On May 8, 8:51 pm, nospamgoingjag wrote:
Please don't think me insane, but when my dishwasher empties sometimes
(of course not all the time) it causes a leak in the floor above it!

This has been working fine for 15 years and the only thing that has
changed recently is the movement of the washing machine about 15 feet
further away than it used to be in a new addition. Because of the
original sill plate being there, the pitch of this waste pipe is
probably a little less than optimal, but seems to drain fine with
about a 1/2" pitch down to it's original connection.

In the area of where the leak shows up is the vent pipe going out to
the roof, but water going up a vent pipe? and leaking besides?

There are two sink drains and a garbage disposal with the dishwasher
connected to the garbage disposal, which is in turn plumbed into the
same pipe as the two drains. there is no air stack connected to this
set of drains, so where the pipe would normally continue up to an air
stack, there is an "air gap". A small plastic valve. used for these
kind of situations.

Since I was sure this was an absurd situation I filled both sinks up,
pulled the plug and let them rush out - no water leak. That had to be
abot 10 times the amount of water that comes out of the dishwasher
during a given cycle. I would have thought it would be at a similiar
or greater preasure as well, but obviously I can't recreate a motor
pushing water out.

I changed the "air gap" just in case, and thought I had the problem
solved, but as I was sitting doing email I heard the drip behind me -
as the dishwasher was emptying on about it's 3rd cycle.

Near the vent pipe is a toilet and near that a sink. Neither cause
any problem when used.

There is also another vent pipe on the other side of the house.

The dishwasher is on the first floor, it's waste water goes into the
basement to go out to the septic. That run for the waste water is
actually closer to the other air stack that the one that seems to have
the leak.

Sorry about the long post, but wanted to include as many facts as
possible.

Thanks in advance for any ideas.

Sounds like a venting promblem your machine is putting the water in to
fast for your drain to handle, is the cheater vent higher than your
drain if not putt it up till it is higher than your sink drain