Every deck I've ever built that the rim joist was going to be covered I've counter bored just enough that the heads on the carrage bolts sit flush when tight and they all passed inspection and it was never questioned by the inspector.
Any composite fascia I've used is to thin to be trying to counter bore on the back side, if I did do it I'd be using Forstner bit, not a spade.
Composite fascia is hard enough to keep laying flat against the rim joist, I just can not imagine using shims so it's not fully supported.
Why would there be anymore chance of it causing rot then a doubled up beam, or doubled up rim joist?
That concerned, then apply a bead of construction adhesive along the back side of the fascia at the top of the rim joist, and or apply a strip of window and door flashing before installing the decking.
Any composite fascia I've used is to thin to be trying to counter bore on the back side, if I did do it I'd be using Forstner bit, not a spade.
Composite fascia is hard enough to keep laying flat against the rim joist, I just can not imagine using shims so it's not fully supported.
Why would there be anymore chance of it causing rot then a doubled up beam, or doubled up rim joist?
That concerned, then apply a bead of construction adhesive along the back side of the fascia at the top of the rim joist, and or apply a strip of window and door flashing before installing the decking.
