I just installed a p trap under my drain in a new shower, previously a tub, water will not flow out of the trap and I have grade, no blockage anywhere, what's happening?

It has to flow, you are saying the water is just not going though the trap, backing up into the shower ?

Right, it just overflows back into the shower

Pure gravity would cause it to flow down hill. So there has to be a blockage somewhere if this is happening. Have you opened the pipe on the P trap and followed to where the blockage is happening. Pure gravity and water flow forces the water to follow to the losest level ( which is why drains work)

i just installed a p trap under my drain in a new shower, previously a tub, water will not flow out of the trap and i have grade, no blockage anywhere, what's happening?
We ended up cutting out the trap and put a hose in the line, no blocks, tomorrow we will use a camera/snake and look for venting problem, plumber was stumped when the line was clear and when we reattached the trap again it wouldn't allow water to flow through it

The trap must be causing a vacuum of some sort not allowing the water to go out. Is there a vent on that line ?

I would have thought so since we just tore out a bathtub, wouldn't that have been vented?

Should have and is, are not always the same thing. Back in the day when I actually did some real plumbing, I did a lot of older homes that had so much patch work, no code plumbing it was a sin.
If this is a PVC trap, they are only a couple of dollars, have you considered using a different design, also if yours has a clean out on it, have you opened the clean out a bit and tried to break any caccum that may be happening.

I am not up on all the fancy things, I am sure one of the "real" plumbers will come on here, I do actually have my license, but it was grandfathered in years ago.

Thanks for your input, it is the cheap pvc type and doesn't have a clean out so we couldn't experiment with it, will let you know what they find tomorrow concerning the vent, it's all greek to me

Yes, I just put one in on my washing machine drain, for some reason, it did not have one. Getting to old and fat to be in the crawl space any longer.

i would have thought so since we just tore out a bathtub, wouldn't that have been vented?
Not so! Back in the old dayze we installed bathtubs with drum traps and vented them through the tub waste and overflow. We didn't run a roof or wet vent on them. Chucks correct, There has to ba a blockage somewhere. If it were a vent problem you would have some drainage. Cheers, Tom

So if they removed the overflow pipe from the wall I would lose my vent? Can they revent, walls are all finished tile, just need to pour cement floor and shower is done

Hi father chuck,

There was no vent attached, fortunately there is a vent for the toilet and sink and the latter is sharing the shower wall, plumber will use a t and attach to this vent, I've got my fingers crosed

John

Let me know, I buy and remodel so many old homes, one did not even have an indoor bathroom once. But this was a new one for me
Hope I did not step on any toes on the plumbing, I don't normally come over here since I am not up on the new codes

Hey father chuck,

Thought you'd like to know how it turned out... we tapped into the vent off the sink, the shower is finished and runs perfectly, glad to learn something new at my age!

John