Before I had a single such roll, and a full one, 20 kg or so. I mounted it with one end stuck under the saddle, tied down firmly, roll straight backwards, protruding like a meter behind the bike. All went reasonable, until I had to ride over a bridge. Halfway the balancing weight pulled the front of the bike up in the air, me included. I quickly stood on the pedals, bending forwards as much I could, and the front wheel went back down.
It was a scary experience though. Since I started to mount such long heavy things diagonally as on the picture.