I have noticed that the ceiling plaster in my front bedroom seems to be crumbling off and the ceiling around where it is coming off feels spongy. My house is a mid terrace over 100 years old I guess. What could be causing this and is it something to worry about? I hope it isn't too expensive to fix. It seems to be the ceiling towards the front of the bedroom towards the window. Certain parts along that side feel spongy and the plaster is falling off.
If the ceiling feels spongy anywhere at all, some sort of deterioration has occurred. The most common cause is moisture seepage—whether from leaky plumbing, roofing, or pets—but plaster deterioration can also occur from sustained high temperature (120 °F/49 °C) as is common above an enclosed light fixture or electrical junction box carrying a heavy electrical load.
Decay could also be caused by certain pests like cockroaches, fleas, or larger critter's waste.
The first step to fixing it is to stop whatever is causing the problem. Then assess what needs to be done.
Wall and ceiling work is somewhat expensive if you hire someone to do it, but it is quite inexpensive if you do it yourself. Even if you don't know what you are doing and have to (re)do it four times, that will still be less expensive than hiring a pro. For a head start, visit your local Home Depot the next time they have a sheetrock demo. Or search for "sheetrock repair" on Youtube.
If investigation or repair require tearing down the ceiling, the price can escalate if you see something else you want to do: rewire, insulate, reroute plumbing, redo siding, etc. Frankly, the add-ons are the most difficult (financially and emotionally) aspects of such projects.