EE was riding in the jump seat of #390 portal bus enroute to 2 South section when a piece of roof material 14' x 3' x 4"" fell approx 8' from between roof bolts striking him in the right forearm, right knee, left wrist & the chin. Resulted in a hairline fracture to his right wrist. Occurred at survey station 300+50 on the mainline. No roof bolts were dislodged.