I am planning to install a 13 x 20 greenhouse on top of red clay brick patio shown in picture. One option is to place the greenhouse directly on top of the patio which is a valid solution. But if I am planning to raise the greenhouse on a brick wall as shown in the picture. (The wall is only going to be 16 inches high in my case). My question is : Can we build the brick wall directly on top of the patio or do we need a concrete footer as foundation for such a short wall? The patio has a 4" base rock fully compacted on top of typical North California clay which seemed extremely hard and never been touched. The green house is Janssens Royal victorian.. 13 x 20' , 2000 lbs total weight, glass panels and aluminum frame.

I left a pot made of some kind of cement for some years on my terrace. I tried to remove the trace with a metal brush rubbing like crazy, but to no avail (see photo). Any idea of a chemical solution to this? Thanks