Re: Question about drainage behind a retaining wall made of landscaping timbers.
The surface drain you're considering is certainly better than nothing at all, but it is still going to allow earth to wood contact, which is going to rot out any type of timber you use for the retaining wall, regardless of what it's treated with.
What causes rot is the inability to dry out. Gravel behind the wall will allow water to pass and not sit against the wood members, where it can create rot. The surface drain will certainly help to divert water before it can penetrate to the retaining wall, but moisture will travel and be held by any dirt that is in contact with the wall.
On one hand, you can look at that fact that the existing wall lasted 15 years. That's not too shabby and well beyond the stay of average homeowner. On th eother, if you're going to be in the house for at least another 15 years, then it's worth installing a drainage system into the new wall so that it will last YOUR lifetime in the home.
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