Nail down hardwood florring parallel to floor joists

P.Ostal -------------------------------------
Hi all
I have open-web engineered floor joists at 19.2" centre, and they run along the longer length of the living/dining room. I have 3/4" particle board subfloors glued and screwed every 8" into the joists..
I want to install nail-down 3/4" solid hardwood flooring, and they would logically go along the longer side of the rooms (and be parallel to the floor joists). I have added 3/8" plywood on top of the 3/4" OSB. Screwed into the subfloor & joists every 6". The reason I used 3/8" ply wood was to get level with areas that were tiled (using 5/8" plywood underlayment). Is this sufficient to allow the hardwood planks installed parallel to the floor joists? Or should I have used thicker plywood, or even add some support for the floors from the underneed (i.e., put 2x4 between the floor joist from the underneath, then glue to the subfloor and screw to side of the joists. Any advice will be greatly appreciated.
P.Ostal
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I hope you're wrong about particle board. It has never been a structural flooring material to my knowledge, especially at 19.2 centers. Particle board has been used as underlayment, but it not even acceptable as an underlayment for nail down hardwood flooring. Hopefully you meant OSB tongue and groove. If it doesn't have the T&G it requires nailers at edges. It would be best if both the OSB and the plywood that you added was glued and nailed. I've not ever run hardwood on a 19.2 center joist system. There are no restrictions on which way the hardwood runs when on sheet goods as far as I know. If it were on lumber decking, it would need to run opposite the subfloor direction.
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That is the best answer given here below and its on 100%...
Hope the 3/8" plywood you used is made of Douglas fir...that's what we use under any hardwood ...But We use 5/8" thick sheets...
Lansing

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The OP didn't mention the sheet good orientation, and whether his underlayment was laid with the same orientation, but it makes a big difference. http://www.canply.org/english/products/comparison/comparison185.htm
NOFMA lists many different subfloor constructions, but they only recommend one way to lay the strip flooring - perpendicular to the joists. The reason for this is that over time, and with load, the floor will creep. The gaps between strips will tend to open up over the joists (the gaps are on the top surface of the strip flooring), and in the "valley" the gaps will be at the bottom of the strips. In other words you will have banding of gaps.
The floor will also have a shortened lifespan if it is laid parallel to the joists. The dip between floor joists will probably be minor (particularly in the OP's situation), but the high points over the joists will require more sanding to get the floor flat. That means the nails in the strip flooring over the joists will shine through after sanding before the nail in the valleys will show up.
I agree with you that the underlayment should have been glued as well as screwed, but it sounds like the OP did a reasonable job beefing up the subfloor. How much the strip flooring will move will depend on the OP's house's particulars - location (for humidity and temperature), what's underneath the first floor, whether the house is conditioned year round, etc.
R
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