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Septic drain field

arnoldk | Posted in General Questions on


I know this isn’t topic isn’t really related to this site but I cannot seem to get a hold of the engineer by phone and I only seen to get one e-mail response from them per week, Monday’s typically. It’s taking a long time to have a simply 5-10 minutes phone conversation. 

The engineer has advised me that I will need to cut more tree at the back of the property to accommodate the drain field. Can anyone explain why the drain field outside (red line) is twice as big as the area that contains the pipes? 

Everything is in meters since I live in Ontario Canada.

Thank you,

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  1. Expert Member
    PETER G ENGLE PE | | #1

    Roots mostly. You don't want tree roots growing into your drain field.

  2. Expert Member
    BILL WICHERS | | #2

    The drain/leach field is the area of ground that "absorbs" the runoff. The pipes just distribute the liquid. Think of the red line as being like the keepout area around a machine on a factory floor -- the area is bigger than the machine, since you need space around the machine to do work. You don't want anything inside that keepout area that would interfere with the proper operation of the machine, the same way you don't want things in the leach field that would interfere with the ability of the ground there to handle the runoff from the pipes.


    1. arnoldk | | #3

      Thanks Bill. I know you can't plant anything with woody or large roots (trees, shrubs) on top and a certain distance of the pipes but what about the bottom portion II believe it's called a mantel) where there isn't any pipe?
      It seems like the drain field is awfully huge for such a small house with only 3 bedroom, 2-1/2 bathroom.


      1. Expert Member
        BILL WICHERS | | #4

        You'll want to run that by the engineer who did the design. There can be all kinds of reasons for the way the design was done, maybe terrain, soil type, etc...


  3. AlexPoi | | #5

    Here, if it's a regular leach field, you need a reserve expansion area for future leachfield repairs. This expansion area must be capable of accommodating a duplicate of the approved leachfield in case the field become saturated and you need to move it.

    This is not the case with a advanced system so that may be an option if you don't have the space but it's more costly.

    1. andy719 | | #6

      I was going to say something similar. Where I am you are required to reserve an area large enough for three systems. You only have to clear the area for what your current system need is though, not the two future backup areas.

  4. arnoldk | | #7

    I finally got a hold of the engineer and the bottom portion of the drain field is what they call a mantle. In our area that is common for raised drain field which is part of the soil filter.

    I was told that these days it's very rare for drain field to be in-ground because of the soil requirement due to tighter regulation. You are pretty much stuck with a raised drain field of some type but they height and size will be determined by the type of system you use (Advance system vs conventional).

    Thank you,

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