Soft washing roof

Soft washed a roof 2 mths ago using 4% hypo. Roof is cleaning up well except for 4 ft × 2 ft patch of white heavy crust substance? How do I treat and remove that. The customer is a bit anxious about it. I suggested leaving it for another while before retreating it?

CkWindowCleaning said:

Soft washed a roof 2 mths ago using 4% hypo. Roof is cleaning up well except for 4 ft × 2 ft patch of white heavy crust substance? How do I treat and remove that. The customer is a bit anxious about it. I suggested leaving it for another while before retreating it?

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Any pics? 

White heavy crust ?.. what is it ? Is it licken?.  Depending what it is I would do  a further application on that area 

Might be salt deposits from where the hypo has broken down.  Did you rinse well enough?

Pjj. I think its lichen. It was on the roof before I started. Its like white paint viewed from the ground. Thanks to all who commented with advice

Forgot to mention. The ' lichen ' is around the chimney

Some lichens take longer to die and come off a roof than others I find orange ones are the worse , wet it with normal water first this will encourage the licken to open up and absorb the water , then give it a 4-1 mix of hypo leave fir a few muinits and re apply do this several times then leave for up to 40 muinits then rinse , this should kill it and it will come off the roof over a period of weeks or months , an alternative method is to spray that area with a biocide mix something like algo clear pro or similar and leave it , again over the next few week / months it will gradually come off the roof