I am thinking about having new Dundee Strip Pre finished HW installed over existing 3/4" HW that has been installed over 1"X8" subfloor. The subfloor has been installed on and perpendicular to the 1'X8" floor joists. The existing HW has been installed perpedicular to the subfloor making that parallel to the floor joist.What would the possible problems (if any) encountered by installing the new HW flooring in the same direction (parallel to the floor joists) on the existing HW flooring or for that matter over existing HW flooring.Refinishing the existing HW flooring due to medical reasons is not an option.

You may install the new 3/4" wood floor over another BUT you MUST lay it perpendicular to the direction of the existing floor. You also may have it laid at 45 degrees to the old floor. One should not lay parallel to the existing wood floor.
There are now floor finishes with very low VOC's that should not bother a sensitive individual. Plus, some finishers are using dust containment systems so that the sanding dust should not be a problem. I would encourage you to look further into having the existing floors refinished.

would this rule also apply to putting hardwood flooring over 3/4" tongue and groove sub-flooring?

so, you're suggesting that the hardwood floor would then be installed in the same direction as the floor joists? (since the sub-floor was laid perpendicular)

*** i should state that the sub-floor is 3/4" tongue and groove planks.

could someone answer my question please?

i'll be installing 3/4" hardwood soon. the sub-floor is 3/4" x 6" tongue and groove planks over 2 x 8 floor joists.

do i need to run the new flooring in the same direction as the sub-floor planks, or can i run opposite?

thanks for any help!!

If you existing subfloor is solid planking, you should run the new hardwood flooring perpendicular to it, which in your case, is parallel to the floor joists. This is not ideal. Ideally, you would install some plywood, 3/8" or greater, over the subfloor, then install your new wood flooring over that.

thanks, gary!!!

For prefinished flooring i would install an underlayment anyway. You want as flat and even a substrate as you can get when going underneath prefinished products.