Installing hardwood over OLD pine floors...
 

I own a 120 yr old row house. The bedroom floors are the original 3/4" x 2 1/2" pine directly attached to the joists. The floor has been sanded and refinished twice. They are in bad shape (big gaps, splintering) and cannot be reasonably refinished again and the new flooring is already purchased.

I plan to lay new 3/4" oak hardwood flooring. What do I need for underlayment? Can I just lay the new floor perpendicular to the existing floor and parallel to the joists?

Many Thanks,
SPL

I'm not sure I would run the new floor in the same direction as the floor joist. Other may disagree but I think you need some plywood first.

SPL,
I would recommend checking with the manufacturer of the new floor material for installation instructions. Improper installation may void any warranty on the materials. In the meantime I found the following website that has a lot of useful information. The section on subfloor preparation addresses your situation. It is important to have a secure subfloor to attach the new floor to, so now is the time to secure any loose boards.
Good Luck
http://www.simplefloors.com/how_to/installation/hardwood.aspx#subfloor

Thank you both for the replies. Simplefloors recommends following the grain of the existing floors, but when I shopped at two local hardwood floor suppliers they advised to go with either 3/8" plywood or to go diagonally... If it wasn't gapping already I would just refinish it...