Drywall is applied to the studs, not sheathing so I'm confused as to exactly what you've done here. But if you glued drywall to plywood, there's your problem right there. Tear it out and start over. Not to mention that if you glued drywall to exterior sheathing, there's no insulation or moisture barrier in place and you are in a world of trouble from that right there.

ilo is correct, plywood moves and is NOT meant to be a substrate for drywall period!

There is a reason roof decks layed with plywod require gaps between sheets.

Plywood moves less than solid wood of the same dimension (NOT including thickness, just the 4 foot and 8 foot directions) but it still moves.

That is why plywood is NOT a good substrate for drywall or tile without a separation layer to let the plywood move without cracking the applied surface.

The walls are insulated with fibreglass batts, and plastic sheeting was tacked over the studs to serve as a vapour barrier and stop air infiltration, before applying the plywood sheathing. The bare plywood was in place for about 3 years before the drywall was applied, and the drywall was in place several weeks before the mudding was done.

In a circa-1915 addition, drywall has been applied directly over the original wooden tongue-and-groove wall board (the old fashioned stuff about 1/2" X 3ý" with the fake groove down the middle, often used for wainscotting). The drywall was put up decades ago before I owned the house, and still shows no signs of cracking.

The plywood is 3/8" unfinished both sides, like what is used for exterior sheathing, but applied to the interior sides of the studs over the plastic sheet.

I won't rip it all down. Since these are only hairline cracks and some joints haven't cracked at all, I'll just go ahead and finish the drywall and paint over. If the cracks re-appear I'll live with them or if they become too objectionable, try covering with wall paper. Wall paper always did a pretty good job of hiding cracks in the original plaster; the only problem, we sometimes get silverfish and they love it.

The only alternative would have been to re-do the plaster and lath work. The studs are irregularly spaced and too far apart (some as much as 2' 4") to support drywall directly over the bare studs. Right after the plaster and lath were removed, I tacked a test sheet of very heavy 5/8" drywall over the studs and sure enough, you could push on the sheet, and it didn't take a lot of force to actually see the wall move inwards. If the studs had been standard 16" o.c., two layers of 3/8" drywall would have worked perfectly. This house was built in the Civil War era using good heavy material, but the carpenters didn't do a very careful job of putting it together. I have even seen what appear to be splinters of old furniture used for shims in the framing. The original fasteners in the old part of the house are square cut nails and wooden pegs.

Another thing, I ran a wood burning stove in the room all winter. I noticed that caused some hairline cracks to appear in the 150 y.o. mantel piece as well, even though the stove is far enough away from the wood according to the manufacturer's specifications. The rest of the house is heated with propane and electricity.

But my original question still is, whether it might help to apply paper tape over the joints before adding the finishing coats of joint compound.

never mind....

This post was edited by millworkman on Sun, Apr 28, 13 at 14:59

Tape won't help. The drywall will just end up cracking elsewhere due to the stresses placed on it. If you won't do it correctly, you're going to deal with repairs twice a year during the seasonal humidity swings when the wood below loses it's moisture and gains moisture. And moves around. As all wood does.

The correct way to do it is to shim the shallow studs and lightly plane the high ones to the same plane. Install some sistered studs or blocking in between if need be so that the drywall seams are fully supported. Then install drywall.

Or, learn to install lath and plaster. It IS DIYable.

If you are going to own an older home, you have some responsibility to it to perform maintenance and repairs properly. Doing it properly would have actually been less work than you've put yourself through.

Read chapter 3.

It sounds like you dried out the mantle and caused checking to appear.

It has nothing to do with "clearance."

The humidity is going to be pretty uniform throughout the room

Here is a link that might be useful: 'Wood Handbook'

It would be way too involved to rip everything out and start over from scratch at this point. I'll just take a wait and see approach. I can live with hairline cracks if they don't get a lot worse than what they are now. Any expansion/contraction cycle in the wood has to have upper and lower limits.

With warmer weather and the stove not running, the crack in the mantle is now invisible.

"With warmer weather and the stove not running, the crack in the mantle is now invisible."

And it will be back next winter when you dry it out again.

It is not gone, just swelled closed.

"Any expansion/contraction cycle in the wood has to have upper and lower limits. "

Depends on humidity.

Read Chapter 3 in the 'Wood Handbook.'

Plywood is more stable in the large dimensions, no better than the base wood in thickness.

Yes, I know that. Also, some of the original doors in the house develop visible cracks every winter that swell closed in the summer.

I took a closer look at the old drywall in the addition that was applied over tongue-and-groove wallboard decades ago. It has cracking at some of the joints, too.

All the more reason never to rip out plaster in an old house if it is at all repairable. Dry-wall is at best a cheap imitation even when everything works right.