When you set up your greenhouse, you want something that is going to be structurally sound. This article is going to focus on those important components that will give your greenhouse the stability it needs. Whether you choose to go for a DIY or a manufactured greenhouse, they will need a solid foundation. You might also want a floor in your greenhouse.

YOUR GREENHOUSE’S FOUNDATION

While you have thought mostly about the greenhouse itself, that only takes care of framing and glazing. You should always remember how important it is to have a great foundation. As mentioned, your building, zoning, and neighborhood rules and regulations might require your greenhouse to have a foundation, but it’s a wise idea anyway to ensure that you have your greenhouse for years to come.

Is it really that bad if you’re not completely leveled? Your greenhouse is like any other building, so you will compromise its strength. You’ll also see some immediate effects. You’ll have issues with your frame and panels aligning (if you use glass), which will see you cutting into pieces that don’t need to be cut. There will also be issues with the door, as you will struggle with the installation or the door will not hang properly. This doesn’t include the effects that you will feel down the road, especially if you don’t account for your frost lines.

Know Your Frost Line Before You Prep Your Site

Frost lines are extremely important to cold climate construction, but it’s still important to know where they are in warmer regions. This might be covered in your building codes, so make sure you understand all the codes for your region.

But what is a “frost line”? This is the depth at which moisture freezes in the ground. Temperatures and soil-moisture content are going to vary, but the best example is that the frost line is far deeper in the northern part of the United States than it would be in a place like Florida. Frost lines matter because when the ground freezes, anything above those lines can move. This is where footings will become necessary for your foundation. The deeper your frost line, the farther down you will need to have your footings or pins. These should be dug below the frost line to avoid them being heaved when they reach freezing temperatures. These shifts can lead to major structural issues that will make your greenhouse unsafe.

While you could look at a frost line map, this is a generalization. The people who are going to know where your local frost line is will be those at the building or zoning office. They will give you a clear guidance on how far down to place your footings or pins.

WHICH FOUNDATION IS BEST FOR YOU?

Knowing your climate, soil conditions, and frost line will determine your foundation or base. Again, this is the wisest idea to avoid issues later on, but before you lay down anything, you want to make sure the area is level. The best way to do this is by digging the top three to four inches off the soil. From there, you can compact the area before you put in additional supports.

These additional supports will be your footings or pins. For example, you can use concrete pier blocks in all four corners to start. Then you will place these blocks every six to eight feet, and you want to make sure that they are resting below your frost line. These are going to be what you construct the rest of your base on, but now we can get into the different ways you can find the right materials for your greenhouse.

Before we get into your foundation options, it’s important to keep in mind that some of these may require extra materials or extra pieces. A lot of this will depend on your frost lines and soil conditions, which is why it is always ideal to check your building codes first. But now, we can go over all the options you have for a greenhouse foundation.

Concrete

A concrete slab is going to be the most durable foundation for your greenhouse. It is going to take more planning and a lot of work to make sure it is set up right, but once you have it in, you really have nothing to worry about except your plants.

Pros:

• This foundation is going to give your greenhouse the most sturdiness.
• It easily makes for great greenhouse flooring.
• It is a better solution in places where you might not get a level area.
• Easily maintainable since you can sweep with a broom or wash when necessary.
• If you are making your greenhouse a permanent fixture, this foundation is the way to go.

You can prevent rodents and insects from tunneling into your greenhouse.

• Concrete is easy to build on, and if you use things like expansion bolts, you can hold your greenhouse in place.

Cons:

• Your garden is only going to come from garden beds, pots, and other containers.
• Standing water is going to pose a problem since it can only drain around the edges, which are going to be sealed off by your greenhouse.
• This foundation is the most expensive of all options, especially if you have to hire professionals to lay it out.
• Concrete is permanent, so you might need permits to lay this foundation down. This differs slightly from building codes requiring a concrete foundation.
• You can move your greenhouse, but that means you’re going to have to lay a new foundation for it.

Wood

A wood foundation is a great base for a greenhouse, and it is one of the most cost-effective methods when you think long term. The best lumber to use is any that is pressure-treated or that is naturally rot-resistant. Keep in mind that if you are farther away from the source, you are likely going to pay more.

If you are working with a smaller greenhouse (under 120 square feet), you can use 4″ x 4″ pieces. Larger greenhouses are better off if you use 6″ x 6″ or 8″ x 8″ timber.

Pros:

• Overall, wood is the more accessible and cheapest option, especially if you intend to not hire a professional to lay the foundation.
• A wood foundation can be adjusted or extended if you want to add to your existing greenhouse or purchase a larger structure.
• Wood foundations are easy to clean, and upkeep is minimal. This can be helped by applying a waterproof coating before constructing your greenhouse.
• Because of the spaces between your boards, drainage is easier.

Cons:

• Treated wood has the potential to release chemicals, which can pose a threat to your plants. Cedar and redwood are rot-resistant, but they are going to be very expensive. It will only be more expensive the farther you are away from the source.
• While wood is strong, it will not be as durable as concrete. Maintenance will be necessary.
• Woods that aren’t cedar, redwood, or chemically treated will be more likely to become moldy.
• If your foundation is more akin to a deck, you will struggle with keeping your greenhouse because of the exposure to the ground below.

Perimeter

You can make a perimeter that you can set and secure your greenhouse base on. These can be made from wood, concrete, bricks, or anything that will provide your greenhouse with some security.

Pros:

• This might be the best foundation you can get if it is impossible to level the area picked for your greenhouse completely. You can also use this method if you are trying to avoid the extra labor of leveling the area.
• With a wooden perimeter base, you can adjust the height of the greenhouse depending on what type of greenhouse you are going with.
• A wooden perimeter is going to be more cost-effective, even more than building an entire foundation.
• This is the most labor-conscious method, as it’s very easy to build.

Cons:

• If you don’t secure your perimeter (usually with cement), your greenhouse might collapse.
• This method should be avoided on sandy soils. This soil can be washed away, which will make the entire base unstable.
• While you save on labor efforts, you have to be precise with your measurements. There can’t be the slightest room for error.

Raised Perimeter

There are many gardeners who, if they are able, prefer to use the raised version of the perimeter foundation. While there has to be more effort put into the initial planning, buying materials, and construction itself, once you have everything ready, it can be a relatively straightforward process.

Pros:

• By controlling the height of your base, you can make planting and harvesting easy tasks by saving yourself back pains from working in your garden.
• Raised perimeter also gives you the option of growing plants that are very tall when they mature.
• Using raised beds will cut down on a lot of issues with pests and weeds since you are using healthier soil.
• You also get all the benefits of the regular perimeter foundation.

Cons:

• With raised beds from the ground, make sure that no weed roots exist in the soil, or they will eventually interfere with your plants.
• The biggest drawback to a raised perimeter is that you will need to come up with a solution for your greenhouse door. You might have to make a custom door or put steps in.
• You are probably going to need to put in some type of flooring for a path to get to your plants, which is going to take away from some of the efficiency of your growing.

Soil

If there are no building codes that require you to lay a foundation, you can just put posts down in all four corners (set in concrete) to brace your greenhouse. You will need to make sure that the ground is level, though, to avoid any compromises to the structure.

Pros:

• For smaller structures, this is the most cost-effective option.
• If you have well-draining soil, you can plant directly into the ground.

Cons:

• If you are buying a larger greenhouse, this is not the best idea for those structures because they can topple or collapse far easier.
• There could be a myriad of problems with your soil from the start, or they can develop over time. These issues will come from drainage, too much compaction, a pH balance that isn’t ideal for the plants you’re growing, and the fact that you are putting all of your plants in competition for nutrients.
• If you just set your greenhouse directly on the soil, there is always going to be a chance that the frame can sink. This will cause structural damage.
• You can easily overwater, which will lead to a muddy, messy environment that can become stagnant or moldy without ventilation.
• Rodents, insects, or other pests can make their way into your greenhouse.

FLOORING

Greenhouse flooring is also going to be important when you’re choosing your foundation. Think about things like drainage, decomposition, price, heat retention, and traction. With your greenhouse, you are going to want these things, and you want something that will still make your greenhouse physically appealing.

Drainage

Drainage is going to be the most important part of your flooring since it is going to see a lot of water runoff. Going with something like concrete might mean that you should add floor drains to keep your floor dry and keep mold from growing and hurting your plants.

With the other foundation options that you’ve read, you can lie down on other types of flooring. You can use gravel or other rocks to provide drainage while using concrete pavers for a walkway.

Keep in mind that this is going to mean that you understand the draining ability of the ground below. If the soil in your area is highly compacted, it can pool water easily. Sandy soils will eventually wash away, which can cause shifts to your foundation that can damage the greenhouse itself.

Decomposition

When you choose organic materials like mulch and sawdust and wood, which we already looked at, you will face the issue of decomposition that attracts pests and algae growth. While organic materials are a cost-effective option in the beginning, they will end up costing you more in the long run as you will have to replace this flooring regularly.

Cost

You want to factor in the cost of the flooring because you don’t want to overspend on something you really don’t need, but you also don’t want to pinch pennies at the beginning for something that will cost you more in the long run.

Concrete is a great material, but you will need to power wash it every year to keep algae from growing, especially in hard-to-see spaces. This will be an investment in time and labor, as you will have to clear out the entire greenhouse when you power wash.

The most cost-effective method that won’t become expensive later would be something like gravel flooring that provides excellent drainage.

Heat Retention

You’re going to want something that will keep you from having to use alternative heat sources as long as possible, so you want flooring that is going to keep heat. This is where concrete, brick, gravel, and other similar materials will be more beneficial. They will trap the heat during the day and slowly release it back at nightfall.

This will be a very important consideration if you live in a cold region.

Comfort

Having a quiet place away from the stresses of life is only going to work if you are comfortable while working in your greenhouse.

If you are using gravel or other rocks, it’s important that you have a comfortable walking path that lets you work with your plants while being gentle on your feet. Or if you are using a concrete foundation, consider having rubber mats laid down since concrete can affect your comfort after enough time.

Traction

Possibly a more overlooked aspect of your greenhouse is making sure you keep your footing while in it. Gravel can easily shift when the ground is wet, but a weed screen below the gravel can prevent a lot of that. The walking path itself should always be clear, which means sweeping off loose soil and stepping with caution if wet.

Keep in mind that the bigger your greenhouse is, the more walking paths you will need to plan so you are only stepping on gravel when necessary.

Weeds

If you are using anything besides a deck-type wood foundation or concrete, think about weeds. While you can try to clear as many roots as possible, there might still be a chance of seeds or roots hanging around. So before you lay flooring down, consider laying down landscape fabric first.

While this might take away the option of planting directly into the ground, you can bring the right soils in for the different plants you want to grow. You will also eliminate the possibility of weeds and pests from getting into your garden.