There’s growing interest in living roofs or green roofs. The difficulty is deciding on the waterproof membrane. Rubber pond liner (EPDM) is the most waterproof and durable material, and the most common choice due to proven performance, but it’s also very expensive and made from nasty petro chemicals.
I think a number of less toxic, more affordable materials are possible, including recycled materials in good condition. One reader suggested pool covers. Heavy duty trucker tarps are another option. You could add 6 mil poly (plastic sheeting) underneath and/or above these other materials for extra moisture protection.
For my dome, I used 2-1/2 layers of 6 mil black poly and have had no leaks so far after about three years in a rainy climate. (The half layer is a small piece on the very top.) You need to cover 6 mil poly carefully to avoid punctures. Some use old carpet or cardboard against it while adding soil. (Screen out rocks.) In our case, we simply packed soil on the plastic, starting at the bottom and working up, with no protective cardboard, etc.
Recycled vinyl billboards are another possibility, but I wouldn’t use them because of the health hazards of leaching chemicals into the soil around the house.
Roof pitch is another consideration. Steep roofs shed water faster and are less prone to leaks. But you need to strike the right balance or top soil and nutrients will wash away, and your roof will dry out too quickly.
Another consideration is the value of the structure. It’s no big deal experimenting with alternative waterproof membranes on a simple $2,000 guesthouse. But you might want to use better materials or multiple layers on expensive homes.
Photo credit: Heartwood Homesteads