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Question DetailsAsked on 9/8/2016

Can I seal a cement basement floor that has water under it due to a high water table?

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IF you mean surface sealing, to limit water transmission through the concrete, yes - there are pool and marine/industrial epoxies designed for wet surface and even underwater application which will work well - run about $75-100/gallon but for a normal basement that is only a couple hundred $ more than normal floor paint, and the application labor cost is not significantly different.

Note that even if you seal it, if the slab is damp looking (darkish color concrete) or surface-wet now, epoxy sealing can limit the infiltration but will not totally stop it (especially along the perimeter) so while it can reduce basement moisture problems, it will rarely (without also lowering the water table) make the slab suitable for any flooring type that can be hurt by water, so if that is the case I would recommend going with a bare epoxied floor without any covering - or at most throw rugs with wide-open weave (like open-weave berber rugs) which can dissipate any moisture getting through.

IF you mean sealing out water that is coming through or around the perimeter of the slab - sometimes, but generally not reliably if the water level is basically up to the bottom of the slab. In fact, if the water table is quite high (say near ground level) trying to grout the water off to stop inflow leakage in quantity can result in buckling or "floating" of the slab. Generally, if you are getting free water through or around the slab (as opposed to just water vapor transmission or damp but not wet slab surface), underdrains leading to a sump pump is the normal solution.

In some cases the underslab soil will allow for just a sump pump to draw down the water table enough to solve the problem without any drain pipes - depends on water level, permeability of the soil, whether the soil is inherently resistant to piping (washing out of the fines), etc. in other cases - commonly in fine-grained floodplain sediment in low-lying or river valley areas, the soil permeability and erodability will make underdrainage infeasible because of the cost of grouting and such - which is one reason why many low-lying river, lake and ocean-side houses do not have basements in certain areas.

In extreme cases, commonly along beaches and rivers with sandy or gravelly soil, if the slab is at or below the adjacent groundwater level it can be grossly uneconomic to try to block off or draw down the water table enough to help. While such measures such as grouting and cutoff walls are used on commercial and industrial jobs, generally if your slab is at or below the normal water table AND you are in fairly free-draining soil, unless you are able to intercept it outside the foundation below foundation level and use french drains to drain it away from the house (thereby putting a permanent dimple or lowered spot in the water table around your house), it can be uneconomic to solve severe basement water problems. Conversely, in fairly impervious conditions (bedrock, silty or clayey soil) fairly minor underdrains leading to aq sump pump commonly work quite well, locally drawing the water table down a couple of feet under the house.

Generally, for sealing a damp floor, your best bet would normally be a Pool and Spa contractor who does underwater pool painting, because a normal painter is not going to be experienced with the materials or methods. Alternatively, a marine structure repair company experienced with underwater epoxy painting on pilings and docks and boats and such would know how.

For drainage of high water table to get the water below the slab to stop infiltration, a Basement Waterproofing contractor would be the normal Search the List category. I emphatically recommend against any drain system that involves above-slab drains or perforating the foundation wall to drain into a basement drain system - an above-slab drain will not get the water down below the slab so it will not dry out, and allowing outside water along the foundation wall into an interior system is an invitation to disaster if the flow exceeds your sump pump capacity in high water or heavy rain events or if you have a power outage so the pump fails or exhausts its backup battery. OUtside water should stay outside - either by waterproofing the outside of the foundation wall (which does not eliminate basement slab wetness) or foundation perimeter drains OUTSIDE the foundation drawing the water level down and draining it away to disposal - or commonly both to provide a "dry" foundation while putting in a french drain system.

Answered 3 years ago by LCD

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