Working at height is known as the most common cause of deaths and serious incidents in the work place. Employers and employees are both responsible for ensuring that any work at height is carried out safely.
Work at height is classed as any work undertaken where there is risk of falling to a lower level, whether this is from a roof, a ladder or a raised platform and even working on ground level with the risk of falling into any voids, excavations etc. Under the regulations employers must ensure that all work at height is planned properly, supervised and carried out by competent people with the skills, knowledge and experience to do the job.
The HSE have produced a free downloadable guide to working at height. http://www.hse.gov.uk/pubns/indg401.pdf