We are redoing the master bathroom in a house we just purchased. We have the washer drain pipe tied in with the shower drain. The shower drain is within 3 feet of where the drain goes below the slab. There is a vent going up to the roof in the middle of the washer drain line. When we run the washer, the water is backing up into the shower drain. It seems to start coming out as bubbles first.
I have tested with running water through a garden hose into the washer drain at full blast and there is no backup. Could it be the detergent bubbles causing the backup?
Would a second vent closer to the washer help? Or is it just the higher volume from the washer not able to go through the drain as it is? We have 1.5" pvc pipe for the drain lines.
Thanks for your help. Any suggestions are appreciated.

It appears that you have a partial clog downstream from the shower and washer. Here's how the scientific part works: the "bubbles" are a sign of water hitting bockage, and then backing up, so that's clue number one. What's more important is that the washer is a pumped appliance (meaning it gets rid of its water using a pump, rather than relying on gravity), which leads to situations like yours. Here's how it works: the drain line is only partially blocked, so gravity-powered wastewater seeps through slowly; but it gets through before enough of it has arrived at the blockage to cause major problems. The washer pump, on the other hand, has two major factors: a) it's pumping a LOT of water into the line, and b) it's pumping it VERY FAST. As a result of so much water trying to get through the narrow opening in the blockage so quickly, the water builds up at the blockage very quickly, sort of like a holiday weekend traffic jam (except that I would never want to spend my holiday weekend in the place where that water is headed ;)). With nowhere else to go, the water backs up out the nearest fixture - in your case, the shower.

This also explains why you could not reproduce the problem with a garden hose: it does not produce the volume and velocity that a washer pump does. It also explains the bubbles you're seeing; they're merely the symptoms of blockage.

Solution: The drain line should be snaked.

Good luck,
Moishe

Oh, and by the way, most codes require 2" pipes for both washer standpipes and shower drains. Why do you have 1-1/2" pipes?