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Thread: Inadequate Main Drain Slope Under Mono-Slab

  1. #1
    DIY Junior Member 2LittleSlope's Avatar
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    Default Inadequate Main Drain Slope Under Mono-Slab

    I have a 5 year old home with mono-slab and about 92' of main line drain (sanitary sewer) under the slab measured by camera and cable from farthest toilet to clean-out just outside the MBR wall. The line is 3" PVC, so the minimum slope to meet code is 1/8" per foot. This works out to a minimum drop of 11-1/2" in 92 feet if the pipe were laid perfectly. Measurement of the difference in the pipe's depth at the clean-out and at the farthest toilet (removed) show that it was installed with just 6" of total drop, 5-1/2" shy of absolute minimums. The builder has gone through Chapter 11, but the plumber is still in business.

    I've been flooded by a sewer backup (major damage to hardwood floors), and had several stoppages. A camera inspection shows a substantial belly under the MBR, and several areas of poor flow or reverse flow elsewhere when toilets are flushed.

    What is the right way to fix this? I don't want a degraded home -- weakened slab, or compromised foundation, or noisy, unreliable pumps, or non-functioning sewer.

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    Moderator & Master Plumber hj's Avatar
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    If you were under the UPC the minimum slope would be 1/4" per foot for 3" pipe. If by Mono-slab, you mean a post tensioned floor, then you cannot do ANYTHING to it except tunnel underneath. Which would be almost impossible for a 92' distance. Since there are sags in the line, measuring at the beginning and end of the line tells you NOTHING about the specific slope of the various portions of the pipe. About your only option is to find a "construction attorney" and see if your problem falls under the "undetectable failures" classification.

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    Forum Admin, Expert Plumber Terry's Avatar
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    In ninety two feet, using 3" PVC and 2% grade, then the drop would be 23"

    I plumb in Washington State, where the "only" time you could use a 1% slope would be on 4" and wth the inspectors approval.
    We would have run 4" with 23" of grade.

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    Nuclear Engineer nukeman's Avatar
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    IPC allows 1/8" per foot (minimum) for pipe sizes of 3"-6". Regardless, what you have is wrong either way. Sounds like you have your work cut out for you. Probably the best bet is to talk to a lawyer and see what you options are in trying to make this right. Do you have any idea if the slope is okay from the house to the street?

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    DIY Junior Member 2LittleSlope's Avatar
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    The slope from house to street was not measured, but water flow down the pipe was rapid -- almost too rapid. I have contacted two construction defect attorneys, and will choose between them tomorrow. This whole thing makes me sick. Hey, at least I wasn't living in Haiti. We are all so spoiled here.

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    DIY Junior Member 2LittleSlope's Avatar
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    The Jan 2005 building inspection reports refer to it as a monolithic slab. I don't really know how it's reinforced -- mesh or rebar. I don't believe that it's post tensioned. This is the typical 2005 Florida slab for a 3000 sf house sitting on fill with a canal/lake in back. Actually, it's a great house, with a very bad plumbing job.

    Another strangeness. The sleeve (6" I think) for the mainline to exit from under the slab at the clean-out extends 4' under the slab. (We put a mini-camera between sleeve and pipe to find out.) Why would the plumber do that? Aren't they usually just long enough to extend through the stem wall?

    Finally, the county plumbing rough-in inspection (PRI - Plumbing R-1) was left as "Incomplete" for many months until the structure was essentially complete. We have a 2-story house in some parts, but there is no upstairs plumbing. The current county inspector said "it was left incomplete pending inspection of the second story plumbing rough in." Does this seem "normal".

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    Nuclear Engineer nukeman's Avatar
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    I asked about the slope to the street because if it is just at the minimum or less than the minimum, you would really be in a pickle and could explain why they used such a shallow slope under the house.

    As long as the slab is not post-tensioned, the concrete and rebar can be cut. More rebar would just have to be added in to tie the new work into the old work. You're looking at a big job. Being in FL, your main living space is probably on this slab (no basement or crawlspace). Finished floors would have to come up, slab cut, dig up pipes, and re-slope everything. Then put back together. You probably won't be able to (or won't want to) to live there while the work goes on. Even worse is you may have to dig all the way to the street to match up to the new (lower) connection at the house.

    If everything is going to come up anyway, you might want to go with 4" pipe (with proper slope) instead of the 3".

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