With only 3/4" ply, and flooring running lengthwise, you will get dips
between the joists over the years. This can be remedied by another 3/8" ply
or blocks between the joists.
Appearance is a matter of taste. Running lengthwise is much harder to
install, but easier to sand. The short way is much easier to install but the
sanding is all edger work.
The conventional wisdom is that flooring should always run parallel to
the long direction of the room. In a hall that means running the long
way. If you sub-floor is adequate the direction of the joists is
immaterial (remember that carpet gets laid directly on sub-flooring
all the time and those floors don't have sagging problems).
I had a hall that was 77 feet by 42 inches. I installed the flooring
cross ways because of the difficulty of nailing the flooring down
without hitting the wall. However, I did not consider wood movement
enough. 77 feet of flooring has to move, even in an air conditioned
house. After a couple of years, I got 40 feet of washboarding, where
the wood expanded (across it's width -- I used 4 inch quartersawn red
oak flooring). Another hall, 16 feet long at one end of the longer
hall has shown no problems, so I would guess that its the aggregate
length that contributed to the problem. Yes, I let the wood acclimate
for 2 full weeks, in the rooms along the hall, with the bundles open
and spread out. I don't know how long your hall is, and only about the
middle of my hall washboarded. I didn't want to surface nail one third
of the oak pieces, which I would have had to do if I had gone
lengthwise, so I went cross ways.
I used a nailer (hammer powered, not air) so maybe the floor just got
put in too tightly as the distance went on, but most of the
washboarding was in the end closest to where I finished.
Hope this helps,
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