There are many different causes of damp patches in the house depending on the situation and where it is located. Damp in a bathroom for example may just be a case of lack of ventilation where a shower is generating steam. However, in a bedroom where damp is seen at the top of the wall where it meets the ceiling could be a result of a roof problem.
If there is a slipped tile or cracked tile on the roof it can be an entry point for water to get in. If this is left unrepaired it can lead to water damage inside and damp patches. If you do discover some damp the first thing to check is the roof. This can be done simply by inspecting the tiles from the ground level. If the tiles look like they may be damaged, it is best to hire a professional to inspect the roof and find out the extent of the damage. They will also be able to best advise you on the course of action to minimise any further damage and get the repair fixed.
The image below shows damp in my house that was caused by a flat roof in need of repair. The roof was sagging and water was collecting on top of it. In my case it was more economical to replace the whole roof, because the damage was already extensive and the roof was 30 years old. The Derby Roofer that inspected and replaced my roof did an excellent job and really minimised any disruptions while the work was being carried out.
It is always better to keep a roof well maintained and catch any problems before they escalate into leaks and damp. It is easier to inspect a slopped tile roof from the ground for any visual problems, like broken tiles, than it is to climb up to see a flat roof. It may be possible to see it by looking from a further distance. However, to check for rips and sagging it most likely will be necessary to climb up to get a better look.