Building a Solid Greenhouse Foundation: A Step-by-Step Guide

Building a Solid Greenhouse Foundation: A Step-by-Step Guide


Constructing a greenhouse is an exciting project for any gardening enthusiast. However, ensuring a stable and level foundation is essential for the long-term success of your greenhouse. In this comprehensive guide, we'll walk you through the process of building a robust greenhouse foundation, step by step, to ensure your structure stands the test of time.

  1. The Importance of a Sturdy Foundation

Before you dive into the construction process, it's crucial to understand why a solid greenhouse foundation is so vital. Our experience has taught us some valuable lessons, especially when it comes to choosing the right foundation options.

When we decided to build our hobby greenhouse in 2015, we initially thought that a simple heavy wooden frame on the ground would suffice for our aluminum frame greenhouse. However, this approach didn't work out as planned. Despite our best efforts to level the frame, we struggled to achieve perfect squareness along its entire length.

Our goals were clear – we wanted a foundation that would allow us to plant directly in the soil inside the greenhouse and remain level during Wisconsin's yearly freeze and thaw cycles. To achieve this, we had to rethink our foundation strategy.

  1. Designing an Uber-Sturdy Greenhouse Foundation

Our greenhouse measures 20'x10′ and features an aluminum frame with 10mm double-wall polycarbonate wall and roof panels. To ensure a stable foundation, we designed it to resemble a frost-protected deck footing. Here's an overview of the concept, followed by a detailed step-by-step guide with accompanying photos:

  • Six 8″ concrete tube footings (one at each corner and one midway down each of the 20-foot sides).
  • These footings are interconnected with 20-foot long 2″x10″ boards (stringers) bolted through each of the concrete tubes on the sides.
  • Additionally, 10-foot long 2″x10″ boards (stringers) are bolted into the 20-foot long boards on the ends.
  • We filled the box frame with dirt to eliminate any space where varmints might want to move in, which also helps maintain a more uniform temperature and moisture distribution around the wood.
  • Finally, the box frame is capped off with 2″x 12″ boards, and the greenhouse is secured to the 12′ wide boards.
  1. Choosing the Right Wood for Your Foundation

Selecting the appropriate wood for your greenhouse foundation is essential, especially for below-grade installations. Using wood that is recommended for such conditions is crucial to prevent rot. Consult with your local lumberyard or hardware store personnel to ensure you choose the right type of wood.

In our case, we opted for pressure-treated wood, as it can withstand prolonged exposure to moisture without deteriorating. Using the correct wood ensures the longevity and stability of your foundation.

  1. Why 12-Inch Wide Cap Boards Matter

You might wonder why we emphasize the importance of 12-inch wide cap boards for your greenhouse foundation. Here's why:

  • Concrete tube forms typically have a minimum diameter of 8 inches. Adding a few inches on each side for the support boards brings the total width to around 12 inches. This size is practical and accommodates minor misalignments.
  • In areas with rocky soil like ours, achieving perfectly straight holes can be challenging. The 12-inch width provides some forgiveness.
  • Wider cap boards distribute wind loads over a broader area, which is especially helpful in regions with strong winds.
  1. Step-by-Step Building of Your Greenhouse Foundation

Before you embark on building your greenhouse foundation, it's crucial to check applicable local building codes. Now, let's go through the steps:

Step 1: Decide Where to Locate Your Greenhouse

Choosing the right location for your greenhouse involves considering four main factors:

  • Sunlight: Ensure your site receives at least 6 hours of sun per day, with more being beneficial in cooler climates. In warmer areas, some afternoon shade may be desirable.
  • Level and Well-Drained: Opt for a level site with proper drainage to prevent standing water in the greenhouse.
  • Wind: Be aware of wind exposure, as greenhouses can act as sails in high winds. Consider sheltered locations if possible.
  • Ease of Access: Place your greenhouse in a high-traffic area for easy utilization.

Step 2: Clear and Level the Area and Mark the Location of Your Footings

Once you've chosen your location, mark the corners and ensure they are square. Mark additional post hole spots as needed. It's essential to have square and level corners to facilitate the construction process.

Step 3: Dig Your Greenhouse Frost Footings

Use a gas-powered post hole digger to speed up the digging process. Consider recruiting help to form a team for efficient digging, especially if you have challenging soil conditions.

Step 4: Prep Your Hardware

Prepare your hardware, including threaded rods and joist hangers, for securing the stringers in place. This preparation ensures a smooth construction process.

Step 5: Pour Your Footings

Pouring concrete footings requires careful planning. Ensure your threaded rods are in place before filling the forms with concrete. Level your forms before pouring to maintain a level foundation.

Step 6: Create the Box Frame that Supports Your Greenhouse

Assemble the box frame that will provide support for your greenhouse. Attach the stringers, align the cap boards, and make necessary cuts to ensure proper alignment and stability.

Step 7: Attach Your Greenhouse to the Foundation

Secure your greenhouse to the foundation using clips and anchors designed for this purpose. Ensure that your greenhouse is firmly attached to the foundation, ready to withstand various weather conditions.


Building a strong and durable greenhouse foundation is the key to a successful gardening endeavor. By following these detailed steps and understanding the essential principles behind each component, you can create a foundation that will support your greenhouse for years to come. Happy gardening!

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