The drain from my washing machine ties into a stand up shower that we have never used. Whenever the washer pumps out, the water backs up into the shower floor to a depth of about a half inch and all that wonderful stuff in my 40 year old drains creates a complete mess.

We would like to replace the shower with a new separate pan and surround. Is there some type of back flow preventer or check valve that we could install to keep the water in the drain pipe instead of the shower floor? And would we need to raise the level of the floor to make room for such a device? My house sits on a concrete slab, so there’s not any access from underneath.

I don't see where there's any type of vent for this run of drainpipe. I have 3 vents on my roof: 1 for each toilet and 1 for the kitchen sink - dishwasher drain (although, again, I've never installed a dishwasher for fear of a similar problem). The pipe for the washer is 32" above the floor. I could put a tee and run another length up for a vent and see if that alleviates the problem. How high should this be so that the washer discharge does not fill up the vent and overflow into the closet?

We’re at a standstill with our remodeling project because the shower has to be fixed before we can finish the floors and carpet. Any help would be greatly appreciated.