Plumbing for slab on grade

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by steve2278, Dec 31, 2012.

  1. steve2278

    steve2278 New Member

    Mar 2, 2009
    North Carolina
    Hello everyone,

    I'm building a new home and it's a slab on grade and all PEX water lines and sewer lines have been run underneath the slab. I also installed radiant heat PEX piping under the slab as well.

    I work as an industrial concrete flooring contractor and I did my plumbing design based on an industrial floor slab design, that is to bury the piping and electrical conduit underneath the slab. I've always preferred this design as opposed to the common residential design of having a crawl space. The whole premise behind having a crawl space is to be able to access, service and repair pipes in the event there's a future problem. Unfortunately, in my experience, crawl spaces are often the route of the problem as pipes are exposed to freeze/thaw cycles, rodents can chew through them; wood is subject to mold, mildew, moisture damage, termites etc. Long stry short, I've had nothing but problems with crawl spaces and I decided to eliminate mine as a result.

    My plumber is here working now, and prior to pouring my slab he insisted that I leave box-outs so he can install my showers and tubs. However, EVERYTIME I pour an industrial floor and I get to the bathroom area I always encounter a managerie of plumbing pipes that are stubbed-out to be 2-3 feet above the finished slab and I have to pour my concrete around them. After I'm done with the concrete, a week later the plumbers come in and do their thing and install their vents, extend their water lines, the toilets, tubs etc. It's nice and simple as all pumbing has already been done and all traps have been installed underneath the slab.

    I asked my plumber why I needed to have a 12" x 12" box-out around my tub and shower drains and he says it's to access the area in the event there's a problem in the future??? My question is "why would there be a problem?" That's like saying I should leave access ways in my concrete every 2 feet in case the PEX tubing in my radiant heat system springs a leak. if it leaks or fails its my problem and I'm willing to assume the risk.

    Granted during the prep stage a lot more can go wrong with plumbing that's encased in a slab and it's crucial that everything is done correctly before the concrete is poured becasue once its poured its permanent. My radiant heat pipes were pressure tested during the pour and I kept the pressure gauge on 3 weeks after the pour and they're fine. I just can't understand for the life of me why it's necessary to provide access to the plumbing fixtures for the tub and shower? If they're done correctly and the slab doesn't settle or crack what could possibly happen to them?

    I owned a 150 year old house years ago and my water and septic lines didn't dissolve, break down or rot?...and I've been in many 100+ year old commercial industrial buildings where pipes buried in the slab were perfectly fine. The concrete floor never had to be cut or accessed. In fact, the concrete seemed to serve to protect and insulate the pipes from damage.

    I've poured hundreds of commercial/industrial floors and I've encased a lot of plumbing pipes causing them to become permanent and on every pour the plumbers and electricians would be present just in case I accidentally damaged something during the pour. And yes, that has indeed happened on occasion and I would have to stop the pour so they could fix the problem, and then I would resume pouring once their finished.

    I just don't understand the reasoning for needing these access areas? My plumber said that the 12" x 12" box-out under my shower/tubs typically get filled with sand/dirt after my top-out inspection, but I don't want that! I never did I don't know why its necessary?

    Assuming my slab never settles or cracks and everything has been done correctly under the slab why couldn't ALL the plumbing pipes be extended beyond the finished slab and then have the fixtures installed afterwards? Sure a drain could become clogged, but that happens with industrial/commercial buildings and it can always be remedied. I don't want a thermal break under my tubs and I don't want a 12" x 12" soft, spongy area under my tub that's been filled with dirt or sand. I want solid concrete everywhere and I want everything encased so that it's permanent...just as it always is done in industrial/commercial applications.

    Any opinions would be greatly appreciated.

  2. bluebinky

    bluebinky Member

    Aug 31, 2011
    Software Engineer
    Santa Clara, CA
    I always assumed it was either to allow access to the traps, and/or to make minor adjustments during final installation. In my opinion, if you ever needed to access a shower trap, breaking up a little concrete would be the least of your worries...

    The 50 or so slab-on-grade houses I've watched being built in north Texas were all done exactly as you describe.

    In my house, fire ants have made major nests in every single cut out...
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  4. hj

    hj Moderator & Master Plumber Staff Member

    Aug 31, 2004
    Cave Creek, Arizona
    ALL slab houses, and other buildings, are done that way. The box outs are to provide room for the tub drain to set into the floor, and enable the tub and shower drains to be "located" relative to the walls when they are ready to be connected. It would be a time consuming thing to "stub" the pipes up, then break out the concrete afterwards to make the connections. Radiant piping is usually installed IN the slab to give faster reaction time, and minimize heat loss into the ground.
    Last edited: Dec 31, 2012
  5. cacher_chick

    cacher_chick Test, Don't Guess!

    Jan 5, 2008
    Test, Don't Guess!
    Land of Cheese
    I would only add that if in 10, 20, 30, or 50 years later, when someone wanted to install a new shower or tub, the trap for the new unit will probably have to be moved a few inches to make it fit.

    At the point where the walls are not yet installed, it would not be feasible to stub up a shower riser exactly where the shower will be, because there will always be some amount of variance in the measurements made during construction. The drain stub for a shower needs to be exactly positioned for it to line up with the fixture, and to do this means repositioning the angle of the trap on the trap arm.

    I would much rather do a fixture replacement where I do not have to break concrete if the option is available.
    Last edited: Dec 31, 2012
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