Hi Holly....There is a Deck Forum, you just didn't think to look under "P"

I'm in no way professional when it comes to decks, but I would use layers of cardboard or black plastic (temporarily!) to kill weeds now. You don't need to go to that much trouble under your deck as to digging and screening soil. I wouldn't leave the cardboard underneath when constructing, you will be creating many months of slug habitat in our climate. Black plastic left in place after construction tends to create a 'pond' effect, will hold rain and could create a moisture issue to affect the life of your wood.

Under a deck or porch is the only place I might use the aerated landscape fabric if building a new deck. ( Check your builder supply stores.) Actually, I replaced a worn deck last Fall, and found black plastic installed under...I wonder if that didn't contribute to the algae growth in winter. It must have been pond-like under there.

This wasn't a new area with anything current growing, so I didn't put anything down to replace the plastic. If an occasional weed finds it's way through the closely spaced boards, I'll pull it...

Here is a link that might be useful: Porches and Decks