Advice on replacing a discoloured wall

John Conlin / 24 May 2017

An outside wall is becoming discoloured – can it be fixed without using a builder?


I had the outside of my house clad with roughcast about 8 years ago. One wall is becoming discoloured with what seems to be moss. A builder says water may be going down the back of the roughcast and it will cost £700 to redo it.  Could you advise?


The fact that only one wall is affected may be significant. All forms of cement rendering (inc. roughcast) will absorb moisture unless they are coated with waterproof paint. If the discoloured wall faces away from the sun or prevailing wind it may not dry out as well as the others and the constant low level dampness will encourage the growth and spread of algae. Pressure wash the wall using an algaecide additive to stop re-growth then, during a dry spell, coat the wall with a colourless silicone water repellent such as “Thompson’s WaterSeal”.

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