We are planning to build a pergola approximately 4x4 m without a roof on a rooftop terrace on the fourth floor and are unable to drill. Our idea is to anchor the posts in large metal boxes about 1700x60 and 40 high, which will also function as planters. We are wondering how thick the metal should be in these boxes. We have also considered using oil drums that we would halve and join together in threes. We would appreciate any opinions on this, thank you!
