8 requires building up the area to square off the greenhouse which will require more concrete or footing showing on parts of the foundation. In many cases, the concrete wall or wood foundation acts as a retaining wall depending on how steep the slope is. If it is a substantial slope, most people will drop their foundation with steps in their stem wall. The good news is that you do not need a perfect spot with our system to make it work as you can see above. This is a custom Traditional 20 x 24 Double Glass greenhouse with an offset ridge beam, extra sidewall height and two jogs in the foundation. Anything is possible = happy customer! construction tip Think about the aluminum sill as a trough to allow water to escape to the outside of the greenhouse.