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Biological staining

I am concerned about a green growth on walls. How can I remove it and prevent it occurring again?

Answer

This is biological staining and the deposits are coloured, often green to brown/black, and result from the growth of algae, fungi, lichens, mosses etc. They only form when the concrete/rendering/masonry is wet for significant periods and is frequently the result of blocked downpipes or leaking gutters. Growths of lichens may obscure mouldings etc and trap water, which may cause damage as a consequence of freeze-thaw cycles. Acidic metabolites may also erode substrates such as limestone and asbestos-cement.

First check for rainwater leakage from pipes or gutters. Thick growth should be partly removed or torn with a stiff brush and a surface biocide brushed well into the surface. Application should preferably be during a dry spell so that they are not washed out before they have time to act. However if the lichen is too dry, lightly moistening the growths with water will allow re-hydration of the tissues and assist uptake of the biocide.

Acknowledgement: The Concrete Society

Other references:BRE Digest 441 : Part 2 - Clay bricks and clay brick masonry
BRE Digest 370 : Control of lichens, moulds and similar growths

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