Doff steam cleaning is probably the safest way to clean any substrate without causing any damage to the surface being cleaned. The steam/superheated water will remove moss, algae, fungi and other biological matter and will also kill off spores. This means there’s then no need to use a chemical biocide during the removal process or as a protection against the further biological activity.
The benefits of using superheated steam on your buildings cleaning project are the fact that no harsh pressure or chemicals are used to clean the substrate and therefore no damage can occur. The Doff steam cleaner is the only machine English Heritage approve for use on its buildings and for good reason.