Interior French Drains in Lancaster, PA

An interior French drain is a channel of perforated pipe and stone set into the basement floor along the foundation walls, feeding a sump pit. It is for Lancaster homeowners whose basements take on water and who would rather fix it indoors than dig up the yard. We install the drain, the pit and the pump as one system.

What an interior French drain does inside a basement

An interior French drain is a channel cut into the basement floor along the inside of the foundation walls. It holds perforated pipe bedded in washed stone. Water that reaches the footing drops into the stone, runs along the pipe to a sump pit, and a pump sends it out of the house. Nothing is dug in your yard.

It deals with water that is already getting under or through the wall. It does not keep water away from the outside of the foundation. If you are still deciding between inside and outside work, the French drain installation page covers when each one fits.

This approach suits tight lots, houses with a deck, patio or neighbouring building against the wall, and basements where water shows up at the joint between floor and wall. In those cases an outside trench is either impractical or would not reach the problem.

How an interior French drain installation goes, step by step

The work is noisy and dusty, so expect to clear a few feet of space along the walls first. The job moves in four stages.

Cutting the channel

A saw scores the slab a short distance out from the wall, and the concrete is broken out and hauled away. The trench is taken down to the footing so water travelling along it can drop straight in. On block walls, weep holes are sometimes drilled so water trapped inside the block cores can reach the drain.

Laying pipe and stone

Perforated pipe goes in on a slight fall toward the sump pit and is covered with washed stone. A fabric wrap or plastic channel liner keeps silt out of the pipe. The stone has to stay clean and open, because that is what lets water move.

Setting the sump pit

The pit goes at the low point, usually a corner. A liner, a pump and a discharge line complete it, with the discharge piped away from the house. A check valve stops pumped water from running back into the pit.

Closing the floor

New concrete is poured over the stone and finished level with the old floor. A seam stays visible along the wall, which is normal. Let the concrete cure before you move heavy items back.

Signs a basement needs an interior drain

These are the symptoms that usually point to water pressure under the floor or at the footing:

• Water seeping at the joint where the floor meets the wall
• Damp patches rising through the slab after heavy rain
• White mineral crust (efflorescence) on block or poured walls
• A musty smell, or mould on the bottom of stored boxes
• Standing water along the floor edge that clears in a day or two

Interior drain or exterior drain: how to choose

An outside drain intercepts water before it touches the foundation. It means digging along the wall, which disturbs landscaping, steps and sometimes paving. An inside drain skips all of that and can be installed in any weather, since the digging is indoors.

The trade-off is that an inside drain manages water after it reaches the footing. It does not keep the soil outside the wall saturated less. Where you have access and want the wall protected, a foundation drain installation outside may be the better fit. Some houses end up with both.

Sump pit and pump sizing for an interior perimeter drain

The pit must be deep enough that the drain pipe enters above the pump's water level, or the system holds water in the pipe. The pump is sized to the water that arrives in a hard storm and the height it must lift to reach the discharge point.

Ask about a backup pump, since power often fails during storms. Also check where the discharge ends. If it empties near the foundation, the same water simply returns to the drain.

Interior drains in older Lancaster homes

Lancaster was settled in the 1720s, and much of the housing is old. Stone and early block foundations are porous, and many cellars had their floors poured long after the walls went up. Those floors often sit on loose fill, which water moves through easily.

The city sits in the Piedmont region and receives about 42 inches of precipitation a year. September is the wettest month, so the first real test of a drain often comes at the end of summer, when the ground is already wet. In older neighbourhoods near downtown, houses sit close together or share walls, and an inside drain is frequently the only workable option.

On rubble stone walls we also look at the mortar. Open joints let water in above the floor, and the drain alone will not stop that.

Keeping an interior French drain working

Once a season, pour a bucket of water into the sump pit and confirm the pump starts and empties it. Keep the lid on, and do not stack boxes or lay carpet over the floor seam, since you need to see if water appears there.

If the pump runs constantly in dry weather, or water pools along the seam, something has changed. Silt in the pipe, a failed check valve or a worn pump are the usual causes. Drainage repair covers fixing a system that has stopped keeping up.