How often should I moss treat my roof? Why? 

We recommend that all roofs are given a KillWell moss, mould and lichen chemical treatment every 2 years.

• Effect of moss, mould and lichen on concrete roofs
  If moss, mould and lichen are allowed to establish themselves on concrete tile roofs, the roots burrow through the paint coating, initially stripping away the paint. Once the paint has gone the tile is porous which allows the moss etc. to establish itself even more. The roots now continuously dig into the surface of the tile, loosening particles of concrete, in effect the surface is constantly being worn off and the tile becomes thinner and more porous. The additional weight of the water in each tile adds considerably more weight to the supports, which have a tendency to buckle. This movement breaks tiles and mortar on the hip and ridge caps. Some tiles can carry their own weight in water.
  
  
• Effect of moss, mould and lichen on terra cotta roofs
  If moss, mould and lichen are allowed to establish themselves on terra cotta tile roofs, the roots burrow through the hardened surface, initially stripping away this surface. Once the hardened outer surface has gone, the inner tile is considerably more porous and softer, which allows the moss etc. to establish itself even more. The roots now continuously dig into the surface of the tile, loosening particles of terra cotta, in effect the surface is constantly being worn off and the tile becomes thinner and more porous. The additional weight of the water in each tile adds considerably more weight to the supports, which have a tendency to buckle. This movement breaks tiles and mortar on the hip and ridge caps. Some tiles can carry their own weight in water.
  

Why is my roof stressing?

The problem with the roof is that the tile coating has deteriorated; leaving porous tiles, the extra weight when wet causes movement and this movement damages the mortar and tiles. Whilst we can do the repairs (i.e. re-lay displaced tiles, replace broken tiles, re-mortar hips and ridges) this will not have got rid of any of the movement, so the new mortar on the hips and ridges will simply crack and break again and new tiles will also break. The only thing that stops the movement is re-coating the tile so that the water, when it is raining, runs over the top of this new surface, instead of sinking into the tile. Some porous tiles can carry their own weight in water so for every ton of tiles there maybe a ton in water (most roofs carry several tons of tile). The water evaporating after each rain produces a very damp house; mould readily grows on the ceiling. The extra weight of the water in the tile causes the support wood to buckle, and eventually the roof permanently takes on a buckled shape.