7 Things to Know Before Building a Greenhouse

Wouldn’t it be amazing to grow big, tasty tomatoes all year round for perfect BLTs or pick bright, cheerful flowers even on the dreariest days? Gardening enthusiasts can have these things and more by setting up a greenhouse in their backyards or acreages. But there are seven things you should know before building a greenhouse.

What is a Greenhouse?

A greenhouse, also known as a glasshouse, is built to shield plants from bad weather while controlling heat and humidity to create an optimal growing space. A greenhouse is typically a frame building covered in plastic or glass used to grow flowers, produce, and other plants that require specialized conditions to thrive.

Greenhouses come in all shapes and sizes, from one-day do-it-yourself projects covered in polyethylene to prefabricated buildings costing thousands of dollars. Here are seven things to consider before plunging ahead with your greenhouse:

1. Location is Key

Location, location, location. It can make or break any venture. Your greenhouse’s location influences building style and the amount of light the plants receive, which determines the plants that can be grown. Additionally, location determines how the greenhouse owners will access the structure, as well as the structure’s risk of storm damage.

In North America, a greenhouse is often located on the south or southeast side of the house, away from shade. To get maximum sunlight through the winter, the best spot is on the east side or along the east-west axis. The southwest or west will work if you don’t live in an area with extreme heat. Avoid the north side; it’s too shady. 

If you want to get specific, try a sun path chart.

2. What’s Your Type?

Choosing the greenhouse structure has a lot to do with what kind of backyard greenhouse projects you plan on doing now and in the future, such as growing enough veggies or flowers to sell at a farmer’s market. You also should consider what other styles of buildings are on the proposed site.

Here are two types of greenhouse structures to think about:

Standalone

It’s a separate building, usually near an outdoor garden. A freestanding structure provides more growing space and can be placed anywhere on the property. Freestanding greenhouses are also easier to ventilate and disperse sunlight evenly across the plants without exposing them to dramatic temperature changes or frigid air.

About 50 percent of Janco Greenhouses customers in Georgetown, S.C. choose standalone greenhouses, and the other 50 percent want the attached lean-to option. “Some people want to come and go into their conservatories from inside their homes,” says Zack Clewer, who is Janco’s sales manager. “Some want that standalone greenhouse near their outdoor garden.” 

Attached Lean-To

These greenhouses are built against an existing structure on one or more of its sides. It is attached to the house or other buildings. Due to the limitations on placement around the home and growing space, lean-to greenhouses are not ideal for everyone, but they are an excellent solution for those with limited space.

The attached greenhouses include:

Window-Mounted: In window-mounted greenhouses, a few plants are cultivated for minimal heating and cooling cost. Single and double designs are available in various sizes to fit most standard window frames, and simple installation tools are needed.

Even-Span: When people think of greenhouses, the attached even-span style is the one that often comes to mind. The even-span greenhouse resembles a freestanding framework. However, it connects to a home or garage with a single gable roof. Even-span greenhouses cost more than lean-to greenhouses, but they provide more flexibility in the design and space.

Remember the Weather

If you live in an area susceptible to wind or storms, having a greenhouse anchored to a foundation with a solid structure can prevent it from blowing away. Those anchored to heavy-duty footers or attached to a building like your home aren’t going anywhere. Some of the glass might break in a storm, but those are replaceable.

3. Pick Your Proportions

The idea is to maximize sun exposure while limiting heat loss, which typically calls for long, narrow structures. Of course, this will depend on your available space, but a width-to-length ratio of 1:3 is ideal.

Many homeowners go for a smaller glass greenhouse when they first start, but as many plant people know, you can quickly fill a space with “one more.” Contrary to popular belief, building a small greenhouse presents a substantial challenge, even for veteran growers, due to their tendency for dramatic temperature fluctuations.

On the other hand, when you build a large greenhouse, it has higher start-up and maintenance costs. The minimum recommended dimensions, by type of greenhouse, are:

• Lean-to: At least 6 feet wide by 10 feet long
• Standalone: From 8 to 10 feet wide by 12 feet long

4. Frame and Form

Two critical factors for your greenhouse: materials and shape. Some are entirely made of plastic or glass, while many are a combination of materials. Some have rounded roofs, while others are pitched. Each has benefits and drawbacks. 

Materials

Your custom glasshouse can be made with an array of materials, either bought in a greenhouse kit or recycled. Finding the right supplies for you and your budget is always the best fit.

Framework: A greenhouse’s structural support system is often aluminum, galvanized steel, plastic, or wood.

• Aluminum and galvanized steel are recommended due to being rust-resistant and lightweight but strong.
• PVC pipes cost the least but are the least durable. They’ll work in mild climates or a temporary greenhouse.
• Wood tends to rot in a damp environment. Make sure to use pressure-treated wood only.

Walls: For the sides of your greenhouse, there are a few options: fiberglass, clear plastic sheeting, polycarbonate, or glass. Companies also sell heat retention curtains for greenhouse walls.

• Fiberglass is light, durable, and hail-resistant. You should opt for greenhouse panels that are translucent or clear.
• Clear plastic like polyethylene (PE), PVC, or other readily available transparent plastic films are options for plastic greenhouses. Depending on the type, PE has a lifespan of one to three years. PVC and co-polymers with ultraviolet (UV) inhibitors will last longer.
• Polycarbonate and acrylic create double-wall plastic covers. Webs separate the sheets of two strong layers of plastic. The double-layer material retains heat longer, resulting in energy savings. Acrylic lasts a long time and doesn’t yellow; polycarbonate yellows more quickly but is usually shielded by a UV-inhibitor coating.
• Glass and an aluminum frame create a low-maintenance, weather-tight structure that minimizes heat loss and maintains humidity. Tempered glass is two or three times stronger than regular glass. 
  
  Glass has the disadvantage of being readily damaged, more expensive initially, and requiring a more robust frame than fiberglass or plastic. It also needs a solid foundation.

• Recycled materials like old windows are picked up from flea markets and recycling centers. 

Shape

The roof is the defining shape of freestanding greenhouses. Gothic, A-frame, Quonset (hoophouse), or gable roof designs have different advantages.

Gothic: An excellent option if you prefer the classic conservatory look. Gothic-style greenhouses have an arched roof that resembles an upside-down leaf. The roof’s shape repels rain and snow, making it ideal if you live in an area with heavy snowfall. Glazed, double poly, PE, and glass are ideal materials for this style.

A-Frame: The A-frame greenhouse is a standard greenhouse design with an even spread at the bottom and two equal-length sloping sides to form a peaked roof. This wide base takes up a lot of ground space. Airflow and maneuverability are lower, but it’s a simple design, can be made with almost any material, and handles heavy snow well.

Quonset: The rounded roof option, like the Quonset, is widespread.  Also known as a hoop-frame greenhouse,  it has a half-cylinder construction formed into a long tunnel. It is less expensive to install and has greater interior airflow control than an A-frame. However, heavy snowfall may cause structural damage.

Gabled: This is a common type of greenhouse. It has straight sides and a triangular roof; it is high enough to move around in and receives the most sunlight. Because of the diagonal design, snow doesn’t tend to build up. The physical structure is easy to build and made from various materials, including rigid plastic panels or glass.

5. Lights, Water, Action

Whatever the style, size, or construction method, there are some requirements for a successful hobby greenhouse. The following components are essential:

Electricity

Whether you wire the greenhouse with outlets (depending on your city codes) or run an extension cord from the house, electricity access can help control temperature and run fans and other habitat control equipment.

Solar-powered products or DIY solar greenhouses may be more suitable if you don’t want to deal with running electricity to the greenhouse. 

Heat

Heaters are necessary for crops such as tomatoes and seedlings to grow and produce fruit and for tropical plants to overwinter in a greenhouse. Some heaters include a thermostat, enabling you to set the desired temperature.

Water 

Availability of water is vital. For the majority of greenhouse crops, hand watering with a garden hose is fine. For those with hectic schedules, a range of automatic watering devices are available.

Air Circulation

Clewer recommends installing oscillating fans in your greenhouse. You must maintain air circulation to keep the temperatures consistent during the winter months when the greenhouse is heated. In the absence of airflow, warm air rises to the top, while cold air collects below near the plants.

Automatic vent openers are a must during the summer. They prevent greenhouses from overheating — plus, the breeze is free, says Clewer.

Flooring

Although the bare ground works as a free floor, it also offers pests direct access to your plants. Water sitting on the bare floor makes it muddy and uneven and may also contain disease-causing bacteria that will infect your garden.

An excellent, affordable solution for greenhouse flooring is pea gravel. On warm days, you may spray the gravel to keep the area cool, and it is easy to stand on. You can build a paver walkway, too. Mulch isn’t recommended in moist, enclosed areas because it encourages fungus.

6. Permits Needed

Whether you need a permit to place or build your own greenhouse on your property depends on where you live. Local building authorities have the final say on whether you need a permit, so check with your local planning department.

• For most rural areas, it’s like adding a shed or an auxiliary building, and you often don’t need a permit.
   
• The closer you get to a big city, the tougher the codes are. Those jurisdictions are more likely to require a permit.

7. Cost

The real question is, “How much will this cost?” With so many options available, you’ll have to decide what you want and how much you want to invest in your greenhouse. These decisions will alter the price, but the two most significant are size and if you plan on a professional installation or opt for a DIY greenhouse.

Professional Install

Always get at least three different contractor bids and quotes. Each bid should include everything — from site preparation to water supplies to finishing work.

The national average cost for most residential greenhouses ranges from $5 to $30 per square foot. So, a 50-square-foot greenhouse averages between $250 and $1,500 overall, depending on your location and the installed amenities.

You might want to consider building a larger greenhouse — more than 300 square feet — because the cost is less per square foot of usable space than for smaller ones, according to the Oklahoma Cooperative Extension Service. However, the initial startup expenses will be higher.

Other Factors to Consider

Type of greenhouse: Each greenhouse style requires different materials and building methods. Prices vary from $5 to $10 a square foot for the Quonset style to $25 to $35 a square foot for the A-frame variety. 

Structure material: Framing materials average $1.00 to $2.50 a square foot, depending on if you choose a wood, PVC, aluminum, or steel framework.

Type of glazing: Glazing materials are whatever you use to cover your framework. Options range from greenhouse plastic sheeting that starts at $0.20 a square foot to glass panes that go for $2.60 a square foot.

Flooring material: You have many options here, from nothing up to concrete floors and pavers. Pea gravel is a good, budget-friendly option from $1.00 to $3.00 a square foot. Depending on size, quantity, and type, concrete and pavers can range from $2.50 to $25.00 a square foot.

Opt for insulation: Insulation can improve the effectiveness of your grow house. Costs for insulation range from $1.50 a square foot for thermal screening to $2.50 to $3.50 a square foot for double-pane glass.

Utilities: Most small hobby greenhouses can run with a water hose and extension cord or attached to your home and its utilities. But if you’re opting for ample growing space, you’ll need to think about installing electricity to run climate control equipment. A greenhouse heater runs from $55 to $270, and fans from $25 to $150.

Solar power is an option, but solar glasshouses have a higher upfront cost and an average of $35 to $45 per square foot, plus the rate a contractor will charge if you’re not able to do it yourself. Contractor rates average between $45 to $150 an hour, depending on the work you need.

Site preparation and permits: If you have to clear land or level your yard before installing a greenhouse, it will require more time, money, and effort. 

Zoning and building laws vary significantly from state to state and from rural areas to urban gardens. Building permits say how you can build, and zoning says where. 

Easy DIY Greenhouse

If you’re the hands-on kind of homeowner, then the make-your-own option is one way to go. A home-built greenhouse costs 40 percent less on average than a professionally built one. Greenhouse do-it-yourself kits start with small, indoor options for around $40 to massive projects ranging in the thousands.

If available, using recycled materials can significantly reduce costs while being eco-friendly. 

Recycled Windows Greenhouse: A great solution for dealing with old and unwanted windows is to use them to build a DIY backyard greenhouse. Such spaces are perfect for growing herbs and small plants, as windows are usually designed for maximum light penetration. 

Tabletop Greenhouse: This is an elementary, DIY type of greenhouse. It is the perfect project for anyone who feels particularly crafty or if you want to grow plants in a well-protected environment. These mini greenhouse designs are unique, personal, and depend on your imagination. 

You can build these out of fish bowls and other glass materials you have around. Picture frames are a good choice for small greenhouses or terrariums. You can get most of the necessary supplies from your local dollar store. 

Why Build a Greenhouse

Zack Clewer says that most of their clients want a greenhouse because gardening is one of their favorite hobbies. But the company also builds conservatory structures designed as complete enclosures, so people can use them for hot tub rooms, dining areas, or additional living areas. 

One client bought a greenhouse because he wanted a room attached to his house in the mountains where he could smoke cigars. Roof vents at the top of the structure allow all the smells and smoke to rise out without leaving any unpleasant after-odor.

Popular reasons to build a greenhouse or conservatory: 

• All-season gardening: You don’t have to worry about the cold or snow killing your vegetables in winter. The controlled environment inside the greenhouse helps everything thrive despite the cold outside.
• Perfect growing space: From potting herbs to flowers and vegetables, the greenhouse environment secures enough heat and water vapor to help greenery thrive. 
• Money-saving: Many households spend a fortune on plants and lawn grass for their outdoor gardens every spring. When you have a conservatory, you can grow as many seeds as you want, and you’ll probably have more than enough to give away to friends and family as well.
• Multi-purpose space: When you have a greenhouse, you won’t need a garden shed because you’ll have enough space to store tools and other equipment.
• Plant protection: Plants that grow in greenhouses are protected from bad weather and pest infestations such as caterpillars, spider mites, locust swarms, and many others. Overall, plants that grow in conservatories are healthier.

FAQ: Building a Greenhouse

1. What is a Deep Winter Greenhouse?

A deep winter greenhouse or DWG uses passive-solar energy rather than an artificial heat source. Gardeners can use them to grow crops that don’t require full sun, like lettuce and herbs, and extend their growing seasons.

2. Where Do You Get Greenhouse Plans?

There are many free resources for glasshouse designs available on the internet. Still, the Mississippi State University Extension offers a wide selection of greenhouse plans to build a do-it-yourself greenhouse in a variety of sizes and styles.

3. How Do You Keep Track of Everything in Your Greenhouse?

There’s an app for that! The UMass Extension Greenhouse Crops and Floriculture Program offers a list of apps and tracking solutions to best use your time, space, and money.

When to Call a Professional

Ultimately, it all comes down to how much time and money you want to invest in your greenhouse. A hobby gardener can easily enjoy a small DIY space, but you may have bigger aspirations or a love for all things green. In that case, getting a free quote from a local general contractor is just a click or phone call away.

Main Photo Credit: Marilyn Peddle / Flickr / CC BY 2.0