Hi @storiesoferne. That's right roof leaks are the result of doing nothing over time and then come the headaches. Among the challenging aspects are:
a) Ensure the quality of the substrate, this means that the slope is directed to the most convenient side and in a sufficient proportion, also that it is clean for subsequent work, this is easy to say but there are workers who do not take care of these details.
b) On the other hand, that the workmanship is qualified. Even the construction companies have workers who want to execute the works as they see fit because they have always done it that way, it is uncomfortable to deal with errors in the criteria of others; for example, not respecting the minimum overlapping areas.
c) The last aspect is the budget, having enough money to achieve an excellent waterproofing system. When the resources are not enough, one starts to look for other alternatives that are not bad but are not the best.