Clay tiles are less absorbent and natural slates not at all. The operator will begin the treatment at eaves and progress upwards with a smaller nozzle pushing the mix into laps and bonds and avoiding excessive dribbling into the gutter.
Natural slates are brushed when a particular brown filamentous growth is present in patches (generally in coastal areas). It adheres to the slate and needs softening by brush on the day.
Large slate roofs will reach high temperatures in summer and cook a heavy biofilm into a hard adhesive one. The hardened film is removed with medium/high pressure water.
The Algoclear Cleaning Process has been tested for the maintenance of thatch to the satisfaction of the Society of Master Thatchers. Removing growth from thatch is a skill best left to professional thatchers.