Ask Me Help Desk - Subfloor and P-trap for basement bathroom

Hello,

I am in the process of finishing my basement. My question is in regards to installing the subfloor and p-trap for the shower. First 2 photo's show the rough in plumbing and subflooring from the front and side. On the side view, the stairs to go upstairs are right behind me. The next photo shows a CAD drawing of the existing and proposed conditions for the bathroom.

As you can see I have a 12" gravel filled box (dap-out) where the p-trap will go. According to the building inspector, once I have the p-trap installed, it needs to get inspected for underground plumbing, then I need to backfill the hole with concrete to cement the p-trap in place. I would then install my subfloor (cut to fit around vents and pipes) and then frame the walls so I can place the tub/shower combo in place. If I do this, and the drain is not in the exact location I need, then I have a problem. It seems like i would need to wait to backfill the hole with concrete until I have the walls up, so I can get the location of the drain correct, but I can't install the wall till the floor is done, and can not do that till the concrete has been poured, so what do i do, in a catch 22???? I want to wait on pouring the concrete so I can have a little play with the p-trap to make sure it is in the correct location.

The subfloor is a 2'x2' square tile, with XPS foam on bottom, and tongue and groove OSB on top, approx 1" thick. What I was thinking of doing was to install the p-trap, lay the subfloor, and cut out a 16"x16" box in the subfloor (larger than the dap out for the tub), finish the rest of the subfloor, frame the walls as much as I can, verify the drain location with the tub/shower, get the p-trap in the correct location, then backfill with concrete, put the subfloor that was cut out back in, finish the walls as needed, then the p-trap drain should be in the correct position for the tub. Any comments or thoughts on my idea or any other ideas on what could be done?

Also, when I lay the subfloor around the toilet drain, how much larger should I cut the subfloor to fit around the pipe, 1/4", 1/2" diameter larger? Do I want to cut the toilet drain pipe to be flush with the top of the subfloor, and then install the toilet flange on the SUBFLOOR, or should the toilet flange be on top of the FINISHED floor?

Thanks in advance,
Steve

I see a couple issues. First is you are going to have your waste and overflow up against an oustide concrete wall. Going to make it a bear to fit the pluming up , just no room to work. Second, you never cement the trap in place, it will be covered with dirt, sand, or gravel, then the riser would have to be wrapped to keep it from being in direct contact with the cement. Plumbing pipes are never to be poured directly in concrete. Normally, we rough in exactly for shower only. With a tub/shower combo, you have the dap out box so you can install the trap and hook up the waste and overflow AFTER the walls are up and the tub/shower unit has been set in place. Something seems a miss either with your interpretation of the inspector, or maybe the inspector himself? Anyway, as for the closet/toilet flange, yes, you want it to sit on top of the finished floor for best job here, okay. And leave roughly half inch all the way around the pipe to allow room for the closet flange to fit. Good luck with your project, and please keep us posted on your progress.

PS. I can see none of the pvc pipes where wrapped, but instead poured directly in concrete, "sigh". Oh well, I guess the inspector isn't worried about that? I will admitt, I have never seen a subfloor laid over concrete, so I may be a bit out of my experience zone here. In the pics, everything is looking pretty darn good so far. Keep it up.

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Originally Posted by mygirlsdad77

I see a couple issues. First is you are going to have your waste and overflow up against an oustide concrete wall. Gonna make it a bear to fit the pluming up , just no room to work. Second, you never cement the trap in place, it will be covered with dirt, sand, or gravel, then the riser would have to be wrapped to keep it from being in direct contact with the cement. Plumbing pipes are never to be poured directly in concrete. Normally, we rough in exactly for shower only. With a tub/shower combo, you have the dap out box so you can install the trap and hook up the waste and overflow AFTER the walls are up and the tub/shower unit has been set in place. Something seems a miss either with your interpretation of the inspector, or maybe the inspector himself? Anywho, as for the closet/toilet flange, yes, you want it to sit on top of the finished floor for best job here, okay. And leave roughly half inch all the way around the pipe to allow room for the closet flange to fit. Good luck with your project, and please keep us posted on your progress.

PS. I can see none of the pvc pipes where wrapped, but instead poured directly in concrete, "sigh". Oh well, I guess the inspector isnt worried about that? I will admitt, I have never seen a subfloor layed over concrete, so I may be a bit out of my experience zone here. In the pics, everything is looking pretty darn good so far. Keep it up.

We did not do any of the rough in, that was all there when the house was built. When I first seen it, I couldn't quite figure out how the bathroom was supposed to be laid out, after doing tons of research and drawing up sketches on CAD, finally started to figure out how to lay it out. We ended up buying the house brand new once it became a spec home, and the contractor just wanted to sell because original people lost financing. There are 3 models of homes in our subdivision, and every house that had rough-in done by contractor, for this model layout, all had the same layout for rough-in, so they all used the same bathroom layout. If I had this house built myself, I would have done it differently. The rough-in was passed for inspection back when they built the house. (4 years ago).

From the autocad drawing, you can see that our back wall of the bathroom will be off the concrete foundation by maybe 6-8", so it will leave some space, but not much, that's just how the rough in was done.

As for the subfloor, we wanted to lay the subfloor on the concrete to give some r-value for insulation and to also create a vapor barrier. We basically followed the info on this site http://www.homeconstructionimproveme...oncrete-floor/. From the website, you can see they recommend to use 1" XPS foam board, 3/4" sleepers and 3/4" T&G sub-flooring, then the finished floor. Doing that we lose 2.5" of headroom for just the subfloor, and we wanted to keep headroom loss to a minimum, so we used a product called barricade subfloor tiles, can be viewed at www.ovrx.com. Only lose 1" of headroom. This gives the finished floor a softer feel, subfloor is quite strong, has r-value of 3.2, and is a vapor barrier. The product we used is similar to Dricore, sold at HD stores.

Would you recommend to layout the entire subfloor, put up the walls, and place the tub in the correct location, and then cut out the subfloor, directly below where the tub will go, so the tub will then sit on the concrete floor slab?

Thanks,
Steve

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My question is in regards to installing the subfloor and p-trap for the shower. I need to backfill the hole with concrete

Here we have a tub dap out that ypu're c0nverting to a swhower. But here,

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so I can place the tub/shower combo in place.

we mhave a tub going in the same place. Can't have it both ways. Either you installm a shower and cement the hole or install a tub and leave it open. So which is it?
We don't wrap our PVC drainage coming up put of the slab but we do wrap our copper water pipes. Can't answer your subfloor question until I know what's going in. Tub or shower.
Venting looks good. I assuming you will tee off to pick up a vanity off the toilet vent. Good luck, Tom

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Originally Posted by speedball1

Here we have a tub dap out that ypu're c0nverting to a swhower. but here, we mhave a tub going in the same place. Can't have it both ways. Either you installm a shower and cement the hole or install a tub and leave it open. So which is it?
We don't wrap our PVC drainage coming up put of the slab but we do wrap our copper water pipes. Can't answer your subfloor question until I know what's going in. Tub or shower.
Venting looks good. I assuming you will tee off to pick up a vanity off the toilet vent. Good luck, Tom

A tub is going in place, which will have a shower head installed (hence the tub/shower combo), that was my bad with regards to only calling out shower. I thought I usually called out the combo.

Tom, Im pretty sure most codes today call for some sort of barrier between the pvc drainage and concrete? Maybe just upc?

If that's the case, then for the 4 years from 2004-2008 that our subdivision was being built, the contractor did not wrap the pvc for any of the rough-ins that I have seen. I talked to the building inspector and he had said all the rough-ins were approved when the houses were built.

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Originally Posted by svaic00

Would you recommend to layout the entire subfloor, put up the walls, and place the tub in the correct location, and then cut out the subfloor, directly below where the tub will go, so the tub will then sit on the concrete floor slab?

Thanks,
Steve

Hi Steve,
Your inspector's correct. The only thing we wrap is the water pipes coming up out of the slab. I can see no advantage to wrapping Schedule #40 PVC.
When we set our tubs the walls are up but we install them on the concrete and then tile around it. Good luck, Tom

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Originally Posted by speedball1

Hi Steve,
Your inspector's correct. The only thing we wrap is the water pipes coming up out of the slab. I can see no advantage to wrapping Schedule #40 PVC.
When we set our tubs the walls are up but we install them on the concrete and then tile around it. Good luck, Tom

Thanks for all the info, have been busy putting up the stud walls to get ready for plumbing, electrical, the tub, etc. Have a few questions wondering if you can answer.

1. By putting the tub right on the concrete floor, wouldn't the tub floor that you stand on (or sit on) get cold due to being directly on the concrete? Is there some way to insulate the underside of the tub, or the tub floor?

2. Would it be wise to install the tub on the subfloor, having just a small box cut out of subfloor (matching the size of the concrete dap out) to fit the trap? That way the tub is up off the concrete.

3. Do tubs ever get small micro cracks in them (the floor part that you stand on), where water can leak through these cracks from the tub floor, onto the floor below? If the tub were directly on the concrete, this water would flow onto the concrete slab, but if I had the tub on the subfloor, this water would flow onto the subfloor, eventually leading to rot, mold, etc. I had a contractor friend mention these possible micro cracks to me, so just wanted to see what would be the better way to go.

Any info you can provide would be great

Thanks in advance,
Steve

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[1. By putting the tub right on the concrete floor, wouldn't the tub floor that you stand on (or sit on) get cold due to being directly on the concrete? Is there some way to insulate the underside of the tub, or the tub floor?

You would be correct if the tub floor actually did set on the floor. It doesn't. There are stryofoam strips from the factory and if you have a good plumber he'll "bed" the tub in as he installs it.

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2. Would it be wise to install the tub on the subfloor, having just a small box cut out of subfloor (matching the size of the concrete dap out) to fit the trap? That way the tub is up off the concrete.

I am no fan of platforming fixtures unless it's necessary and this isn't necessary.

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3. Do tubs ever get small micro cracks in them (the floor part that you stand on), where water can leak through these cracks from the tub floor, onto the floor below? If the tub were directly on the concrete, this water would flow onto the concrete slab, but if I had the tub on the subfloor, this water would flow onto the subfloor, eventually leading to rot, mold, etc. I had a contractor friend mention these possible micro cracks to me, so just wanted to see what would be the better way to go.

Going back to,answer #1 This is why we "bed" plastic and fiber glass tubs. So that doesn't happen. Uptight about cracks? Install a steel or cast iron tub. Good luck, Tom

Lee, It's a little late but in my area the only time we wrap PVC in on a toilet stub out. Regards, Tom

Since it relates to my previous post, thought it would be good to post it on this thread, so that it would be easier to reference what was being talked about previously. I didn't know it would have been a problem to re-post on this thread

Steve

Where gave you the impression that there was a problem? I answered your questions and left an little note for Lee (mygirlsdad,) I fail to see any problem with that. Were you not happy with the answer? Where's the problem? Tom

Tom, I think you may be right about the wrapping. It must be an area thing. Our old inspector wanted us to wrap every pipe coming up through the concrete. His reasoning for this is if the concrete ever heaves, it won't break the pipes. Of course we put a few extra wraps for toilet and shower stub ups (dont use dap outs for showers here, just do exact rough in's for showers). I may have to break out the old code book and see if we actually need to wrap every pipe around here. You got me thinking now. Ill ask our new inspector what his thoughts are on this when I next see him.