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All Things Garden

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 fabriclike 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.



A two way image collage, the first image shows a side yard with brush and dirt piled up with various potted plants mixed in and the end of a chicken run in the background. The second image shows the same side yard once it has been fully cleaned up. There is now only fresh dirt remaining with the end of the chicken run still in the background.
I can’t believe this is the same space it is now!


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. 


A rectangular space has been marked out with stakes in each corner and three different lines etched out in the ground in the shape of a rectangle. The chicken run is about a foot away from the outermost line.
The two outer lines show where we intend to dig the trench for the block foundation, while the inner/middle line represents the actual greenhouse base. As you can see, it is positioned on the inner portion of the foundation.


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.


Three lines are now gone after the soil has been dug down to make a trench the width of the lines that were once drawn. The center of the rectangle is now higher ground after the trench has been dug. There are two chickens observing the work from the chicken run in the background.


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. 


A two part image collage, the first image shows the rectangle and trench being covered with contractor grade landscape fabric, it is grey in color and has been attached to the ground using landscape stakes. The second image shows the foundation area after the trench has been filled with road base material and tamped down. The three or four inch deep trench will be the foundation for the block foundation to sit on.


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.


The foundation is being laid with concrete pavers. The first layer of blocks has been laid while the other pavers are sitting nearby when it is time to attach them on top of the first layer. A rubber mallet, level, and right angle measuring tool are visible.
A two way image collage, the first image shows the concrete block foundation is shown three layers high. The front of the foundation still needs to be finished since it is only one block high. This was done to make it easier to get in and out of the inside of the foundation during the construction of the foundation without having to step over the tall foundation during construction. The second image shows the foundation after the inside has been filled with pea gravel and the area outside the greenhouse foundation has been landscaped with smaller river rock gravel. The first layer of block is now covered in gravel with only two layers of brick visible.
If you look closely, you can see that we had to turn one set of blocks sideways compared to the rest (towards the back) in order to get the foundation dimension we desired. You’ll also note that we added the pea gravel flooring and exterior river rock gravel before securing the greenhouse in place.


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. 


A photo showing the same side yard area that has been worked on and renovated as part of this tutorial. It looks like a brand new, clean, pretty space. The concrete block wall foundation is in place with a small 6x8' greenhouse kit on top, with two purple and gold pots with succulents around the greenhouse front door, and clean river rock covering all of the exposed ground. Green vines are one side of the greenhouse, and a house with string lights on the other.


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!


DeannaCat signature, keep on growing

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