Why are there cracks all over my upper floor ceilings and walls?
This is very very common in my area as I live in a part of the Midwest that can have 60 days straight of 10-20 F in the winter and 60 days straight around 100 F in the summer.   I have bought more than a few houses with the issue before - on purpose.  Because what looks like some major structural issues going on, it is really no big deal other than the cracks in the drywall.  I know it is a big deal to you because you shouldn't have to deal with issues you didn't cause.

Why the issue just appeared had little to nothing to do with the new roof and a lot to do with the new ventilation and insulation the attic area got.  One of two things or both certainly happened:


The attic had a lot of loose insulation before and they got rid of a lot of this to increase ventilation.  It is very likely that your attic was full to very full of insulation before.   
The attic did not have a great thermal barrier and they added something to create a break from the house.


There are a variety of reasons why they may have done this.  There could have been condensation/mold issues due to lack of ventilation or they could have determined that there was a considerable amount of heat loss so they wanted to redo the attic.

Most likely though the roofing company did what they usually do or made the change based on making money.  The architect/engineer that designed it originally allowed the drywall crews to nail/screw directly into the trusses at the corners.  Any architect working on a project like this would be all over this as it is a huge aesthetic thing.  It likely that the architect planned on keeping the attic "warm".  The fact is it was working just fine for a long time and someone changed it.   The roofing company should be on the hook for this period.  They probably got all kinds of money installing baffles and mushrooms and whatever, so that there was "new" insulation.  

Let's talk about what is going on.  The bottom of your trusses are below insulation.  They are conditioned and dry.  They stay the same.  The top of your trusses are above the insulation.  Now that the ventilation is "better" they get even more cold air and moisture in the winter.  So they are sucking in water and elongate during cold seasons.  As they elongate they push down on the truss bottom plate on each side.  The bottom plate is stretched and bows up in the middle.  Given that your drywall was installed the same throughout your house I would guess that your biggest cracks are in the middle of the truss.  Also the vertical cracks you are getting are for the same reason - movement.  Albeit some more flexible mud and keeping your home at more consistent humidity levels would probably fix those.



Just know that seasonally your truss will pull up the highest during the winter and flatten back out during the summer.  This will just keep on happening every year with some years being worse than others.

So how do you fix your cracks for good?


Have the attic reinsulated.  Meaning less airflow and ventilation and more insulation.  If there was a ventilation related issue that they fixed there should be pictures to back it up.  You don't just change something that was working fine.  Housing systems are very unique.
Have all of the ceiling drywall redone.  Basically you would need to install the ceiling drywall and not attach it to the trusses anywhere within 12-20" from each wall.  This will allow the corners the ability to move with the trusses.
My drywall guys never screw within a foot as it is but this was not common practice until the past 10-15 years.  




Installing truss backing angles everywhere would fix your issue.  This is just a messy alternative but what needs to be done.  




The last choice I offer is just to install some crown molding.  However you can't just install the crown molding using the traditional method or you will be back on here next winter asking "how do I fix a big gap at the top of my crown molding?"  What you need to do is install a wedge around the entire ceiling and attach this wedge only to the trusses.  This actually doesn't add much install time to the project as the crown molding goes up super easy and is firmly secure.  And it will move up and down with the top of the ceiling all year long.