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Reclaimed Window Greenhouse Part One: Foundation

Updated: Oct 17, 2023

I have a love of gardening that spans back to when I was a child. There is just something about the sun-warmed soil and nurturing a plant from a seedling up to harvest. The longer I garden, the more plants I start from seed. I love watching them emerge from this tiny seed with the promise of a season of growth and plenty.

Building a Reclaimed Window Greenhouse Part One: Foundation

greenhouse foundation

A number of years ago, I picked up a small plastic greenhouse that worked initially, but I have outgrown it. Now, every spring, not only my greenhouse but every south-facing window I have is bursting with trays of green life. My teen daughter has been particularly vocal about the botanical takeover of the house each year. Now that we have our root cellar near completion, we decided to build the reclaimed window greenhouse I have been dreaming about.

Selecting the location to build our greenhouse and preparing the ground for the foundation.


The First step was to choose a good location. I chose a spot near our garden where we still had electricity from a previous garden shed we had removed when we expanded the garden. Our greenhouse has the greenhouse portion to the north, but the ideal would be to have it to the south so the shed portion doesn’t shade it too much. Because of our setup at that location, it didn’t work well, so ours is a little less than ideal in that way.

Next, I planned out the greenhouse itself. I had been collecting antique windows for some time with this project in mind. So, I inventoried all my windows and pulled out the graph paper. (You could use a computer, but I am a little old-school.) Because all the windows are slightly different sizes, it can be tricky to figure out which ones go the best together. The key that I found was to make sure that the vertical supports were regularly spaced. The horizontal ones can be wherever you need them for the window, so you want to cluster similarly sized windows in columns.

For my greenhouse, I knew I wanted some storage space in the back, so I had a place to keep all my pots in the off-season. Plus, I wanted to have a location outside of the house for my loom and spinning wheel. So, I created a six-foot-deep shed at the back of the greenhouse. After planning for the shed and windows, my footprint was 10.33ft x 19.125ft. This worked out well because where we live, you do not need a building permit as long as the structure is under 200 ft. Check your local codes to see if the same rules apply to you.

After I had my location and my design, I cut out the lawn that was in that location. A sprinkler ended up right in the middle of where my building was going to be, so my husband had to move that. We marked our corners with metal posts and strung a line between them to mark the wall locations. To be sure it is square, you measure from the corner to one side 3 feet and then from the corner to the other side 4 feet. If it is square, the measurement between those two marks should be 5 feet.

Creating the footing trench for the reclaimed window greenhouse.

create footing trench

Because we were doing a trench foundation, we cut the walls of the hole to the exact measurements. We went off our highest corner and made the string 1/2 an inch higher than we wanted our stem wall to be. We then used a level to adjust the string height around the entire rectangle to match that corner. This gave us something to measure off of.

We cut down 14.5 inches from the string to allow for 4 inches of gravel, 4 inches of footing, and 6 inches of the stem wall. Once down the correct depth, we made the trench 12 inches wide for the footing.

We used a spacer board to make sure the walls of our trench were 8 inches deep on the inside allowing for the gravel layer and the footer portion.

Add gravel to the base of the footing trench.

add gravel base

Our next step was adding the 4 inches of gravel into the trench we had dug.

Prepare the rebar in the footing trench.

prepare rebar

Before we could pour the concrete, we needed to make our #3 rebar framework. We did this by attaching them to each other just on the inside of the trench and toward the outside of the trench. (Essentially, two side-by-side rectangles.) We connected them with an “L” shaped piece of rebar That will stick out into the stem wall and added another row of rebar that would be in the stem wall.

Pour the footing the the reclaimed window greenhouse.

pour footing

As we poured the footing we would lift the rebar to center it in the concrete. You don’t want the rebar exposed anywhere but where the stem wall will be or it can rust out and weaken the foundation. We then allowed it to dry for 24 hours.

Build the stem wall form.

build stem wall form

We used spacer boards to hold our stem wall form which worked pretty well for creating the correct size. We ended up doing it 5 inches wide to accommodate our rebar but 4 inches should be strong enough.

Pour the stem wall for the reclaimed window greenhouse.

greenhouse foundation – pour shed stem wall

In retrospect, we should have added the shed divider foundation at the beginning but we made it work later as well. The most important thing would be to tie it into the outside foundation with rebar. Initially, we were just going to have dirt floors but we revised our ideas as we went along. We decided a wood floor would keep it cleaner in there.

Remove the stem wall form.

remove form

Next is my favorite part, removing the concrete form. This is where you find out if your concrete work was adequate. I really recommend using a concrete vibrator when you do your pour as it gives you the smoothest finish. An alternative would be to use a small rake or even a gloved hand to agitate the wet concrete.

Use concrete anchors on your stem wall to adhere the walls.

concrete anchors

These wedge anchors make it so much easier to attach the walls to the foundation. They are a little spendy but well worth it in this case.

Add pressure treated wood to the stem walls to begin building the walls of the reclaimed window greenhouse.

add pressure-treated wood for the wall

We just cut pressure-treated wood to fit the foundation and then using a concrete drill we dropped holes about every 2 feet through the wood and into the concrete. This allowed us to sink in the anchors and tighter them up so that our walls were securely attached to our foundation.

The next step: framing. (Watch for our next post.)

For helpful videos, check out our YouTube channel:

To see how to make a root cellar, follow this link:

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