If the foundation is not level the greenhouse won’t sit correctly and there will be gaps In the glazing especially if it is any kind of glass.
Simple suggestions would be to shim the foundation or pour more concrete to level it out. We can't emphasize enough how important it is for the greenhouse to be level and square before starting the install.
We frequently receive calls that the door or the glass around the door will not fit. This is because the foundation is not level. We typically ask that the customer measure the existing foundation and we often find that it is off by up to 3/4" at times.
The foundation MUST be level and square.
There is a tolerance of 1/8" accounted for around the door so any more than that and the door or glass will not fit.
