Some old materials can be sealed over others require removal.
Waterproofing of old roof.
The roof must be dry to bond with the roof.
Waterproofing is the process of making an object or structure waterproof or water resistant so that it remains relatively unaffected by water or resisting the ingress of water under specified conditions.
On the other hand while breaking the old waterproofing layer on the ceiling the slab also gets damaged to a certain extent.
No schedule complications or disturbing the occupants and contents of the building.
The roof must be clean or the material will adhere to the dirt and not the roof.
Pay as you go repair small areas of the flat roof at a time due to molecular welding new to old.
Water resistant and waterproof often refer to penetration of water in its liquid state and possibly under.
Such items may be used in wet environments or underwater to specified depths.
To avoid such scenarios waterproofing your roof again makes total sense.
By waterproofing as you go no water damage to new roofing materials or the existing building can occur during construction.