Drywall should have never benn installed without a stud at the seams. I do not know of anything else to do but have it installed correctly, sorry.

Without a stud there to fasten the drywall to, it will sag over time no matter how much kiudgework you do to try to "repair" it. The only solution is to remove the drywall, install proper blocking, and apply new drywall. Hire out the project if you don't want to DIY, as it won't be that expensive. In a stairwell, you need decent scaffolding and extra pairs of hands to do it right. It's worth paying someone else to do so you don't end up in the hospital from a slip and fall.

"Drywall should have never benn installed without a stud at the seams."

Not all seams are studded or backed up, and they do not need to be.

A typical installation uses 4x8 sheets, and only the 4 foot edges are on studs.

The long edges are perpendicular to the studs and when correctly taped and finished are not going to sag (ceilings are done the same way).

Hi, Maybe think out of the box on this problem. How about covering the seams with a fancy moulding (think chair rail) Then possibly paint the sections some neat colors. Make it look like it was planned, you are only limited by your imagination.
Good Luck Woodbutcher

Woodbutcher is right, as is op - the house is settling, which is normal. Where the drywall joins near the upper floor level, that bulge will happen. You need to actually cut away half an inch or so of the drywall - a little off both sheets - and affix some sort of trim (presumably top only) to allow for movement.

Problem solved - and it's the only way to solve it, it will keep happening as the house settles.

Once you've done what I've suggested, cut away the edges of the other boards exhibiting this and tape and re-mud - if you leave the gap under the board at the floor level, the others should be ok.

So when you say cut away the edges of the boards, do you mean just trim the 1/8" that is pressing against the sheet below it and then tape and mud? I love the chair rail idea but this is in a stairwell and the seams occur all the way up the 18' tall wall at different intervals.

Are you sure this is not that chinese drywall? Why would the drywall bulge if the house is settling? I would also look for water penetration.

If there really isn't any support behind the edges...and you want to add support at the joints without removing the drywall, how about cutting an access hole at each joint (eg. 4" high x 6" long), then insert a 1 x 4, 1 x 6 or similar size plywood piece behind the edges and screw them to the board. You may need to drill a couple finger holes in the board to pull it in place as you screw...or alternatively put a long screw in it as a temporary handle to grab onto. Then reinsert the patch you had cut out and screw that in place. (you may want to clean the edges of the patch with a sharp utility knife at an angle).

That should firm up the drywall at the edges enough to keep settlement from pushing out the edges.

Next, use a utility knife (with sharp blade) and cut away the high stuff along each edge. Next, retape (I wonder if there is tape to begin with), followed by subsequent wider coats.

It bulges at the seams because it's affixed at multiple points along the upper part, it moves slowly downwards and pushes against the lower sheet.

Where I am, it's code or at least standard practice to run that trim or picture rail across that seam, to cover the gap they allow for expansion/contraction/settling.

You don't need support behind the edges provided it's properly done, and adding such might create more issues.

Really? Code or standard practice...and a gap for expansion/contraction/settling? Come on. It's just a bad drywall job (probably with insufficient framing underneaith) that needs neither excuses nor moulding to justify it.

Absolutely it is in British Columbia. I don't know for sure if it's code, but it's absolutely standard practice in stairwells (usually the only place where you get your whole two floors height in one wall.

I'm an alarm technician, and I see it (the gap and the trim that covers it) in every new house. Mine has the same bulge at the seam at the level of the upper floor, because it was built before that trick was introduced.

I assume it's not a rule in the US since nobody seems to know about it, but it's standard here, and it WILL fix the OP's problem (assuming it's only one seam, but if it's several, once the gap is introduced, the others can probably be fixed.)

What 'framing' would fix it, in your opinion? Clearly it's downward pressure. Sure, a heavier point load may have reduced the effect, but the direction of pressure certainly appears to be downwards.

I guess that's why we leave a 3/8" gap at the bottom of a wall.

A wet 2x12 joist is a good 3/8 wider than a dry one. When it shrinks underneath, the non-compressible drywall has to go somewhere, as two objects can't occupy the same space. A slip joint or molding shroud is the no-brainer solution.
Casey

Thanks. I should add that they now put slip or expansion joints in drain pipes from the upper floor. I've only noticed them on the main larger pipes, I don't know if they put them on the smaller ones.