Scaffold planking must be able to support, without failure, its own weight and at least four times the intended load. [1926.451(a)(1)]
Solid sawn wood, fabricated planks, and fabricated platforms may be used as scaffold planks following the recommendations by the manufacturer or a lumber grading association or inspection agency.
Tables showing maximum permissible spans, rated load capacity, and nominal thickness are in Appendix A(1)(b) & (c) of 1926.451(a)(1).