crumbly drywall on the ceiling

I was just patching a hole in the ceiling where a fixture used to be. The drywall is very weak. For example I put some furring behind the hole, mounted to the drywall. When I began to attach the patch to the furring, the drywall screws holding the furring pulled right through the drywall. The pressure I applied to drive the screw was enough to easily pull the furring loose.
I'm wondering what this means. Does old drywall become weak and crumbly? Is this water damage? Inferior product? Is the ceiling going to drop on me if I don't replace it?
If it is water damage, is there any good way to find the extent of it? I mean, I hate to poke holes all over the ceiling, but how else can I tell?
Btw, the roof was replaced a couple years before I bought the house. I don't think it's leaking now, but it may have been a few years ago.
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On 10 Mar 2006 10:41:59 -0800, snipped-for-privacy@thecraftstudio.com wrote:

Can you poke your finger through the drywall without too much effort? If yes, It's probably water damage. If there's no sign of dampness then it doesn't matter what the extent is. Just cut the drywall back to the nearest joists, and screw the patch directly to them.
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Goedjn wrote:

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Like Goedjn said, but it's probably better not to disturb the existing drywall screws/nails. I'd cut back flush with the sides of the nearest joists, then attach 1x wood blocking to the sides of the joists and attach the patch. It's far easier to cut the old drywall out that way and you're not trying to run your knife/saw/rotozip over the fasteners.
R
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could be water damage, but if it's behind a light fitting it could be heat damage. ive seen what you descibe and can only guess its heat damage of some sort.

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