I take it you mean the rafter is sagging slightly and not twisting. If twisting fix some noggins between the offending rafter and the next to help it stop twisting further.
Can't do much about sagging.
I put tiles on bent [sagging] rafters as the tiles hook over the battens not the rafter. If its the batten thats bent cut it out and replace it.
Can't see so thats all I can offer.
Rob
July 2007