I'm looking for the way to make the pressure treated wood in my project last the longest possible. I want to put a piece of plastic (black) sheeting to protect the 2x12s being used for a new retainer wall. It will then be backfilled with dirt against the plastic, holding the plastic in place. Will this make the 2x12s last longer, or will it increase the rot time of the PT wood?
Will plastic sheeting against a wooden retaining wall preserve the wood
landscaping
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Wood exposed to earth, or even gravel, as you discovered, is a compromise at best. You can hedge your bet by using treated wood. Use wood with the maximum amount of preservative. "Outdoor wood" is not enough, the preservative levels must be at least adequate for ground contact. Preferably wood treated for foundation service should be used. Doing something to ensure any water that does contact the wood can easily drain away is also important, as well as trying to prevent water contact in the first place.
9" of soil does not impart much of a lateral load, so almost any retaining structure will suffice. Still, it would be better to not involve the fence if possible IMO. How far back from the existing structure would depend on what you end up doing to retain the extra earth. You could extend the existing structure, which would obviously have 0 offset. At the other extreme, relying on gravity to retain the earth, such as placing a row of large stones, you would want to see a minimum 45 degree slope between the existing wall and the tops of the new structure.
By using an impervious retaining material like stone or concrete blocks, you can backfill with plain soil, no gravel is required for minimal height walls. Taller structures often backfill with stones to prevent hydrostatic pressure from pushing on the wall. When back filling against wood, do everything reasonably possible to prevent water from contacting the wood, even if it is treated.
That depends partly on the fence design. If you're doing something edge-to-edge, installing water-swollen pickets will result in gaps once the wood dries out.
Otherwise, it's more likely that the pickets will warp if left to dry quickly in the sun. The side that dries out first contracts, causing curvature.
That said, pressure-treated lumber is extremely wet to begin with, so it may not matter much. If you get any severely warped pickets you might just have to change them out.
I'd buy an inexpensive tarp and keep it all dry until it's up. Why make the project more challenging than it already is?
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I'd go with dimple board instead of the plastic. The dimpleboard will allow water to drain to the bottom of the retaining wall.
If you just go with plastic, assuming the retaining wall has air on one side, I'd expect it to last longer than without plastic. I assume the soil being retained is wet more often than dry.