Are you talking from experience?

The roof truss are the main beams. These you don't touch. The planks I mentioned above are those 1x2 inches planks (that are for supporting the roof tiles).

When the tank leaks or overflow, the ceiling may be water stained. The ceiling are thick boards, attached to the roof truss by means of nails and ceiling beams. They can be repainted.

Ceiling beams are those thin pieces of long wood attached to the ceiling/roof truss with nails. Have to remove them first if changing the ceiling boards. This is a more major job, as the thin ceiling beams would be damaged.

And surprise, surprise... There would be a thick layer of dust on the ceiling boards. You would regret removing them.

Anyway, the job is for contractors, not diy. The steps listed are just for reference and as guide what the job entails so that you have a rough idea how much to pay the contractor.

Need to find the source first… whole roof kena?

basically, roof trusses like this, you cannot detach.
papan ceiling however are not load bearing or rather they only bear the load of the ceiling.

Ceiling boards and associated boards can be repainted, but if you wanna stick a new tank inside, might as well do everything one shot.
take out the boards to put tank in, then replace ceiling board

just changed my water tank earlier this year, forget about patching, if it starts to leak means time is up, unless you are very free to patch up every single time, easiest way will be replacement with 316 stainless steel tank, not the 304 and 316 more suitable for indoor.

cut open ceiling just enough for the new tank, get plumber to cut off old tank piece by piece, install new tank, re-do plaster ceiling. that's about it, too troublesome to replace it back with FRP tanks

my plumber just climb onto the roof with the new tank, dunno how he did it

How much change?

Same as mine.

plaster jer la..

Later water kena cut due to external pipe bocor or local water dept maintenance, how ?

The tank will act as storage, just like how the under roof tank does.

ask your plumber , varies on sizes for the tank

Nowdays no more using fiber tanks, mostly use Polyethylene tanks, ask Bangla or Indon guy to patch with aditional pvc sheet and pvc glue, better patch in and out just to be sure it stick well.

bypass it, install new tank on ground floor+pump
or take the risk and no tank at all.

Last time why kemahiran hidup class did not teach this