On Jan 6, 6:44*am, "
wrote:
On Jan 6, 9:37*am, Robert Macy wrote:









how to repaire [and prevent future cracking] WITHOUT adding a big LUMP
to the seam?

The wall run is around 60 feet and 10 feet tall. the junction right in
the middle is wanting to crack. Trying to keep from forming lumps on
the wall, I cut down, used paper tape with a layer of mud under and a
layer on top. [Many may remember the problems I had posted earlier. I
tried to wet the tape first thinking of wall paper and how that
shrinks to fit, but wetting CAUSED the paper to 'slide' as it dried
and thus a crack, so in response I took all that out and did it again
with dry paper tape, which did work better. Only had the tiniest of
hairline crack form all summer long only about four feet of run on the
wall only.]

Well this winter as we isolated that section of the house - meaning
cooler temperatures and probably contracting drywall sheets, the
crack(s) opened up with a vengence! I mean over 3 mil separation!, but
worse the crack is the full floor to ceiling AND even now goes up
along a ceiling section which I never had trouble with before. I
suspect in the heat of the summer the crack will close back up, too.

Yes, I know houses change shape with time, but this seems too much to
be caused by 'settling'. My conjecture is is that this is more caused
by 'flexing'. Oh yeah, the seams are ON a stud(s), so there should be
no reason for movement there.

So, my question is
How do you repair/prevent cracking at drywall seams WITHOUT creating a
huge lump on the surface? I already have built up 1/8 inch thick to ++
on these stupid seams.

I don't think the thickness is going to do much. *There
isn't much tensile strength to drywall compound. *If the
drywall moves, it's going to crack. *And it sounds like the
movement is related to temperature and possibly humidity
changes. * You say the room is "isolated". *What does
that mean and how cold does it get? *For small cracks
that are persistent in 90deg corners or similar, I've used
a flexible caulk type filler. *don't know how practical that
would be for a long, mid-wall joint though.
But

Drywall "moves" more than steel but less than aluminum based on
temperature changes.

It it also wants to move quite a bit based on relative humidity....

Thermal Coefficient of Expansion: Unrestrained: 40-100 °F (4-38 °C):
9.0 x 10–6 in./in./°F

Hygrometric Coefficient of Expansion: Unrestrained: 5-90% r.h.
7.2 x 10–6 in./in./% r.h.

So keeping the the drywall within a narrow range of both temp & RH
will minimize the tendency for the crack to open & close.

As T4 mentions, thinkness of the "patch"/ coverage wont help much.

With finishes... if brittle, they will crack with movement. So there
needs to be minimal movement OR the materials need to accomodate
movement.

Flexible caulk is an option that I use at corner joints, I've also
used it with success on narrow (less than 1/16") field cracks in
plaster.

You might consider cutting the corner tape & using caulk in the
corners.
A 60' wall is quite long and suffers a huge amount of dimensional
change with temp or RH... if all the movement is forced to be
accomodated at single line....you'll get a wide crack.

You mention 3 mil...did oyu mean 3mm or .003"?

..003 " can easily be fixed with flex caulk
3mm (.125") is huge

cheers
Bob