# Subfloor and P-trap for basement bathroom



## svaic00 (Jul 2, 2012)

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 pdf's are photos which 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 pdf shows a CAD drawing of the existing and proposed conditions for the bathroom.

As you can see I have a 12" gravel filled box 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 can 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 cant 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 square opening for the shower), 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, ann then install the toilet flange to the subfloor?

Thanks in advance,
Steve


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## svaic00 (Jul 2, 2012)

Not a single response??? I can insert pictures, if you are not able to view the pdf's, Anyone, Anyone????


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## oh'mike (Sep 18, 2009)

Post pictures PDFs sometimes mess up my computer--


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## oh'mike (Sep 18, 2009)

When installing a basement tub drain---pack the pipes with gravel---then top with a thin,easily removable layer of concrete----

Your tug should have a set of rough in locations--post a link to that tub---

Consider a solvent weld drain set---if you do blow the location,it will be easier to shift the tub drain using the solvent weld set--they use standard 1 1/2" fittings.


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## TheEplumber (Jul 20, 2010)

I can't open the files but why fill the hole in. It's under the tub. Do it last. Walk the inspector through it too. Ask him how he would do it.


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## svaic00 (Jul 2, 2012)

TheEplumber said:


> I can't open the files but why fill the hole in. It's under the tub. Do it last. Walk the inspector through it too. Ask him how he would do it.


Attached are the photos. First 2 photos which 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.

Original post:
As you can see I have a 12" gravel filled box 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 can 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 cant 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 square opening for the shower), 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, ann then install the toilet flange to the subfloor, or should the toilet flange sit on top of the finished floor?

Thanks,
Steve


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## oh'mike (Sep 18, 2009)

I'm with E-plumber---no real need to cap the gravel---the tub drains are often below the floor level--so check that hole size and make sure the drain arm will clear your concrete---

As to the flange on top of the finished floor or mounted to the subfloor---that debate goes on for ever---I like mine attached to the subfloor or tile backer---so the flange is flush with the finished floor--but every time I say that someone comes on with an opinion that the flange belongs on the top of the finished floor---


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## svaic00 (Jul 2, 2012)

oh'mike said:


> I'm with E-plumber---no real need to cap the gravel---the tub drains are often below the floor level--so check that hole size and make sure the drain arm will clear your concrete---
> 
> As to the flange on top of the finished floor or mounted to the subfloor---that debate goes on for ever---I like mine attached to the subfloor or tile backer---so the flange is flush with the finished floor--but every time I say that someone comes on with an opinion that the flange belongs on the top of the finished floor---


Yea i read an article about flange attached to floor from a year ago, and half said flush, half said on top of finished floor, and I can see both sides of it.


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## oh'mike (Sep 18, 2009)

I've had instances
where the flange hit the toilet horn when mounted on top of the finished floor--that's why I keep mine flush--theory is one thing --real life experience is all I go by---


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## svaic00 (Jul 2, 2012)

See my post above dated 7-18-12 for more info and description of the job.

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 anyone 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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