But there are some safeguards that must be taken to assure that the garden is safe and won't jeopardize the structure of your house or roof. Calculate the load your roof can bear. Discuss the weight-bearing potential and drainage of your roof with a structural engineer. An engineer will determine how much weight a roof can tolerate, where the heavy plants should be placed and ways to drain moisture. Without re-enforcement, an average roof can withstand about 16 pounds of weight per square foot, and no more than 1,000 pounds over the entire roof, says Berkeley architect and engineer Paul Endres.
Plants can be quite heavy.
A five-gallon decorative pot holding a plant with saturated, compacted soil, may weigh as much as 150 pounds. The most important thing is to protect the drainage of the roof and not to compromise the watertight environment. Wood decking, outdoor carpeting and tiles are good materials for roof gardens. Before putting down flooring, the roof should be tested for watertightness, Lawson cautions, and it should stay easily accessible for servicing. Workers installing new floor surfaces should take care not to damage the waterproof membrane.