Q: Runes that proc a barrier like Scourge

I was reviewing the list of new runes that proc barrier like Rune of the Scourge and suddenly wondered whether the barrier proc'd before damage on the hit was calculated or after damage on the hit. That is, does the damage taken come out of the barrier, first, or does the barrier happen after taking damage?

Barriers do not last long at all so, getting one after soaking full damage from a hit seems almost worthless and counts on taking another hit right after the one that proc'd it to provide any benefit.

Does anyone know how in what order things happen with rune barriers?