I have concrete tiles on my bungalow,they are full of moss,but I ve been told not to jet wash off because it makes the tiles porous and therefore causes leaks.
Bit of an update. The guy came round shortly after I posted this and he said a tile was needing moved above the bit that stained the ceiling. To be fair, it's rained pretty heavily today and a good few times since and it's been fine in the other areas as well.
