Homeowner Forum / Construction / October 2005

The short answer is that most pros will tell you that you must run the
flooring across the joists, somewhat to protect themselves from your not
being satisfied with the end result, but mostly because NOFMA/warranty
recommends it.

The long answer is, anything less than 3/4" plywood/1" plank sub floor on
16" centers, or if your sub-floor is not level or flexes any at all when
stepped on between the joists, you'll either need to add bracing under the
sub floor if running the hardwood parallel or run the hardwood at a right
angle to the joists.  If you don't, you'll see every valley/hump between the
joists when the light hits the floor (washboard effect), and that doesn't
look pretty.  Also, when run with the direction of the joists, flexing
between the joists can lead to the hardwood splintering/cracking at the
edges and working the nails loose causing premature squeaks.  Important, if
the hardwood is installed parallel to the joists and the sub-floor is not
level, sanding will take out too much material on the high spots while not
taking any on the dips.

That said, I installed a 14' X 24' room parallel to the joists because I
felt the hardwood looks better when installed with the length of the room.
It was installed over 3/4" plywood on 2 X 10 SYP which was blocked on two
feet centers underneath with 2X6's.  The rest of the house has smaller rooms
or were perpendicular to the joists, and as such, I installed the flooring
across the joists (changed direction from the larger room at the doorway).
If your sub-floor is OSB or less than 3/4" plywood, I personally would not
install the hardwood parallel even with blocking.

Another alternative that is somewhat common is to run the flooring at a 45
degree angle.

That's my .02, but worth every penny!!

http://www.nofma.org/faq.htm#installationq4

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