, which allows for better airflow and temperature regulation, with most plants getting 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 in, including vining plants. Plan on floor space to grow many plants but also areas to hang baskets that your flowers and even indeterminate tomatoes can hang from.
At bare minimum, consider the height of the people entering the greenhouse, including yourself — you don’t want to hit a beam every single time you walk into your gardening space.
Also, take into account the plants you’re looking to grow there: trees need significantly more room, even dwarf varieties, than plants like Swiss chard and even tomatoes.
As odd as it might sound, you might also consider an open roof for your greenhouse structure. This allows gardeners to make use of the elements better when growing their plants. With these greenhouses, the open roof panels can be opened or closed, depending on what the weather is like.
For example, if there is rainy weather or snow, the panels can be mostly closed to allow plants to still have airflow while be 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 rather than just simulating outside conditions, the plants are being exposed to the elements. The plants acclimate more readily to their new growing locations when they are transplanted.
Preparations if You Have a Flat Roofed Greenhouse
If you have a greenhouse with a flat roof, you need to make sure that the roof has a 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 roof off during severe weather conditions.
You also need a way to remove precipitation. Even having a slight angle to your roof will aid rain and snow in sluicing off of 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
There are many places in which you can buy a greenhouse. The prices can go from a few hundred dollars to thousands. If you can afford to pay those prices then maybe consider building your own greenhouse as I did in the video below.
This has been with me for the past 8 years and has stood the test of time and 100 MPH winds. Watch the re-build below when I moved it to a new location.
Can a greenhouse have a solid roof?
No, greenhouses need to be able to let light transmit the plants inside, so they cannot have a solid roof. Similar to greenhouses, shade houses may have covered roofs, although they typically still let light transmit across a barrier. They are useful for plants that need to be shaded from the sun.
Does a greenhouse need to have an airtight roof?
Greenhouses do not need to have an airtight roof, although controlling the areas that air can enter and exit help control the structure’s ability to act as a heat sink and keep the temperature up. Many greenhouses are designed to allow airflow, with roofs or wall panels that open.
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. They are usually made with the lean-to style of the greenhouse. But take into account your local weather, curved roofs and arches tend to release snow much better.
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Remember folks you reap what you sow!