This is a tough one with out actually seeing it.

some ideas...

Were the joists wet/moist when the drywall was installed? Moisture might have finally made it's way through. The drywall will suck the moisture out of the wood. Im sure its not wet but it might of softend the ceiling paint a little and made it darker.

Is the home settling and put a little pressure on the ceiling were it might be bulging and you seeing shadows from it?

Looking at it from the floor gives a different perception than if you get a ladder and look at it real close. It might give you a better idea of what is going on.

Just a thought, I might be way off.

It might just need a couple coats of premium paint. Use a primer sealer on the dark areas first.

Joe

I have seen a highly detailed and technical explanation of this on another site, but I can't remember where. I believe it all revolved around different temperatures between where the joist met the drywall and the insullation and some kind of moisture difference that collected dust and dirt over years...

I remember reading that too..something to do with the cooler temps over the rafters or studs actually slowing down the molecules and the slower things move the more likely they are to "stick" and become visible over time. Someone brought up "Brownian Motion" or some complex explaination...can't quite remember.

I've seen this before and it had to do with 24" center trusses, and 1/2" drywall.

Is it 1/2" drywall?

Mike Finley said:

I have seen a highly detailed and technical explanation of this on another site, but I can't remember where. I believe it all revolved around different temperatures between where the joist met the drywall and the insullation and some kind of moisture difference that collected dust and dirt over years...

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