What brick cleaner should be used for the black stain on external brick wall? All manner of questions and solutions to problems and issues surrounding the subject of brickwork, blockwork and stonework

[rocksoccer]

The black stain as in the attachments on an external brick wall seems to get darker and darker over the past few years. Not exactly sure what it is. Is it kind of mould? But I never see any external walls have mould. There are a lot of trees around the house, and some moss grows in the joint between the concrete slabs at the top of the wall. This wall faces north east, but there is a giant tree in neighbour's garden which I think blocks a lot of sunlight in the morning.

Is brick cleaner the right thing I should use to remove the black stain?

I checked some brick cleaners, but really struggled to decide which one is suitable. I asked in store and the staff did not have any ideas about the differences.
https://www.wickes.co.uk/Sika-Brick-and ... L/p/133793
https://www.wickes.co.uk/Bond-It-Brick+ ... L/p/253125
https://www.diy.com/departments/bostik- ... 078_BQ.prd
https://www.diy.com/departments/bostik- ... 681_BQ.prd

Any comments are appreciated. Thanks in advance.

[stoneyboy]

Hi rocksoccer,
You have rain water from either a defective pitched roof verge or from the flat roof drainage, causing this problem.
All of the brick cleaners you list are based on hydrochloride acid and you would need to apply this over the whole stained area. Due to the height involved this is quite a dangerous operation.
Even once you have applied numerous coats the stain may remain - you have an organic growth (the black mould) and a mineral stain (the grey coating). There is no easy one action solution and you may have to try different chemicals, even then you will still be left with a stained wall.
Regards S