Setting grade on a patio with retaining wall

Discussion in 'Hardscaping' started by Stanwood, Jul 11, 2018.

  1. Stanwood

    Stanwood LawnSite Member
    Messages: 3

    First post here on LawnSite (be gentle). Hopefully this is a really obvious (stupid) question.

    I'm building a dry-laid patio that will be bounded on two sides by the house and two other sides by a dry-laid retaining wall. Part of the wall is pre-existing (lighter grey) and will be extended as indicated (darker grey). The retaining wall intersects existing concrete steps. The patio surface will be flush with the top concrete step and the retaining wall cap. I'm looking for feedback on how to grade this patio for drainage.

    In the attached image I've indicated the elevation (in inches) relative to the top step. Based on my survey work this is at the same level as the existing retaining wall (within 1/2"). I plan to extend the existing wall on a level grade and slope the patio up towards the house by 4". This will create an 1/8" to 1/4" slope for drainage away from the house and flat patio at the wall cap. Does this look correct?

    I'm a little nervous about puddling in the lower right corner of the patio which will be quite flat. Anyone have experience with this sort of configuration? I could lower the existing wall by one block (5") but I think this just creates problems elsewhere.

    Patio grade.png
  2. zedosix

    zedosix LawnSite Silver Member
    Messages: 2,678

    You've got a large area that's relatively flat, is it possible to slope to the middle of the patio and install a center drain?
    What about installing a drain in the right corner if your worried about water pooling there.....
  3. Stanwood

    Stanwood LawnSite Member
    Messages: 3

    I hadn't thought about using a drain. A center drain 2" below the patio edge would give between 1/8" (N-S) and 1/4" (E-W) slope per ft. Draining to daylight out the side wall is manageable. The lower overall grade means more digging and maybe 2 risers for the steps to the doors. But there's a nice symmetry to it. Thanks for the suggestion!

    Anyone with experience sizing a drain for run-off on a dry-laid patio (minimal seams)? From what I gather I should use a runoff coefficient of 0.70-0.80.
    Patio grade 1.png
  4. Mr. Jon

    Mr. Jon LawnSite Member
    from Jersey
    Messages: 155

    Any type of pavement I figure 100% runoff. 540 SF 3" per hour rain = 17 gal/min., 13 sq. in. grate open surface area connected to a 2" pipe.
    2018-07-12 08.33.25.jpg
    hort101 likes this.
  5. Stanwood

    Stanwood LawnSite Member
    Messages: 3

    Thanks! I like the old-school calculator.

    It's 450 SF (not 540). With your other assumptions that gets me to 14 gpm. The paving stones will be big (2'x2') so I may cut in something larger like a 9" sq or even 12" sq catch basin to 4" smooth pipe. That could handle a 100-yr peak (5 min) deluge in my area: 9.1 in/hr --> 42.6 gpm.
  6. Mr. Jon

    Mr. Jon LawnSite Member
    from Jersey
    Messages: 155

    You're welcome. I got the calculator from NDS at a trade show. Yeah a 9 or 12" catch basin would work great.
  7. AGLA

    AGLA LawnSite Bronze Member
    Messages: 1,796

    I'd pitch it at 1% (1/8" per foot +/-) away from the house and catch it in either a channel drain or trench drain on the opposite two sides.

    A channel drain is a manufactured product with a grated top. A trench drain is an excavated trench filled with gravel - I'd put a 4" perforated pipe in it to carry the water to a drywell or to daylight to make sure that it will keep up.

    I would not pitch it to a drain in the middle. If the drain clogs or can't keep up you'll have a puddle or icing depending on the climate.

    You might have a building code issue to deal with as well which is whether or not you are required to have a safety railing (patio at or above 30" next to adjacent grade. You could just berm up a planting bed on the lower side of the wall so that it is less than 30" lower.
    hort101 likes this.

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