Anyone a commercial roofer?

[Got Stripers]

I have a 4 foot by 55 foot upper deck, which causes issues below when it rains, so I'd like to pull the decking and lay down some thin plywood and rubber roofing. The current decking is already too close to the shingles, so I don't see me working any under the shingles, unless I put down thinner decking; but I hope I can reuse what's there. I wouldn't think with 4 feet I need to pitch it. I'm thinking with rubber roofing, the plywood might not need to be weatherproof, plus I don't know that they make a thin weatherproof plywood.

Anyone know someone on the south shore or southeastern mass I might be able to get some remnants from, with maybe a small amount of primer and glue? I don't know if Loews or Home Depot sell rolls and if they do, it might be far more than I need for this small project, thoughts?

ps: Looks like I can get what I need from Home Depot, but the pool liner is a third of the price of roofing EPDM and I'm wondering why that wouldn't be sufficient, or maybe a roll on coating over the plywood.