It typically needs to go across the floor joists so it can be nailed to them.

We have put new sub flooring down,so thought we would just lay some both ways an see which we like, but thought maybe there was a right way it should be done. Thank you

These are quotes from various hardwood floor manufacturers. Subfloors usually have too much deflection to run the flooring parallel to the joists.

"Building codes establish requirements for structural support components of flooring systems which may not provide adequate rigidity and support for proper installation and performance of a hardwood floor. Whenever possible, install flooring perpendicular to the floor joists for maximum stability."

"When installing parallel to the floor joists it may be necessary to stiffen the subfloor system by installing an additional minimum of 3/8″(9.5mm) approved underlayment."

”(subfloor)-19” Joist Spacing: Use an APA approved 1-1/4” minimum thickness, exterior grade plywood, or 1-1/4” minimum thickness, OSB, following manufacturer guidelines, including expansion spacing between the sheets. The joist manufacturer's recommended span must not be exceeded.
Joist spacing in excess of 19” is not recommended for solid hardwood installations unless additional layers of directionally opposed sub flooring are installed with sufficient thickness to eliminate sub floor deflection."

This might help, but you should always follow the requirements of the manufacturer so that you have a warranty when you are done.


http://www.hoskinghardwood.com/Department/Hardwood-Floors/All-About-Subfloors--installing-flooring-over-various-sub-floors.aspx?dId=7&pageId=2

Right Fred, perpendicular so there is something to nail to is what I thought.... I see flooring laid diagonally sometimes though, what is the allowable span between nails?

It isn't exactly required to go perpendicular, but you need a much thicker subfloor than normal with a deflection rating of less than L/640 according to some. Some still require a very low deflection even if going perpendicular. These thicker subfloors and dense hardwood floors could most likely require more joists to be added due to the added dead load. Nailing etc. should all be a minimum of what the manufacturer requires. It is usually more than every joist. Here is an example;

http://www.hardwoodamerica.com/index.php?id=56
The floor joists and subfloor should be checked with some quick calculations by an engineer or other qualified person before proceeding.

For reference, many floors with carpet were only designed with a deflection of Length/ 360.