Ok is there some deck guru out there that can answer this for me. I want to do Fiberon Horizon decking on my new home. I'm doing a mostly (all but 1.5') covered deck in the very rainy climate of Oregon. The deck is being constructed using a waterproof system (uh theres probably a better name for this) where there is a waterproof membrane and a sleeper system above it. Come to find out Fiberon wants to dictate 6" or clearance underneath their line of decking..and we don't have it with this type of install.

So my question is...why does Fiberon require so much darn space under their capped composite product when something like Earthwood Evolutions (Timbertech) says they just require a minimum of a 2" (There must be a minimum of a 2" (nominal) sleeper system supported by and connected to the substructure over which the deck is to be built.)

Am I missing something obvious? I know which deck product I want and like the look of but I don't want to void my warranty right out the gate >