Well, he can tell me where the webbing is located and I can tell him if it's correct. I've seen all kinds of homade trusses and unless they bought one to copy or had a blue print made I'd assume it's not correct.

Atleast it's a starting point as to why only one side is sagging. If the webbing is different on the sagging side vs the non sagging side etc.

Will do more checking and measuring. Thanks for help and advise wish there were contractors in this area that needed work. Crazy all the building here no way you can get any help.

Can you post pics?
With pics I bet you could get a definitive answer.

Run a string line about 12" down from the peak on the sagging side then run another string on the opposite side in the same spot with a 2x4 spacer to raise the string and see its off that way as well. Your gonna shingle it anyways so scew holes shouldnt matter at this point.

See if the trusses are off laterally. That would be nailed on more to one side off center line than the other.
There could be a outward bow in one long wall that they nailed the trusses to first and then moved the other wall to the same spot on the truss instead of string lining both walls.

If this is the case your stuck with it and it's cosmetic.

Need pictures of building showing that front and rear walls are straight and that ridge is straight both vertically and horizontally. Need pictures of the sag in the plane of the roof from eve and from two quartering directions, Need to know how many trusses are sagging and if the are adjacent to each other or randomly placed. Need picture sowing the web placement in the truss. hedd you to determine if the lower chord of the sagging truss is straight or sagging also.

As a note; any jacking between the top and bottom chords as has been suggested will only compromise the the bottom chord and the truss as a whole, added structure in form of added webs or knee walls is just added load unless it is additionally supported. Removing the webs and supporting the bottom chord before jacking would work just the top, and is plausible, with replacement of webs after adjustment is made.

If the ridge is straight horizontally as well as vertically, and the top of the walls are both straight in both vertical and horizontal, and if the ceiling is not sagging in the same place as the roof; then it is reasonable to guess that the sag is built in. (either through inverted crowns or miscut/misplaced webs).
In this case shim between truss and sheathing accompanied by scabbing/sistering the top chord could be a solution. If the run from ridge to plate is not over long, an add in rafter would straighten the deck with not much added load,but only if it is supported at the plate and by a header at the ridge. Is the " 28 foot span" wall to wall? If so 16' 2x6 should be easy to find. Make a pair of rafters and collar tie them in beside the truss.
I have shingled many old roofs with sags and they kept on shedding water.

Sounds like a lot of guys posting have some solid experience behind them. But as White17 posted one really needs more information and that would include pictures.
One poster soundly advised checking level and straight using a string. Several things that may have happened: If the builder of the homemade trusses did not understand how to build a truss, they may not be standing up. If they were built in a jig and then they did not keep the same end (say the right or the left) on the same side when installing, it could variation.
If they did not run a string on the outside walls when installing the trusses, the walls could have spread. It is possible that the way the trusses are installed they do not keep the walls from spreading.

Only an experienced builder should be making and installing trusses, and then most experienced builders buy manufactured trusses if they can. That way it does not cut into your time, and it is hard to beat a professionally built engineered truss that comes with a guarantee.
Good luck

I agree.
I have straightened a few roofs too.

is it the south side that sags?

Yep south side and to boot no vents! Im really thinking since its on a shop to leave it but it is just not right and I know when I go to sell it the inspector will get it, so pay now or pay later. No sag in the ceiling and the walls are plumb. Im thinking what I might do if anything at all is rip off the whole mess and get engineered trusses and do it right. I am the kind of guy whether trapping or building something build it right, and then over build it, do it right and not have to mess with it again. Problem being this area for a lot of reasons you can't get any help and for sure not a contractor for any kind of remodel, all they want is to build new. So might have a project this summer and hopefully not this fall.

Without pictures, the rough assessment is if the sag is just on one side of the roof, to rip off that side to the rafters, then star screw 2 by 8's onto the common rafter (that would be the top board that holds up the roof, and then run a strut off of that at angle to the ceiling joist or ridge beam, depending on what you call it.

If the sag is on the pinnacle of the roof, run a king post up the that from the ceiling joist and then run spanner ridge boards between the rafters. Anchor it all with metal brackets and star screws. Put the roofing back on, screw on metal work and it should look Euclid.

Being a Truss Designer, engineering trusses since 1985, I would say that the problem that caused the sag was poorly fitted joints in the web members and plywood gussets that weren't of adequate size to restrain the forces at those joints.If it's a clear span application, I doubt there is a reasonable "fix". The best you could hope for is to prevent it from getting any worse.

south side no vents , hard to say without pictures but you had a steam box going , and the wood bent to the pressure under heat and humidity.

I am no expert but your wanting it to look right so that no one questions it later and make it not get any worse.

you already have to pull the shingles , then pull the sheathing then you will have a better idea of what really happened.

if you sister up trusses making a strait line for the roof again add extra vents a vent between every truss and make sure to add soffit vents or fully vent the sofit.

the north would have the same load but could be cool enough to not get the wood to the point it will bend .

you can drive down most any street and see roofs showing damage or premature aging because of inadequate venting