I’m looking at doing raised flower beds around my garden, it will be 24 meters in length, I have been pricing different woods such as treated scaffolding boards and sleepers, I’m going to do them 18 inch from the ground so will be stacking them on top of each other. Even using even scaffolding boards will cost about £300. I have looked into doing it with fence boards and it will only cost me £120, I’m not putting soil all the way to the bottom just roughly 8 inches deep. Will fence boards be strong enough to do this or will I need to buy thicker wood like scaffolding boards? Thanks