I have been fixing nail pops in the ceiling of my girlfriend's townhouse and found that 3 of the beams are flexing causing my screws to crack the mud.A little background first, the house was built in 82 so the construction is not good to start with. The builder seems to have put up the ceiling first and then the interior walls. The truss structure is a 5 beam joint. One runs straight down from the roof, the 2 ceiling beams but up to it, and then there are 2 30 degree beams running up to the roof. There is a steel plate holding it all together on each side. It is prefabbed . The previous owner then stripped of the instillation off and screwed down 3/8 or 1/2 plywood from truss to truss (there are 2 of them per ceiling beam) down the center of the attic. The drywall is joining right under the truss. 2 of the pops are because the glue has failed so I can shoot in liquid nails under it and compress and screw them to fix the cracking problem. But the third is because the truss is flexing. I have cracking up to 2 feet around the truss. What I want to do is sister it to stop the flexing, shoot liquid nails under truss, sisters, and beam, and add a lot of screws running down the beam to hold the drywall.

My questions is, is this the right thing to do? I read the one hit for flexing ceiling so I think this is the way to do it. So assuming it is:
1. Should I sister one side using a 8 feet by 2 x 4 where the truss is in the middle or on both sides? The truss isn't cracked, but is is flexing because of all the weight.
2. The other post said nails or screws. Is that right? I want to use 4 inch screws.
3. I am planning to use liquid nails on sister beam(s) but how much is enough? How many screws should I do down the sister beam? 1 every foot, 2 foot? 1 row to 2 or more?
4. How many screws should I but into the sister(s) and beam to support the drywall ceiling? Since I am using liquid nails I am not sure if more is better. I was thinking 1 every 2 feet would be good enough.
5. Should I tape the crack areas or just mud it again? I think tape is correct but if the flex is fixed, do I need it?

"but is is flexing because of all the weight" What weight?

If you have a truss roof system there is little reason to have flexing as you describe. Is there weight bearing from above? You really can't floor a truss system, at least the design you explained, a tied king post. It is not designed for down load except through the roof. Attic trusses are different, they are designed for some loading.

They did not reinforce any of the 2 by 4s in the attic which are not really able to suitable to support a floor. Plus the previous homeowner left a lot of heavy crap in the attic that I have been removing out. There are nail pops all over the place in the ceiling and the glue seems to be separating on many of the beams where they created large stacks of blown insulation when the scrape it off between the trusses. The home inspector didn't report any of these problems when the home was bought. Basically they didn't know or care what the hell they were doing when they did it. But she is stuck with it now and something has to be done to stop the cracking in the ceiling. I don't think removing the floor is really a option at this point.

Well, the problem is improper use of a truss. As I noted before, a truss that you have is not designed for loading down on the chord. That type of truss is designed to transfer load to the walls. Remove as much load as yoU can. Sistering will help, one 2X is sufficient, screwed to the chord plate. However, if you currently have a bow in the truss,sistering won't remove that.

I am not sure what at a chord plate is. Is that is the mending plate that holds the truss together? Or does it refer to the 2 x 4 beam that the ceiling to attached to? If it's the plate, I think I would have to drill holes it in before I can screw anything into it unless I hit one of the slots. Is that really OK? Also how many screws into the plate?

The chord is the lower truss piece that is across the room. The drywall screws to it. If you screw drywall,do Not go through the paper level, otherwise you will indeed have pops.