The ability "Iron Barbs" does 1/8 damage; I didn`t tested Rough Skin, but im pretty sure it does the same amount of damage.
So, I tested the "Rugged Helmet"
Pokémon Black and White have introduced a new item with the name Rugged
Helmet (rough translation). This item, when equipped, deals 1/8th (of full HP)
damage to the opponent when they use a contact move. For all intents and
purposes, it gives the bearer an additional ability in Rough Skin.
Well, it`s not 1/8; it need to take effect 7 times to faint the opponent:
I also tested Desseed (Iron Barbs) with "Rugged Helmet", the ability activate first, then the item; it takes 4 times to faint the opponent: