A green or black-streaked wall on a shaded elevation is not just cosmetic dirt, it is active biological growth with roots and spores that a simple wash can spread rather than remove. Coating straight over it, even after a wash, usually means the growth reappears through the new paint within a season.
Not reliably on its own. A coating with a fungicidal additive helps resist regrowth, but proper biocide treatment of the existing growth before coating is what actually deals with the underlying problem.
Shade and reduced airflow keep that side of the building damper for longer, which is exactly what moss and algae need to establish and spread. It is a genuine difference in exposure, not just appearance.
It depends on the product and the weather, but we allow enough time for the biocide to work and the wall to dry properly before coating starts, rather than compressing both into the same short visit.
A wash removes the visible growth from the surface but can leave spores and root structures intact in the pores of render or masonry, ready to regrow once conditions are damp and shaded again, which many north-facing and tree-shaded walls always will be.
Once a wall is treated and clean, specifying a masonry system with a built-in fungicidal or algaecidal additive gives ongoing resistance to regrowth, which matters far more on shaded elevations than on a sunny, well-ventilated south wall.
A wall shaded by trees, close to damp ground, or facing away from prevailing wind and sun will always be more prone to regrowth than an exposed, sunlit elevation, whatever coating is used. We would flag this honestly rather than promising a permanent fix on a genuinely high-risk wall.
Biocide treatment needs to be finished and the wall properly dried before coating starts, and we build that lead time into the programme rather than trying to treat and coat within the same short visit.
Ask us to assess which elevations need biocide treatment before your exterior repaint.
Moss and algae need treating at the root before coating, not just washing off the surface, or they simply grow back through the paint.
Speak to a Hampshire commercial decorating contractor that turns up, communicates clearly and delivers a finish that lasts.
