In 2015, we decided to build a hobby greenhouse to extend our growing season. We thought the aluminum frame would be strong enough that a simple, heavy wooden frame on the ground could be used as the base of our greenhouse. (Most greenhouse kits recommend a concrete slab foundation, or railroad ties or cinder blocks for small greenhouses. ).
Unfortunately, that didn’t work out so well. There was no way we could get the frame perfectly level along its whole length, no matter how hard we tried.
I didn’t want to sacrifice the ability to plant straight in the dirt inside the greenhouse. There also had to be a foundation that would stay level during Wisconsin’s yearly freeze and thaw cycles.
A greenhouse foundation is crucial for ensuring the structure remains stable and level over time. An uneven or weak foundation can lead to issues like doors and windows not closing properly, glass breaking, and even collapse during extreme weather. Follow these steps to build a sturdy greenhouse foundation that will support your structure for years to come
Assess the Build Site
- Choose a site with good drainage and full sun exposure for maximum heat and light. West-facing walls work well.
- Check if the ground is level or sloped. Add soil to level a slight slope.
- Identify the soil type – clay, loam, sandy, etc. This affects how well the ground will hold posts and piers. Wet clay or loose sandy soil may require extra stabilization.
Prepare the Ground
- Remove grass, weeds, and debris.
- Dig out the top 3-4 inches of topsoil around the perimeter, at least 12 inches wide.
- Excavate deeper holes at the corners to add concrete pier blocks for stabilization. Place these every 6-8 feet.
Build a Wooden Base
- Construct a frame using pressure-treated lumber in the dimensions of the greenhouse. Cedar naturally resists rot.
-Anchor the wood frame with steel rebar hammered 2 feet into the ground. This secures the base. - Allow space between boards for drainage. Elevating the greenhouse prevents waterlogging.
Add Concrete Footings
- Pour concrete into the holes for pier blocks, about 12 inches deep.
- Let concrete cure fully before continuing, at least 48 hours.
- Pier blocks provide extra stabilization on uneven or sloped sites.
Level the Base
- Check that the wooden foundation is perfectly level using a spirit level.
- Add shims beneath boards or pier blocks to adjust as needed.
- A level base prevents structural issues like doors and windows not closing properly later on.
Secure the Greenhouse
- Set the greenhouse frame onto the leveled wooden foundation.
- Pre-drilled holes along the base allow securing it with bolts into the wood.
- Consider using metal corner plates for extra strength.
Additional Tips
- Gravel the center floor for drainage and to deter weeds.
- Install gutter drains around the perimeter to direct water away from the structure.
- Seal any gaps between boards with silicone caulk to prevent rodent access.
- Maintain the wood regularly by cleaning, sealing, and replacing any rotted boards.
A proper greenhouse foundation requires planning patience and quality materials. But the investment of time and effort up front will pay off for many growing seasons to come. The steps above outline a sturdy wooden foundation secured with concrete that will support your greenhouse structure and keep it level for optimal functionality.
A Note on Greenhouse Foundation Wood Choice and Why We Went With 12″ Wide Cap Boards
When you pick out the wood for the base of your greenhouse, make sure you pick wood that is meant to be installed below grade.
Talk to the people who work at the hardware store or lumber yard near you, and they’ll point you in the right direction. You’ll need something pressure treated, or specific wood types.
If you don’t use the right stuff, this wood will rot because it may have been wet for a long time.
On the choice of the 12″ cap boards:
Do we really need a 12 inch wide strip to bolt the greenhouse to? Yes.
The concrete tube forms’ smallest default size is 8 inches in diameter. Two inches on each side for the support boards brings the width to about 12 inches.
The 12 inch width is also more forgiving. It’s challenging to keep everything exactly square over long distances, so 12 inches allows for minor misalignment.
In our area, we have glacial till that has dumped a lot of rocks in the soil. Digging a perfectly straight hole can be a big challenge.
The 12″ wide cap boards help to distribute the wind load over a wider area. This helps when you have to deal with sustained winds of 20 to 30 miles per hour or with high winds once in a while.
#6 – Create the Box Frame that Supports your Greenhouse
First, get your screws, flat washers and lock washers in place on the 20′ stringers and 10′ stringers. Then, mount the interior 8 foot stringer.
This may need to be trimmed slightly, depending on the placement of the earlier stringers. (See diagram above.)
Once the stringers are in place, line up the 12″ cap boards. Cut the boards so they meet at a roughly 45 degree angle at the corners. This allows you to drill directly into the concrete, screwing the cap plates down with 3/16″x4″ Concrete/Masonry Screw Anchors.
There’s no need to do this because the cap boards are attached to the stringers every 12 inches with 3′′ deck screws. BUT – if you want one more layer of holding everything together, it’s an option.
To get everything to line up right, you might need to change the cuts where the 12 inch boards meet. Put them in place, mark, and then cut. You can see an example below.
It can be a little tricky to get everything to line up, so having several people helps. All four of us worked on getting the boards in place. By the time we finished, we all had sore hands and scraped knuckles. The boys screwed in all the 100+ 3′ screws on the cap plates in one evening.
A Beginner’s Guide to DIY Greenhouse Foundations: Pros and Cons for FIVE Different Styles!
FAQ
What is the best foundation for a greenhouse?
How deep should greenhouse foundations be?
How do you prepare the ground for a greenhouse?
How do I choose a foundation for my greenhouse?
Durability: Consider the longevity you want from your greenhouse. If it’s a long-term structure, a more permanent and durable foundation like a concrete slab may be a better choice. A short-term or hobby greenhouse won’t need as solid of a foundation as larger, permanent structures.
What are the different types of foundations used for greenhouses?
There are several types of foundations commonly used for greenhouses. Here are two of the most common: Concrete slab: This is a solid concrete base that provides excellent stability. It’s ideal for larger, permanent greenhouses. Pier foundation: This foundation consists of concrete or wooden pillars that are sunk into the ground.
How do I install a greenhouse foundation?
If the ground surface that you are installing your greenhouse foundation on is on a slight slope, dig the trench deeper or more shallow in some areas in a manner that creates a level trench. While creating the trench, we did our best to keep the marking stakes in place in the corners. Yet it may be necessary to remove them and re-measure later.
What is the foundation of a greenhouse?
The foundation of your greenhouse can vary based on personal choice and local bylaw restrictions – make sure you do your research for your area. The choice is yours but regardless, the preparation is all the same! For our greenhouse, I wanted the feel of a finished structure (a tiny home vibe), so we decided to build a raised wood floor.
How to build a greenhouse?
All you need to do it to build a solid perimeter to sit the greenhouse base on. You can make it out of breeze blocks, kerb-style slabs, bricks, paving slabs or concrete. That way you leave the central floor section free and can plant directly onto ground. Also, you can gravel the central part to keep it neat and tidy or make a pathway.
Does a greenhouse need a foundation?
The foundation of a greenhouse is a key to a successful structure. If you don’t have a good foundation your greenhouse won’t function properly. The problems like doors which can’t close and windows which don’t open are quite popular if the foundation is not stable.