This type of dampness is found on external walls. In the case of roof leaks, it will be on ceilings.
It only appears because of a defect outside letting water pass from the outside into the home. For example, a missing tile or slate, spilling from a blocked gutter, badly fitted windows or missing pointing.
This type of dampness gets worse after rain and will usually appear as a well-defined ‘damp patch’ which looks and feels damp to the touch. Once the cause has been fixed, the damp patch should fade and dry out over time.
Black mould is rarely seen on areas of penetrating dampness. This is because the affected area is usually too wet and the dampness contains salts picked up when passing through the wall which stops its growth.