How to Build a Concrete Block Greenhouse Foundation

Are you planning to add a greenhouse to your property? If so, congratulations on that awesome decision! We have had our hobby greenhouse for about 4 years now, and our only regret is that we didn’t get a larger one. When it comes to installing a greenhouse, one very important consideration is creating a level and sturdy foundation for the greenhouse to sit on. The greenhouse foundation also provides a surface to anchor and secure the greenhouse to, which is essential to prevent damage during harsh wind and weather.

As we explored in our “Hobby Greenhouse 101” guide, a greenhouse foundation can be created from wood, concrete, bricks, blocks, or other similar durable building materials. We chose to build a concrete block wall foundation for our 6×8’ hobby greenhouse kit. I liked the look, durability, and also the fact that we could build it up high. By installing our greenhouse on top of the block wall, we gained over a foot of height and headroom inside!

Keep reading to see how we prepared and built a simple concrete block greenhouse foundation.

SUPPLIES NEEDED TO BUILD A CONCRETE BLOCK GREENHOUSE FOUNDATION

• Concrete or cement blocks of choice. We sourced our concrete blocks (called Morro Stone) from a local landscape supply company – Air Vol Block in San Luis Obispo, CA. Using the length and height dimensions of your blocks, do the math to see how many you will need for the given size greenhouse and desired height. For example, our greenhouse has an approximately 6×8’ perimeter base, and we wanted to build up the greenhouse foundation to be a foot tall – or three rows of 4” tall blocks high.
  
  
• Adhesive.  While you could opt to secure your concrete block greenhouse foundation with traditional mortar, we chose to use a concrete construction adhesive or glue instead. The professionals at our local landscape supply company advised that this was the best and easiest way to attach the blocks together. Our concrete blocks happened to be marketed as a “mortarless wall system”. Choose an adhesive that is described as durable, waterproof, and capable of permanently bonding concrete, masonry, stone, or brick. You’ll also need a caulking gun to dispense the adhesive.
  
  
• A Level – used to assess both the level of the ground surface during prep, and to check your work as you go when laying the blocks.
  
  
• Road Base. Depending on your native ground surface, this may not be absolutely necessary but is suggested. Creating a firm and stable surface to install the concrete blocks on top of will help prevent the greenhouse foundation from shifting and settling in the future. Choose a compacting road base or similar material that is used for stabilizing foundations – NOT play sand or other fine material that erodes and moves with moisture. 

• Soil Tamper – used to compact the soil and suggested road base material below the block wall foundation. 

• A measuring tape, stakes, and rope or string – to measure and mark your foundation area. Stakes or small flags are especially helpful in marking the corners to keep everything perfectly square. 

• Recommended: Weed barrier landscape fabric for below the greenhouse floor, and possibly even below the foundation. The space we installed our greenhouse was extremely weedy, so we chose to add landscape fabric below everything, extending far out from the greenhouse perimeter as well. I highly suggest using a commercial-grade durable landscape fabric – like this one, which we use extensively in our garden. It will last a long time and does not easily rip or become a hot mess like other thin black plastic weed barrier material does.
  
  
• Power drill, a concrete/stone drill bit, and concrete screws

INSTRUCTIONS TO BUILD A CONCRETE BLOCK GREENHOUSE FOUNDATION

Step 1: Choose Your Greenhouse Location

First, choose a location for your greenhouse and the greenhouse foundation. If possible, select a location that is already fairly level. Ideally, a greenhouse should be located in a place that receives as much natural light as possible. Take into account how the sun’s path and any shadows will change during various seasons.

However, if your area receives high wind, I would opt to install the greenhouse and foundation in a semi-sheltered location away from known wind tunnels or open spaces. Unfortunately, the only space we had available for a greenhouse was on a side yard that gets partial shade. We do have to use supplemental lighting at times, but it is also nicely protected from the wind.

Step 2: Prepare & Mark the Greenhouse Foundation Space

Clear the future greenhouse foundation space of all weeds, old ground cover, etc. If there is loose top soil, rake it as level as possible. Remove any large rocks or other obstructions.

Next, stake the corners of the future greenhouse foundation. Measure from several angles, including from corner-to-corner diagonally across the middle to ensure that everything is perfectly even and square. If needed, run rope or string between the corner stakes to create straight lines to follow.

Helpful tip:

When deciding on the foundation size, take into account both the size of the greenhouse frame, the width of your blocks, and where the frame will sit on top of the blocks. Do you want the frame to sit exactly in the middle of the blocks, or perhaps slightly different?

For example, we chose to build our concrete block greenhouse foundation in a manner that would make the greenhouse frame sit on the inner one-third of our wide concrete blocks. This way, the majority of the block “bulk” protrudes outside the greenhouse instead of inside. The greenhouse frame still had plenty of space to rest securely on the foundation, but the blocks don’t take up too much room inside, which would have otherwise obstructed space for tables and shelving. 

Step 3: Create a Base Trench

Because our native soil is very soft and sandy, we chose to create a trench to fill with compacted road base to install the blocks on top of. Using an edging shovel, dig a shallow trench (about 6” deep) around the marked space where the blocks will eventually go. Make it a couple inches wider than the chosen blocks themselves. This gives you some wiggle room for error and adjustments.

Again, keep everything as level as possible. The ground in our greenhouse area was already fairly level. Plus, the road base added to the trench later can also be worked to level. 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.

Step 4: Line & Fill the Foundation Base Trench

If you plan to add weed-blocking landscape fabric, this is a great time to do it. Lining the trench below the road base and into the interior greenhouse area creates one seamless and effective weed barrier. Weeds inside a greenhouse are not only a nuisance, but also can introduce pests or disease.

Next, fill the trench with road base material. We added about 4 to 5 inches of road base, and then compacted it well with a soil hand tamper. Check for level. Add more base and/or tamp certain areas as needed to get it as level as possible. 

Step 5: Build the Concrete Block Greenhouse Foundation

Before you start to lay the blocks, double-check your measurements and reposition the flag stakes if needed. I also found it helpful to put the string back between the stakes to provide a straight line to follow as we laid the blocks. Furthermore, plan to clean off excess dirt from the concrete blocks. The glue adhesives are most effective on “clean”, dry, dust-free surfaces. We used a small hand broom to brush off the blocks as we worked

Next, begin laying the blocks, checking for level as you go. We installed the first layer all the way around the perimeter before going upward with more rows. Apply the concrete adhesive between all touching sides with the assistance of a caulking gun. Follow the instructions on your chosen adhesive in regards to the recommended glue bead thickness or amount.

Note that block walls are most sturdy if the blocks are offset and staggered. Given the size and shapes of our blocks, we were not able to stagger ours for this project. However, with glue between the blocks on all sides, they are extremely secure and completely immobile still – 4 years later. 

Pro tips: We went through WAY more glue than we anticipated and had to go back to the hardware store for more, twice. Also, if you are going to add pea gravel or other flooring material to the interior of the greenhouse, it is easiest to do this before you set the greenhouse on top of the foundation. Allow the foundation to dry for 24-48 hours, and then add the gravel or other flooring material before proceeding.

Step 6: Secure the Greenhouse to the Foundation

Most all greenhouse kits should have holes along the bottom perimeter of the frame as a way to secure the greenhouse. If yours does not for some reason, you may be able to add holes to the frame using a drill and appropriate drill bit for the material (e.g. for an aluminum frame).

Once the adhesive has fully dried (usually 24 to 48 hours), set the assembled greenhouse on top of the foundation. Get it centered and positioned the way you want it permanently installed. Go around the perimeter and assess the holes provided in your greenhouse frame. We marked through the holes onto the concrete block with a sharpie pen.

Before grabbing the power drill, it is best to pick out the concrete screws best suited for the project first. Choose ones with a wide enough head to catch and hold the holes in the greenhouse frame. Then get a concrete/stone drill bit that is suggested for that size concrete screw, usually a tad smaller. Drill pilot holes in the concrete block greenhouse foundation, in line with the greenhouse frame holes. Finally, add the screws themselves, securing the greenhouse in place. 

Ta da!

Congratulations. You have just constructed a very durable and attractive concrete block greenhouse foundation. I hope you found this tutorial to be helpful. Please feel free to ask questions, and share this article! Also be sure to check out our Beginner’s Guide to Using a Hobby Greenhouse for important tips about ventilation, heating, cooling, and more. You may also be interested in our Seed Starting 101 tutorial. Thanks for tuning in, and happy growing!