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Question DetailsAsked on 11/30/2014

How can I locate covered basement floor drain under subfloor?

i am getting a sewer smell in area of home in always same area of home. the floor is floating floor over a sub floor. I am pretty sure there is a dry floor drain in this area somewhere under the floor because there are no visible floor drains anywhere in the basement. Any suggestions?

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2 Answers


This could be what you say (in which case, as you know, some floorer or DIY'er really blew it), the smell could be from stagnation of ground moisture wicked up through the slab causing rotting in the flooring padding (or underside of the flooring if no padding), stagnation in an underfloor drain system or sump pump (if you have such check it first), leakage from septic system through foundation or around slab (if on septic), or a leak in the sewer pipes under the slab - but anyway, to your question:

Several ways - depending on what sort of padding is under the flooring to block detection:

1) using a $10-15 stethoscope (also very good for looking for pipe leaks in the future), listen around on the floor while someone taps with a metallic item like a small wrench on the drain pipe - preferably as nearby to the smell area as possible, and will carry further if done on pipe a couple of feet above where it enters the floor (preferably on cast iron section if available) or at another floor drain. Remember both normal plastic sewer pipe and cast iron are fairly brittle, so you want just a gentle tap, not hard thumping because you are talking a lot of taps, so I would not use a hammer unless a childs toy hammer. Just the metal part of a larger size screwdriver tapping sideways might work fine too. Just as hard as you would tap a table top if frustrated is fine, don't get too aggressive with it. If working alone running water in a tub (so there is no overflow risk) and listening for the water flow might well work too, plus if smell gets a lot stronger while doing that it will tell you that you are probably on the right track.

2) if that does not work, try putting a loud (boombox ?) speaker with very loud, low-frequency (lots of bass) music right tight (held in place with something so in good contact) against the sewer pipe and listen for that - some Sousa or hard rock would be best probably. Of course, source has to be far enough away in house that you do not hear the music directly through the house despite the stethoscope being in your ears.

3) if that does not work, rent a thermal infrared camera for about $40 or so half day (if that is an available plan) or about $75 full-day at Home Depot (a new program for them so not all stores will have yet) or a tool rental place or some auto parts stores. If cannot find a rental place locally, then some insulation contractors and many home energy auditors have them too - probably about $100-150 for one of them to come do a single floor area scan - if going that way, find out how much to scan entire house and provide you with imagery on disk - might not be a whole not more and gives you a real good idea of what your energy loss locations are. Whether it will see the temperature difference between slab and an open (or maybe grated) floor drain through the flooring and underlayment is anyone's guess (especially if less-conductive plastic) because unfortunately it is all in the same environment (ground contact under, flooring over). If you have water stagnating under the flooring it should show that unless it is everywhere so there is no difference to show across the area. If using the camera, then maybe running very cold water (if your water supply is quite cold) is your best bet as that will be about maybe 20-30 degrees cooler than the floor surface, because you would have to run an awful lot of really hot water to show a positive temperature difference. However, because the floor drain is embedded in the concrete (hopefully cast iron so transmits heat better) and therefore about ground temp (which is close to the cold water temp is, roughly) it might take hot water for a LONG time - 5-15 minutes say - to heat the pipe enough to show a difference against the slab temp - so if a quick scan does not pick it up with cold water then running full hot water at a moderate flow rate (like normal sink faucet flow) to give it a fairly long heating time before the water heater runs out might do it. And run in the nearest available source so it does not cool down in the pipes more than necessary getting to the presumed drain.

Once you find it, if there, then you can consider what to do about it - you could leave it and put a removeable cover plate over it to add water t times (so it would be able to drain water in event of serious flooding), or plug it off entirely with an expanding rubber drain plug.

Answered 5 years ago by LCD


Hi Thanks for getting back to me on this...turns out that there was a floor drain underneath the sub floor. I managed to get my hands on the original house plans, and sure enough, there was supposed to be floor drain in the vicinity of the bad smell. Proceeded to cut open the floor drain and there it was! ....Unbelieveable that whoever originally finished the basement would do this!

Answered 5 years ago by daone77777

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