A single joint will not jab the roof of the mouth unless the horse is over flexed, the reins held and the bit too wide for the mouth. Pain on the bars/tongue (too much unilateral action as well) is much more likely. That said, the bit should be high enough (esp when first bitted) with two wrinkles. The horse must draw the tongue quite far to get it over the bit. Also, making the sure the horse is (allowed to be) up/open is important. If a handler is going to lunge a horse (with side reins), it is important to attached them to the caveson first, so the horse learns about routine light pressure/hh w/o the attachment to the bit/mouth. Teaching the horse how to properly respond to the bit with work in hand is also helpful for such horses.