How old is the roof? Methinks there IS water damage, and wishing it away won't work.

OTOH, the "water" damage maybe from steam, and from the INSIDE, i.e. do you have any bathroom fans that vent into the attic space, instead of out through a wall or roof? That is actually allowed in certain jurisdictions, and gets things soggy in a slow sneaky way.

I saw the underside of the plywood and there was no discoloration or any other evidence of water damage. Also, there have never been any leaks. It is possible that the top side was wet when they put the roofing materials on and, over time, the wood got soft and sagged. The roof was put on in late 1997 so it isn't very old at all. Also, there don't seem to be any sags or dips on the east facing side of the roof or, if there are, they are much smaller and barely noticable.

Thanks tak2w. That is exactly what one guy I had in suggested. How many areas did you have jacked up and what did it cost you? What ares do you live in? Upstate NY by any chance?

fwiw
I was just stumbling around the florida solar energy center and one item discussed was not enough or proper attic ventilation and how it causes wavy roofs - from the increased humidity.

Do you have OSB board or plywood? I am willing to bet that when they built your roof they didnt use plywood clips. Plywood clips go in between the trusses to keep this from happening.

Right, my thinking keeps coming back to humidity issues too, from whatever source. That will weaken plywood, or most woods, without noticeable discoloration, and even without permanent damage. After all, isn't STEAM-EXPOSURE the preferred method for bending wood on purpose?

More details please, i.e. roofing/shingle materials, SPACING between trusses, prevailing wind direction, gable and soffet vents, etc. Unless you've got heavy SLATE shingles, I don't think plywood will sag under normal conditions. In So-Fla. we've got heavy concrete tiles on 5/8" ply (post-Andrew) w/ trusses on 24" centers, with no sag, and of course many roofs still have only 1/2" ply from the pre-Andrew codes. They don't sag unless water/moisture gets to them. (I've got 5/8" tongue-in-groove, VERY stiff, knock-knock ;')

PS: Of course the above bracing methods will correct the sagging, but you still need to identify and CORRECT the underlying CAUSE of the sagging. I doubt the cause is under-spec plywood. Your moisture problem may be seasonal, and not apparent at the present time, e.g. involve freeze-thaw.

In my first house, the whole neighborhood had 3/8" OSB roofs with trusses 24" on center. Eventhough it was almost like a trampoline we didn't get much sag, but some. The house was built in the early 70's and was in OH. My neighbor ended up removing and replacing everything with 1/2" OSB or plywood. I didn't have any problems selling it.

7/16 OSB properly installed (staggered joints and sufficient number of nails) with the H-clips shouldn't sag for a sloped (4:12 or better) roof with a 24" span between trusses. Lots of high end homes are built like this today. If you don't have the H-clips or you have moisture problems, you can get sagging.

If moisture is the problem, you have to correct that at the source. If its the lack of H-clips, just cut a bunch of blocking to block the seams. If the sagging isn't too bad, you should be able to push it out with the blocking as you nail it (preferably with a pneumatic nailer - otherwise, you'll need to grow a third hand which is not easy to do unless you live in the shadow of TMI). No jacking should be necessary.

I am coming backt o this issue as I am hoping to get it worked on soon.

I have plywood.
The plywood shows no discoloration or other signs of moisture.
I have soffet vents and the vent on the roof peak that runs the length of the roof (name escapes me).
I think the ventilation is OK. Nothing appears to be blocked by insulation, for example.
I don't know the truss spacing.
The wind direction is north-south and the front of the roof faces west and the back faces east.
The sagging only appears to be near the peak of the west side.
Venting from bathrooms are carried to the soffets.
I'm not sure about the clips. What exactly do they look like and what is their pupose?

"Venting from bathrooms are carried to the soffets. "

Where the moisture is pulled right back into the attic through the soffit vents.

I realize this is an old thread, but I am having similar issue however it is BRAND NEW CONSTRUCTION. No interior humidity, and we started noticing and complaining within the first week of the roof being complete. Construction continues, but we can't come to an agreement with our contractor as to his level of responsibility in the matter.