Flashings are the junctions between the roof slopes and adjoining abutments and they are generally formed from lead.
The picture was taken on a recent survey and shows the sort of things we surveyors are on the look out for when carrying out a survey.
In the case of this London property, the flashings were mostly satisfactory and seem to be watertight apart from a slightly lifted section on the parapet wall.
This will need to be fixed or the already resultant issue with damp will worsen.
Periodically they will require re-dressing tight to the roof to ensure a permanent and watertight joint.
Flashings must be properly wedged and pointed in order to ensure that they are fully secured into the masonry above the roof covering.
They should be bedded at least 25mm into the appropriate mortar joint and lead wedges should generally be provided at approximately 450mm centres in order to ensure that the flashings are properly secured.