A drain has to sit below the water it is meant to intercept and above the level its outfall can accept. For a wet yard that is usually a foot to two feet. For a foundation it is at or just below the footing. Depth is set by those two constraints together, never by a rule of thumb.

How deep the water sits here

In Layton the shallowest mapped seasonal water table sits 29.9 inches down, in ground mapped largely as Parleys loam, 0 to 4 percent slopes. Two and a half feet is deep enough that a yard drain at a foot to eighteen inches sits above it, and shallow enough that a foundation drain at footing level may not. That is the practical dividing line on this ground: a surface job and a foundation job here are asking about different water, and only the second one has to reckon with the seasonal table.

The reasoning behind that

Water travels to a drain because the drain is lower. A trench above the level the water is moving through catches only what falls into it from the surface, which is why a shallow drain in a wet yard so often disappoints.

At a foundation the target is the footing. An exterior drain set at footing level intercepts water before it reaches the wall. Set higher, it lets water through beneath it and collects what is left.

The ceiling is the outfall. A drain must fall continuously to wherever it ends, so the depth at the far end is the depth at the start plus the fall. If the outfall is shallow, the start cannot be deep, and the honest answer becomes a different design rather than a deeper trench.

Every depth also has to clear whatever is buried on the line. That is a locate call before a shovel, not after.

More on this: Exterior French Drain and Waterproofing in Layton, UT.

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