The first thing that the employer should know is that, any worker have to perform the job 2 meter or more from ground level. Because it is going to be counted as a Working At High by law. Next thing is that any possible hazards due to falling, dropping objects, or collapsing of the structure, soft soil, etc. have been assessed. The other part is having a safety plan and have a written record to proof that you have trained the workers. Just one Item is enough to put many of employer in serious legal issue.