The best hydroponic system for a rooftop greenhouse would depend on the size and layout of the greenhouse, as well as the climate and the types of plants you plan to grow. A popular option for rooftop greenhouses is a deep water culture system, which consists of a large container filled with nutrient-rich water and floating rafts for the plants to sit on.
This system is simple to set up and manage, and is ideal for growing a variety of plants. Other hydroponic options include ebb and flow systems, aeroponic systems, and wick systems.
