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The term Underflow has been created by The SpoutOff™ to explain the observable fact that rain water will flow off a roof into a rain gutter, down the downspout and away from your house even when the gutter has debris in it . . . IF THE OUTLET IS CLEAN.

Because water always follows the path of least resistance, rain water will fall from a roof into a gutter and flow through debris in the gutter to the downspout. Water always flows with the pitch of the gutter, under the debris (Underflow), to the downspout.

If the outlet (hole in gutter at top of downspout) is clear, water will fall from the gutter, through the outlet, down the downspout and away from the house… as it is supposed to.

HOWEVER, if the outlet is clogged:

  1. Water will NOT fall through the outlet into the downspout
  2. Water will back up into the gutter
  3. Water will wet debris in the gutter causing that debris to become “mud-like”
  4. Water will overflow the gutter at both the front and back of the gutter
  5. Water will run down the face of the house, damaging the house
  6. Water will wet the ground at the foundation of the house, soaking and damaging the basement of the house

The lesson of Underflow is this:

As long as the outlet is clear, rain will flow down the gutter, through the outlet, and down the downspout, even if there is debris in the rest of the gutter. Water will move away from that house or building and will not cause damage to that structure.

However, if the outlet is clogged, water will never flow out of that gutter even if that gutter is perfectly clean! Water will back up, overflow the gutter and, over time, cause damage!

By simply keeping the outlet clear, water will always flow out of the gutter, down the downspout and away from the house. Keep rain gutter outlets clean and gutters and downspouts will always work as designed.