Hardwood flooring is to be run perpendicular to the joists for structural integrity of the flooring system to prevent rolling or waviness between the joists over time.
Floating floor have to be perpendicular to floor joidts.
If the old subfloor is wood planks you should install the finish floor perpendicular to them which will be parallel to the joists.
This provides added strength to the floor and helps cut down on bowing in the laminate.
Just make sure the subfloor is flat and the proper thickness e g.
Installing them parallel creates the possibility that the floor will sag.
If the subfloor is stiff enough there is no reason why solid nail down strip flooring can t be run parallel to the joists.
Since the floor joists typically attach perpendicular to the rim joists and roof rafters typically bear perpendicularly to the rim joists as well then the joists and rafters are typically parallel.
If you have an older house with a shiplap floor the shiplap is perpendicular to the joists so the flooring is parallel to the joists.
Some houses have the shiplap on a 45.
If the subfloor is plywood of marginal quality then the finish floor should be perpendicular to the joists to stiffen the overall assembly.
That said some times it s not possible to do this.
If you go parallel to the joists you may end up with areas of flooring that aren t fully supported.
First renail the existing plywood subfloor so that the old nails are tight and add new nails to achieve a maximum nail spacing of 6 inches.
Flooring experts recommend installing flooring boards perpendicular to the floor joists in a house with a plywood subfloor.
The nofma specifically states this in their instalaltion guidelines for nail down hardwood flooring as does almost every hardwood manufacturer.
Generally floor or ceiling joists run parallel to the rafters.
Most directions recommend running laminate flooring perpendicular to floor joists.
Plank should be stiff enough to span the joists with no problem.