-1 toughness rolls

The K'daai Destroyer and the Bale Taurus, both monsters coming out of the Chaos Dwarves, Throne of Chaos book have a rule with non-magical weapons. It is stated that nonmagical weapons have a -1 roll to the wounding roll when it is Str vs. T for those monsters. Does this mean if you need to roll a 6 in melee for wounding, that it will automatically not hurt the monster, since you subtract -1 from the roll?

I have played it where 6's always count against the monsters, but I want to know for sure.