question on T&G flooring

[Bob Rowlands]

Nice platform. Thanks for the pic. If that were my platform I'd install some 2x X bracing between the posts on the down hill side.

Since you are using beams on four foot centers it wouldn't hurt to eyeball the tops for plane from the edge of the platform. I'm thinking you won't need to plane anything unless any individual beam is radically off relative to the others. But you'll need to factor in how flat the overall beam framing is. If there's a big old crown in the top or the top is just simply out of plane somewhat, I'd forget planing and deck it. The tongue on the 2x deck will add massive strength to the floor. 2x6 T&G is absolutely gonna work great for you. Just screw it down well with plated screws.

Couple off topic comments about floor flatness. Fwiw floors need to be really out to feel the variation in flatness. The slab in my basement is all over the map for flatness I know because I built my house and bailed rainwater from the low spots during early house framing. TOOO many times. Lol. All poured residential basement slabs are radically uneven relative to the floor framing above.

Also, century old redos can have floors that are radially out. I've seen two inches in twelve feet redoing kitchen. You CAN feel that, but the occupants are totally used to it. As long as the cabs are plumb and level it really isn't that big a deal. Not an apples to apples comparison but thought I'd toss it in.

As for plastic, go for it. If it rains during decking you'll get to poke some holes.