According to the Work at Height Regulations 2005, the duty holder must ensure that no one working under their control goes onto or near a fragile surface unless that is the only reasonably practical way for the worker to carry out the work safely.
For example, cold water storage tanks and other items of plant are often located in loft spaces and in order to access the area for servicing, inspection and maintenance there needs to be a safe route across any fragile or un-boarded surfaces.
If anyone does work on or near a fragile surface, it is vital to ensure that suitable platforms, coverings, guard rails or similar are provided and used to minimise potential risks.