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  1. #1
    Join Date
    Oct 2011
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    Default Question about drainage behind a retaining wall made of landscaping timbers.

    .....I am in the process of replacing a retaining wall made from 3x4 rounded landscaping timbers. Its about 4ft high and 50ft long with a 4x4 dead-man structure built into the yard. We're replacing it because some of the facing landscaping timbers are rotted out.

    .....The wall had no drainage system when it was built 15 or more years ago. I was going to simply install new facing timbers and put sheet plastic along the inside of the wall as a water barrier along with a drainage ditch at the top (12-18" deep & wide with leach-bed pipe and downspouts) to control run off.

    .....Will this work or do I have to dig the entire structure out and put gravel and drainage behind the entire wall? Or should I just put it back the same way it was?

  2. #2
    Join Date
    Dec 2010
    Location
    Houston Texas
    Posts
    2,969

    Default Re: Question about drainage behind a retaining wall made of landscaping timbers.

    The sheet plastic will do no good. It will trap water between the plastic and the wood.

    Dirt - landscape fabric - 3/4" or larger stone - timbers

    The landscape fabric keeps the roots and soil out of the stones so the stones can drain well and be as dry as possible

    The stone should be the type used for septic systems. Crushed 3/4" or larger works well. Too small and it will hold water, too large and the fabric can't do its job.

  3. #3
    Join Date
    Oct 2011
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    3

    Default Re: Question about drainage behind a retaining wall made of landscaping timbers.

    Quote Originally Posted by HoustonRemodeler View Post
    The sheet plastic will do no good. It will trap water between the plastic and the wood.

    Dirt - landscape fabric - 3/4" or larger stone - timbers

    The landscape fabric keeps the roots and soil out of the stones so the stones can drain well and be as dry as possible

    The stone should be the type used for septic systems. Crushed 3/4" or larger works well. Too small and it will hold water, too large and the fabric can't do its job.
    .....What I was going to do was have dirt - plastic - timbers; with a french drainage ditch at the top - 12-18 inches deep and wide with drainage hoses to carry the surface run-off down behind the timbers and release it out the front into the existing ditch.

    .....I'm trying to avoid digging the whole wall out and back filling it with gravel. Which I'll have to do by hand.

    .....I could skip the plastic and just put a drainage ditch at the top of the wall to catch the majority of the run-off. But what I don't know is whether or not that will handle the majority of the water that is actually moving toward the wall due to the slope.

  4. #4
    Join Date
    Aug 2007
    Posts
    7,195

    Default 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.
    I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!

  5. #5
    Join Date
    Jul 2010
    Location
    SoCal
    Posts
    6,736

    Default Re: Question about drainage behind a retaining wall made of landscaping timbers.

    You have two problems here.

    1. If your landscape timbers are sitting in the ground, they will rot and decay. Some slower than others, but they will all end up the same way. And this is the reason why plastic won't help you here.

    2. to collect rain and other water and to divert it away will require building a concrete U shape channel, along the 50 ft fence, with enough slope to carry the water away. This is better than laying gravel in my opinion, because the rock will allow the water to stay while a slopped channel won't.

    I once had to solve a similar situation: a driveway below street grade, allowing rain water down into the garage. My solution was to cut 12" off the driveway, where it met the garage floor, install a concrete channel with metal grates on top. The water ran down the driveway, into the channel and off to the side of the driveway. Problem solved.

  6. #6
    Join Date
    Oct 2011
    Posts
    3

    Default Re: Question about drainage behind a retaining wall made of landscaping timbers.

    Quote Originally Posted by dj1 View Post
    You have two problems here.

    2. to collect rain and other water and to divert it away will require building a concrete U shape channel, along the 50 ft fence, with enough slope to carry the water away. This is better than laying gravel in my opinion, because the rock will allow the water to stay while a slopped channel won't.
    .....I was going to install a french drain at the top of the wall to handle the run-off and reduce the amount of water heading for the wall, I just don't know if that would be enough or how deep I should run it. Would that serve in the same capacity as a concrete ditch?

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