If you want your terracotta roof cleaned without the risk or stress of flooding your roof space the only method is to use a service provider that will clean your roof using a soft wash process. This is ideal for slate and terracotta roof tiles because they can be brittle. In some cases we don't even need to walk on your roof to do the work. Lightweight carbon fibre poles can be used to deliver the product on to your roof. The spray will get in between the roof tiles killing off the moss and lichen but also killing off the spores that live between the roof tiles. This is the area that the high pressure spray can't get to. If your tiles are strong you might be able to remove some of the lichens etc with a less than maximum high pressure spray. But bear in mind that this process uses thousands of litres of water whereas a soft wash process might only use 200 litres.
When the product is first applied the organic growth is killed off in the first 48hrs, then the dead material is washed off over the next few months with the rain and wind. This will eventually get your roof looking like new but you will have to be patient as it can take a bit of time.

Roof repair Melbourne by Fix ya Roof