The space between live rolls shall be entirely filled either with planking 2 inches or more in thickness if of hemlock, or 1 1/2 inches if of hardwood, or with metal sheets or plates so supported that they are not displaced visibly by a load of 150 pounds applied at any point. If bolts are used to fasten the planks, the heads should be set in countersunk holes. Sheet metal or plates should not be less than 1/4 inch in thickness.
34 Pa. Code § 45.122
This section cited in 34 Pa. Code § 45.143 (relating to live rolls).