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Until recently, I had no problems with a washing machine plumbed into the
kitchen waste via a standpipe as follows:
http://tinyurl.co.uk/cee3
.... standing in for ...
http://s28.photobucket.com/albums/c2...gAnch=imgAnch2

From bathroom
| |
| | Main Soil Stack
| |
| |
| |
| |
| |
| |
| |
| |
| |
| |
| | | | Washing
Machine
| | | Sink | | |
| | +----+ +-----+ | |
| | | | | |
| | | +---+ | |
| | | | | Standard Sink U-Trap | |
| | | | | | | |
| | | | | | |
| | +---+ | | |
| | | | | |
| | | | | |
| | | | | |
| | | | Standard U-Trap | |
||||----------/ /-------------+--------------------------------/ |
||||------------------------+ | +--------------------------------/
| | | |
| | +---+
| |
To sewer

When I acquired a small dishwasher, I adapted the waste plumbing as follows:
http://tinyurl.co.uk/33lv
.... standing in for ...

http://s28.photobucket.com/albums/c2...gAnch=imgAnch1

From bathroom
| |
| | Main Soil Stack
| |
| |
| |
| |
| |
| |
| |
| |
| |
| |
| | | | Dishwasher Washing
machine
| | | Sink | | | | |
| | +----+ +-----+ | | | |
| | | | | | | |
| | | +---+ | | | |
| | | | | Standard Sink U-Trap | | | |
| | | | | | | | | |
| | | | | | | | |
| | +---+ | | | | |
| | | | | | | |
| | | | | | | |
| | | | | | | |
| | | | Standard U-Trap | | | |
||||----------/ /-------------+------------------------/ /-----/ |
||||------------------------+ | +--------------------------------/
| | | |
| | +---+
| |
To sewer

The discharge flow seems to be adequate in that I don't get *water* coming
out anywhere, even when both machines are running together. However, I do
get *soap froth* seeping out of the top of the dishwasher standpipe, and all
over the cupboard under the sink, as the washing machine empties.

What have I done wrong, and what is the cure?

Any help gratefully received ...