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.
Floating floor have to be perpendicular to floor joidts.
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.
Plank should be stiff enough to span the joists with no problem.
In these cases the conservative choice is to go perpendicular across the floor joists and allow them to support the new floor as well as the sub floor.
That said some times it s not possible to do this.
Installing them parallel creates the possibility that the floor will sag.
If you go parallel to the joists you may end up with areas of flooring that aren t fully supported.
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 the subfloor is plywood of marginal quality then the finish floor should be perpendicular to the joists to stiffen the overall assembly.
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.
The nofma specifically states this in their instalaltion guidelines for nail down hardwood flooring as does almost every hardwood manufacturer.
Some houses have the shiplap on a 45.
Most directions recommend running laminate flooring perpendicular to floor joists.
If the subfloor is stiff enough there is no reason why solid nail down strip flooring can t be run parallel to the joists.
Generally floor or ceiling joists run parallel to the rafters.
I guess you wouldn t have to lay it parallel but the shiplap has a lot more movement then plywood or osb.
A flooring contractor howard brickman responds.