I've got garden beds that are elevated off the ground which I filled with 1/3 compost, 1/3 coir, 1/6 vermiculite and 1/6 perlite. It's definitely much lighter than topsoil (maybe only 0.5 T/m^3 when dry), but as it's so much lighter, plants can't root themselves in as firmly, so if installing on a roof, you'd want to be wary of tall plants being torn right out by strong winds. |