The roof is inert. The roof is a pitched surface left to its own devices for long enough and it becomes a slow changing rock pool with all the flora and fauna that the wind and rain can bring to it.
At first there is nothing to see, just some airborne algae which settle on the surface of the roof tiles as a faint greenish discoloration often mistaken for weathering. The algae retain moisture on the surface of the tiles and this retained moisture aids the establishment of moss spores that are found everywhere. The moisture retaining layer of algae is thickened by the growth of the moss which also penetrates under the tile laps and into the pointing between the tiles. As the freeze-thaw cycles continue to occur the moisture retaining layer of algae and moss is further softened by the tiles and the pointing.
Lichen – This is one of the hardest types of growth to remove from masonry. Unlike moss, lichen does not simply sit on the surface of masonry, rather it digests the masonry from within, using strong acids to break down the materials of which it is composed. The same process that is used by lichens chemically bond to masonry to dissolve and break down organic matter, is used by lichen to slowly etch its way into the very fabric of masonry. Over long periods of time this can cause serious damage, slowly pitting the surface of slate tiles and eroding the glazing of roof tiles. These processes can be prevented by a competent surveyor prior to the installation of any Solar Panel Installation Clevedon //redbridgeandsons.co.uk/solar-pv-panels/solar-panel-installation-clevedon/.
Birds then add a dash of nutrients to the surface of the roof in the form of guano. This is particularly a problem for a roof covered in tiles as it acts as a fertilizer to the algae and moss that are trying to colonize the surface of the tiles. In addition to the nutrients in the guano, roosting birds can also be a real problem as they can loosen and even pull tiles off the roof as they climb up and down the surface. Seeds in the droppings of birds can also penetrate the gaps in the pointing and grow into larger plants such as trees and shrubs.
On a coastal roof, the problem is further complicated by the salt spray from the Bristol Channel. Chlorides from sea salt can penetrate the coating of exterior metalwork on a roof and, when combined with moisture, can cause the metal to corrode at a rate greater than on inland sites. The deposits left by the salt can also create surfaces that encourage the growth of particular species of algae that are not typically found inland.