I'm not sure if I've quite got your plan but is there a soil pipe running down that small bit of exterior wall?
I (well fortunately downstairs mainly) had an issue where the soil pipe had corroded and flushing it would splash water on the wall and eventually caused damp on the inside.
Edit: just seen those pictures and looks like there is.
I'm not sure if I've quite got your plan but is there a soil pipe running down that small bit of exterior wall?
I (well fortunately downstairs mainly) had an issue where the soil pipe had corroded and flushing it would splash water on the wall and eventually caused damp on the inside.
Edit: just seen those pictures and looks like there is.