You have to think and take care of three things before starting a terrace garden. 1) Weight of terrace garden 2) Drainage system 3) Wind flow.                You must have a strong roof to take a load of plants, soil, pots, etc. Also, you must have a proper drainage system to drain water.  Then it will be safe to make a terrace garden. Avoid making a terrace garden if your roof having exposure to heavy wind flow.