Raising Hero attributes to 16

I've been wondering how to increase an attribute line to 16 on heroes. So far, I have only been able to raise it to 15 by adding a superior rune. How can I get it to 16?

Thanks.

Are you putting the superior rune on the headpiece? Their helm has +1 to whichever attribute rune you place on it.

Well, that explains it. I always equip the sup rune on the gloves. Thanks.