new home construction, master shower slab floor. We get an earthy smell from the drain. Builder/plumber have removed gravel/debris twice. Trap water is clean, but you can not see any water unless you use a camera. We have bleached, backing soda and flushed the trap several times. Smell returns in 24hrs. My plumber and an independant plumber by the builder found that the pipe coming off the "P trap" slope backwards slightly and water lays in it. Everybody is at a loss for the smell, but all agree its not sewer. Any thoughts would help.

Just because I've run into this many times before - and it's always the same thing:

If the liner for the shower base was installed on a flat subfloor surface with no slope to the drain, or if the drain ring that crimps into the liner was slightly raised - either will cause water to pool between the liner and the underside of the mortar base. It will usually take a few months to start to smell and if you get in there with cleaners and bleach it will go away for a while, but it remains a mix of stagnant water and soap scum and can get quite "fragrant".

The bad news: The only way to tell if this is the problem without tearing everything out is to confirm with whoever installed the liner. If this is the problem, there is no solution other than removing the pan down through the liner and building it back correctly.

Not what you want to hear but like I said above, I've seen this quite a few times.

thanks for the reply. the builder is trying to avoid what you are suggesting. they are going to attempt this friday to again clean the drain and after that they are out of option. the smell stops when we cover the drain with a rag. when we remove the rag it smells like the drain. i know that the rubber ring that is around the drain and sits below the drain cap had to be banged down several months ago. i wonder if that rubber seal could be bad?

All that rubber ring does is create the seal between PVC drain and the PVC rough-in pipe that was set in the floor prior to the shower build. If it were leaking then drain water would bypass the drain and soak into the ground (you are on a slab...right?). The thing is, unless that rubber ring got cut or damaged, it's pretty foolproof and makes a tight seal. Any doubts can be fixed with a liberal application of plumbers Goop on top of rubber ring itself.

Shower pan drains drain from two areas. The main is the obvious open drain on top - run your shower, water runs down the drain and you're done. But pan shower drains also have small grooves and holes to allow for pan drainage. These holes are now surrounded by the mortar of the base and are really just 'weep' holes. Tile and mortar are porous and will allow moisture to travel through the tile and mortar to collect on top of the pan liner. This is the area that must drain through the weep holes in the drain. If those holes are clogged with mortar, or if the drain was installed so that the liner has to rise up over the lip of the drain (even by 1/8"), then water is going to pool between the underside of the mortar base and the liner. The weep holes that allow for drainage also make a path for any odor to enter the drain. This path is above the trap so the odor will go into the shower.

This is why it is important to have a sloped surface UNDER the shower liner before it is installed. A slight (1/4" per foot) slope will allow the water trapped between the liner and mortar base to drain towards the drain weep holes. All too often, the liner is installed directly on top of the flat concrete (or other subfloor). While the liner will not leak if installed this way, it will cause the odor problems that you are experiencing.

The only permanent solution I know of is to completely rebuild the base and lower portion of the shower.

The only non-invasive solution I know of is fairly short term: You can plug up the drain about 6" down, then fill the drain with bleach and let it sit for several hours. The bleach will flow back through the weep holes and treat the stagnant sludge that has collected above the liner. If you are lucky, you only have to do this 2 to 3 times a year to keep the smell in check.

thanks for the input. i will forward your thoughts to the builder for additional discussion.

(1) it is sewer. you said in the first post the line from the trap slopes backwards and water pools in the line. this is the first thing to fix... bust out the floor, fix the drain line so it has a slope of 1/4 to 1/2 inch per running foot DOWN to the main sewer.

(2) if they can't get a mud bed right, go to a Schluter base and drain system. you are likely repeatedly blocking the over-liner weep path into the drain through little weep holes. there is supposed to be fine gravel around the drain system protecting these seeps from the mortar and/or thinset. wife and I had to build one of these mud pans for the first time in our basement, and we were surgically careful (read as "slower than death by clean living") to get all those parts right.

(3) have you been bringing the inspectors in and up to date on this? eventually the contractors are going to stop returning your calls. they can't stop returning the inspector's calls.

update....the builder tore the whole shower out including tile walls. The plumber did not glue the drain pipe just below the shower pan drain hole. this pipe is actually a curve piece. Of that is a 12" straight extention leading to a "P trap". Our local county approved this. At first we thought this had fixed the smell issue. The finished the full work in March As soon as it got warmer and humid, the smell returned. Now I have noticed black mold on the pipe just visiable leading down the drain. I will say that when the whole problem 1st started, I had an independant plumber look with a camera. The said that the pipe leading out of the trap has a backward slope and water is laying in there. Could this water be fouling and the "P trap" is not sealing the smell? I have bleached and used other cleaning methods and nothing is working. The builder is talking about tearing the fiberglass pan up again. FYI....the plumber who created this mess filled for bankruptcy.

Thanks for the update. Sorry that you are still having problems, it sounds like you have eliminated most possible causes. So....

1. You have a fiberglass pan? If so, then much of my older advice from above is not valid in your case.

2. You found one PVC connection not glued - could be more further down the line encased in concrete.

3. That backwards slope out of the trap could be the problem. Consider this: Your shower drain runs into the larger drain from the toilet somewhere. If that junction is close enough, could be that some of the toilet runoff is backwashing into the backwards sloped shower drain on the downhill side of the shower trap. A trap can block some odors, but not everything.

4. Are you experiencing any other mold issues in this bath? Do the towels smell moldy at all? Just wondering about the mold collecting in the drain - if excessive moisture in the entire room could be contributing. Maybe try running a dehumidifier in the bath and see what happens.

You have eliminated most everything - not too much left to troubleshoot.

update: Builder brought in another plumbing company. The inserted a camera and did not see any leaks. He did not some supect areas, where the pvc had rough edges at the glued joints. The water flow looked correct from shower, toilet and sinks to the main. This company could also smell the musty odor and has no idea what to do. Since this is a slab issue and the shower sits to the exterior wall, the builder now wants to dig from the outside under to see if he can find water at the "P trap". He's trying to avoid tearing the whole shower floor up. Again, I'm the only slab floor home who has this issue. He wants to do this in the next 2 weeks. If there is anybody on this board who has heard of this musty smell issues, I welcome all suggestions. I bleached the drain and flooded the panagin Sunday to get a good flush, but the smell is returning.

I can offer no help with your problem, but would like to make an observation that you have a great builder. It's unusual that any builder would go to the lengths he is