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standing water in gravel beneath slab.

Posted by veloguru (My Page) on
Thu, Nov 10, 11 at 20:32

I am building two additions to my raised ranch. Both are framed up and roofed ( no gutters yet! ). I was festidious about waterproofing new foundation and installing footing drains which are working fine. Whilst cutting doorway in existing block foundation wall to connect the two basements, i was disapointed to see standing water in the gravel about an inch below the bottom of existing slab. I need to concrete the 1" x 3" piece between slabs but realize that preventing the moisture from permeating upwards is near impossible. I have no sump pump as being a raised ranch, never thought i needed one.
My thought was to install a sump pump in a nearby utility room, and use XPS insulation on walls AND floor ( sealing joints with spray foam/ tape) and installing laminate flooring over electric radiant heat. I mentioned gutters not being completed yet as this is probably not helping matters. My existing basement was finished with carpet on the slab and fibreglass batts/poly sheet on wood framing, none of which was done by me and all of which was a bad idea and is now a rotted moldy mess. Am i on the right track with pump/ sealed XPS ? any other solutions? Any help appreciated.


Follow-Up Postings:

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RE: standing water in gravel beneath slab.

My first concern would be the water in the underslab gravel. Standing water there can indicate abnormal water inflow, a high water table, blocked or no drainage or even not enough gravel to hold the water until it drains off (there should be at least 4-5 inches.)

The existing weepers should be connected to a drain or outflow somewhere. If not, they serve no purpose.

A sump pump is designed for placement in a sump pit, which can be connected to under-floor drains and/or weeping tiles.

XPS (extruded polystyrene) is neither dampproofing nor waterproofing. It is insulation that is not affected by the moisture in a basement. So don't expect it to stop water flow.

Laminate on floor
If you use XPS on the floor as insulation, you will need one or two layers of plywood over it, followed by electrical radiant heating and the laminate. Only use laminate warrantied for basement use and which is compatible with radiant heating; check that the radiant can be used under laminate.


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RE: standing water in gravel beneath slab.

Thanks for the reply. I don't expect the XPS to waterproof the space, just want to try everything to eliminate future mold problems. I intend to use a Heat recovery Ventilator to continuously supply fresh conditioned air, which should help along with a dehumidifier.
I have the Laminate/ Radiant compatability figured out.
You are correct, i do have a high water table and have ground water seeping from the earth off the hill behind my house. ( Just installed new curtain drain to help with that). Will install pump in sump pit before filling in void between slabs to observe if water retreats downwards.. Thanks again.


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RE: standing water in gravel beneath slab.

The key is to keep the water level lower than your floor slab. As long as that is the case, you will not have water intrusion. Natural drainage is preferable but the pump is the second-best option. Polyethylene sheet will serve to restrict moisture coming up through the concrete. My floor has 4" of washed stone under the slab with 4 layers of 7 1/2 mil poly between the stone and the concrete. The stone drains naturally on the daylight side of the basement.


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RE: standing water in gravel beneath slab.

I have a similar issue.. I have slab on grade (no basement), had some moisture damage to hardwood, so opened up the slab and found water accumulating about 20 inches below the slab. The house is at a good level, but some rain water may be seeping in... I have added french drain now.

Is it normal to have a water table at that depth below slab? should I do more like add a sump pump?


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RE: standing water in gravel beneath slab.

There are many soil types that if you dig a hole in them do not allow water to easily drain away.

As long as it remains well below the bottom of the slab it is likley not problem.

You might be able to install a sump and lower it even more, or you might find your sump running almost continuously.

Try using a small pump and see if you can remove all (or most) of the water.


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