While many urban authorities in Europe are confronted both with increasing demands by urban dwellers for allotment gardens, vacant urban soil tends to be scarce and/or polluted by past industrial activities. A possible solution for local authorities could therefore be to promote rooftop gardening. Little technical information exists however on certain forms of rooftop urban agriculture, called Z-Farming. In 2012, a pilot experiment was run in Paris (France). Simple and cheap systems of rooftop gardening were tested on a rooftop, using as crop substrates only local urban organic waste so as to contribute to the urban metabolism. Production levels, physical and chemical evolution of substrates and heavy metal contents in cropping substrates and edible vegetables were measured. Available results show (i) high levels of crop production compared to the reduced use of inputs, and (ii) very low levels of heavy metal pollutants in the edible parts of the crops. These encouraging results allow us to consider that rooftop gardening is possible. It will nevertheless be necessary to identify more precisely the types of roof that can be used, and to assess more fully the generic result of the low level of pollution, as well as the global sustainability of these cropping systems.