First, I had to pick a site that I could get a lot of winter sun. Luckily, I have a spot near my garden that is perfect. The only problem is that I had a cross slope of about 2 feet - Nobody likes a crooked greenhouse, so I did a little digging with my tractor and leveled out a spot.
That's one day gone - anyone who lives in New England knows that there are no such things as small rocks!
Unlike some folks, I made the decision to bury the steel foundation instead of building it on top of pressure treated wood. I wanted to be able to enter the greenhouse without kicking the base plate and to be able to push a garden cart through the door. We'll find out in 20 years if the steel rotted out.
Since I wanted to try to keep as much heat as possible during the winter, I bought some 1" foundation foam and laid it directly on the dirt, then built the foundation on top of it. I then drove rebar into the ground at each corner and attached the foundation to it. I'd hate to have my greenhouse turn into a giant kite during a storm.
This past year I put in a new sidewalk and patio and had a bunch of gravel left over...perfect fill for the inside of the foundation. I filled the inside with gravel and back-filled the outside at the same time to prevent the foundation from warping or shifting. One thing I noticed when filling the foundation was that the gravel packed into the "c" shape of the foundation and helped to weigh it down better. I'm now confident no one will see this greenhouse go flying through their yard.
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