Hi @yh18,
That is definitely a watermark, which, considering it is growing, indicates that there is an active leak somewhere above that point.
If there are no pipes above that point, then plumbing is unlikely to be the cause, which means the issue is likely related to the roof in some way.
Roof leaks do not always come from broken tiles. They often come from cracked mortar, ridge capping, valley flashings, nail penetrations, or flashing around vents. Water can travel along rafters or battens before soaking the plaster, so the stain may appear far from where rain actually enters, making roof leaks difficult to locate from above.
The best thing to do is check the ceiling cavity directly above the stain, ensuring you are safe when doing so by switching off the power at the switchboard, wearing protective clothing and a mask, and only stepping on solid framing. While in the ceiling cavity, you want to look for damp timber, dark marks, mould, or wet insulation as these are indications of moisture. If you can find these things, you may be able to follow them back to their origin to get an idea of where the entry point is. You can then check the tiles, ridge capping, roof penetrations and flashing nearby that location.
Another common cause is condensation in the roof space. Warm, moist air from the house can condense on the underside of the roof or insulation and drip onto the ceiling over time. This is common in bedrooms, especially if roof ventilation is poor or insulation is compressed or missing in spots. If you can't identify a leak in the roof, then this could be the cause, in which case you may need to look into how the ceiling cavity and roof are insulated, as well as what kind of ventilation you have.
If the stain is still growing, the moisture source is still present and repainting alone will not fix it, so it is essential that the cause of the leak is found and dealt with. If you can't locate the source of the leak, a leak detection specialist should be able to help.
Let me know what you think, and if you have further questions, please don't hesitate to ask.
Jacob
