A damp patch under the roof need not be a leak
Condensation leaves the same brown ring as rainwater. Telling them apart decides whether you need a roofer or have a building-physics problem.
A brown ring on the sloping ceiling reads as one diagnosis: the roof leaks. Sometimes that is right. But there is a second route by which water reaches the same construction, and it comes from the inside.
Warm, humid indoor air meeting a cold roof surface condenses there. Dutch consumer authority Milieu Centraal warns that insulating a roof incorrectly carries a high risk of moisture in your roof and rotting wood or mould — so that is not rainwater coming in, but indoor air cooling in the wrong place. Often something is then wrong with the vapour-retarding layer on the inside, or it is missing.
One frequently heard piece of advice actually makes it worse. Milieu Centraal specifically advises against a ventilation space between the insulation layers, because it removes too little moisture and can even introduce extra moisture. So having a cavity made "to let it breathe" does not solve it.
Two clues help tell them apart. Rainwater follows the weather: the patch grows during and just after a shower, and is wet seasonally. Condensation follows heating behaviour: it appears in the cold months, more strongly in bedrooms and bathrooms, and often across a larger area rather than at one point. If in doubt, have it professionally traced (€ 90-250) — the cheapest step, because a roof repair (€ 175-600) on a roof that is not leaking solves nothing. Every guide rate is in the price index.