Depth of french drain?

[Karen]

Contractors are recommending different depth for installing floor drains, ie french drains. One says 12" to better drain the water, another says 4" as anything deeper affects structural integrity of the foundation walls. What's the "industry standard?

[TJ1962]

Standards are borrowed from plumbing specifications, which makes no sense.

Pitch in plumbing pipes are designed to keep flow of solid waste. yet these pipes are rarely laid horizontal with foundation footer, and only undermine a small section going out to the street.

If footer is 10 inches and pipe trench is 20 feet long, it is laid level. Why undermine to foundation to produce a pitch when there is no solid waste. Water will seek a level and flow even if laid level.