"Colbyt" wrote:

-snip-

First of all I don't consider 5/8" over 1/2" to be under-layment at all.
That is called a floor. Half inch is nothing.

Under-layment is something you put down to provide a smooth clean surface
for tile or vinyl and can be run either way. Usually it 1/4" and added to at
least a 3/4" sub-floor.

I would install it with the 8' running across the joist and try my darnest
to hit the joists. I would stagger the seams so that the new does not line
up with the old on either direction. I would use a minimum of 8D sinkers to
secure it. Now you have a floor.

To minimize potential squeaks I would use a single layer of 30# felt or a
double layer of 15# felt. Alternately you can use liquid nails or the like
in lieu of the felt.

What he said. Though he didn't make clear that all those 8D nails
go into joists.

*THEN* you put your 1/4 subfloor in if you need it and use the short
ring nails.

Jim