Joist Size:
4 x 2, 6 x 2 (Nominal)
45 x 95, 45 x 145 (Finished)
 
Lengths:
3.6, 4.2, 4.8
 
C24 graded structural timber is stronger than C16 timber, it can handle higher loads & it much more durable. They're the obvious choice when it comes to structual timber as it can take 50% longer for the timber to grow.