Waterproofing a wood porch directly over a garage

[Chris Heeren]

I'm closing on my first Rental Property this month, a Duplex that is currently 100% occupied. The biggest concern with the property is the wood plank porch was built directly over the garage without any waterproofing. The owners then installed insulation and drywall on the Garage ceiling and allowed rain water to seep down between the porch boards and soak up in the insulation/drywall. I'm planning to pull up the porch boards to repaint and was wondering what type of waterproofing would be recommended to ensure that when I replace the insulation and drywall that it doesn't turn into it's current state in another 10 years.

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Is there a type of spray on water proofing that would look okay on a front porch? Would pulling up the wood boards and placing a plastic membrane layer directly to the joists before reapplying the boards work? Any help would be appreciated.