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What is the best material to use behind a wood retaining wall for water? - toxic roofing material florida

I install wood walls. The boys enter the area behind the walls, and I have the best material behind the wall to use to prevent moisture and / find water. I was told, tar paper / cover, but you do not know how "eco". Furthermore, the application should not be painted, want to tar as parentheses. We think this is very toxic.

Any help is welcome!

Thank you!