Loft conversion flooring

[Rooftop]

Hi

Have a 6 x 7m loft space currently with 4” x 2” ceiling joists

Looking to have access for storage and a model railing way setup and maybe a desk to use from time to time

 

Which option is best

 

1) add new joists in parallel with existing from wall plate to wall plate with load bearing walls in middle (2.4m joists joined with M12 + joist connectors) 18mm OSB board for flooring which size C16 could I get away with?

 

2) add another 2-4” on the existing ceiling joists (at 90 deg angle on existing) with 18mm osb flooring

 

Thanks in advance