Very smart, dpach. You installed a membrane to prevent water penetration between members of the built-up beam.
I also put Blueskin on every joist as well since most joist rot occurs when water runs off the floor and under the deck boards and soaks into the joist around the screw holes that hold the deck boards on. The membrane is rubber with a tar layer underneath to seal around the screws. Also used the Bakor glue first as extra adhesive.
It cost a whooping $250 to do a 20 x 14.5 foot deck......peanuts really.