The minimum thickness / strength of the boards depends on the spacing of the platform beams below them:
Board thickness | Axis distance between platform beams |
25 mm (9.8 in) | ≤ 55 cm (22 in) |
30 mm (11.8 in) | ≤ 65 cm (26 in) |
If you choose thinner boards, they will bounce and sag uncomfortably, which does not feel safe.