An interior drainage system, often called an interior French drain, is a perforated pipe laid in stone along the inside of the footing, beneath the edge of the floor slab, and pitched to a sump. In Buffalo it is the most common permanent fix for water at the wall-floor joint, because it addresses the water table directly and does not depend on what is outside the house.
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The cove joint, where the poured floor meets the wall on top of the footing, is not sealed in any Buffalo house built before the last few decades, and it is the lowest opening in the basement. When the water table in the clay rises above the slab, as the Erie County soil survey says it does in Odessa and Niagara soils from December through May, the pressure under the floor drives water up through that joint. It also comes through floor cracks, around the furnace pad and anywhere else the slab is broken.
This is hydrostatic pressure, and the important thing about it is that it acts on the whole floor at once. Building Science Corporation's guidance on basements notes that where groundwater tables are consistently high the only real strategy is drainage that relieves the water before it reaches the structure. Sealing the cove joint from above moves the entry point to the next crack. Relieving the pressure at the footing removes the reason the water is coming in.
The floor slab is cut and removed in a strip roughly a foot wide along the affected walls. The soil beneath is excavated to the top of the footing or slightly below it, depending on footing depth. A layer of washed stone is placed, perforated drain pipe is laid on it and pitched toward the sump location, and the trench is filled with stone to just below the finished floor. On block walls, weep holes are drilled through the bottom course into each core so the wall can drain into the trench rather than hold water. On stone walls, a dimpled drainage board is fixed to the wall and lapped into the trench so seepage from the wall runs down behind the board and into the drain.
A drainage flange or gap is left between the new concrete and the wall so water running down the face has a path into the stone. The pipe enters the sump liner, and the pump does the rest. The trench is then capped with new concrete finished flush with the existing floor. The result is a floor that looks nearly the same and a wall that no longer has water standing against its base.
Not every basement needs the whole perimeter done. When water enters only along one wall, that wall faces a downspout or a high neighboring lot, and the other walls have never shown a stain, a partial system on the wet wall with a sump at one end is a fair solution and costs proportionally less. Where the problem is the water table itself, which shows up as dampness along several walls and around floor cracks, a full perimeter is the honest recommendation. Water under pressure will find the walls that were skipped.
Lake-plain houses in Buffalo, Kenmore, Tonawanda, Cheektowaga and Amherst tend toward the full-perimeter case, because the clay holds water evenly around the house. Houses on the sloping ground toward Orchard Park, Hamburg and East Aurora more often have a single uphill wall that takes the water, and partial systems are more common there.
An interior drain manages water that has reached the footing. It does not stop water from coming through the face of a wall above the drain, so a poured wall leaking from a crack four feet up still needs that crack injected. It does not fix a wall that is bowing; that needs anchors or carbon fiber first. And it does not correct the surface water problems that are loading the backfill, which is why we address gutters, downspouts and grade on every job even when the drain is the main event.
It is also not a substitute for a backwater valve where the problem is sewer backup through the floor drain, which is a plumbing matter. On the assessment we separate the groundwater from the sewer water so you are not paying for a drain to fix a problem it cannot touch.
One more caution. Because the interior approach creates a path for soil moisture and soil gas into the drain and sump, the sump lid needs to be sealed and the system needs to be closed. Building Science Corporation is emphatic that the interior approach must be airtight; we build it that way.
Questions
Same principle, different location. A yard French drain collects surface and shallow groundwater outside. An interior system sits at the footing under the basement floor edge and collects water that has reached the foundation. On Buffalo clay the interior drain does the work the yard drain cannot.
A strip about a foot wide along the walls being drained. The rest of the floor stays intact. The strip is re-poured and finished flush when the drain is in.
Washed stone and perforated pipe are chosen to keep fines out, and a well-built system on the lake plain runs for decades. Clean-out access is left at the sump so the line can be flushed if it ever needs to be.
Yes, but the bottom of the finished wall along the drain path has to come off and be rebuilt afterward. If you are planning to finish the basement, do the drain first; it is easier and cheaper in that order.
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