Yard drainage in Rockland County

Why Rockland yards stay wet

Most of the county sits on glacial till and clay. Water does not soak through clay the way it soaks through sand, so after a storm it sits on top, runs downhill, and collects at the lowest point on the lot. On a half-acre colonial in New City or Nanuet that low point is often the back corner of the yard or the side of the foundation.

The second problem is roof water. A 2,000 square foot roof sheds more than 1,200 gallons in a one inch rain. If the downspouts dump that at the corners of the house, the ground next to the foundation gets more water than any other spot on the property. That is the first thing to check before any drain goes in.

The third problem is grade. Builders backfill the foundation with whatever came out of the hole, and that fill settles over ten or twenty years. The result is a slight dish around the house that slopes toward the wall instead of away from it. A one inch dip is enough to hold water against the foundation for days.

Regrading: the fix to try first

Regrading means reshaping the soil so water runs away from the house and off the low spots on its own. The target is a drop of at least 6 inches in the first 10 feet away from the foundation, and a steady slope after that toward a swale, the street, or a wooded edge of the lot.

A swale is a shallow, wide channel cut into the lawn along the natural drainage path. It is graded at a slight fall, sodded or planted, and carries surface water across the yard without pipe. On flat lots in Pearl River or Monsey a swale is often the only practical way to move water because there is no elevation to work with for a buried pipe.

Regrading is the least expensive drainage work per square foot because it is mostly machine time and topsoil. It handles surface water well. It does nothing for groundwater or a high water table, and it cannot be done within a few feet of a neighbor's line without checking where that water will end up.

French drains: for water in the soil

A French drain is a trench with perforated pipe at the bottom, surrounded by washed stone, wrapped in filter fabric so soil does not clog the stone. Water in the surrounding soil moves into the stone, drops into the pipe, and follows the pipe downhill to an outlet. The trench is typically 12 to 18 inches wide and 18 to 30 inches deep, with the pipe laid at a fall of at least 1 percent, which is about 1 inch per 8 feet.

The pipe should be 4 inch rigid perforated PVC, not the corrugated black pipe sold in coils. Rigid pipe holds its grade, can be cleaned out, and does not collapse when a truck drives over the lawn. The perforations face down so the pipe drains the bottom of the stone bed.

A French drain is the right tool when the ground is saturated, when water runs under a patio, or when a slope above the house sends water toward the foundation. It is the wrong tool for roof water. Downspouts should go into solid pipe, not into the perforated line, or the roof water floods the drain that was supposed to relieve the soil.

Dry wells: only where the soil takes water

A dry well is a buried chamber or a pit of stone that holds water and lets it seep into the surrounding soil. It only works if that soil can absorb water faster than the well fills. On sand and gravel it works well. On Rockland clay it often does not, and a dry well in clay turns into a buried bathtub that is full for a week after every storm.

Before a dry well goes in, the soil should be tested with a perc test: dig the hole to depth, fill it with water, and time how fast it drops. A drop of less than an inch an hour means a dry well will be undersized no matter how big it is. In that case the water needs to go somewhere else.

Where a dry well does make sense, it is sized for the roof area feeding it. A common residential unit holds around 50 gallons per chamber, and several are linked together for one house. A pre-filter or a catch basin ahead of the well keeps shingle grit and leaves from filling it in.

Where the water is allowed to go

This is the part most homeowners do not think about until a neighbor complains. In New York and New Jersey, a property owner generally cannot collect water and discharge it onto a neighbor's land in a way that causes damage. Piping a downspout to the property line and letting it run into the yard next door is the fastest way to end up in a dispute.

Acceptable outlets, in rough order of preference: daylight on your own property where the ground falls away, a swale that spreads the water across your own lawn, a dry well if the soil percs, or a town storm drain or curb outlet. Most towns require permission for a curb cut or a tie-in to a storm sewer, and some do not allow tie-ins at all. Stream-side lots in places like Sparkill and Tappan may have buffer rules that limit what can be built near the water.

The outlet is the most important part of the system. A drain with no working outlet is just a longer trench that fills with water. Every design on the drainage and excavation page starts by finding the outlet and working backward to the problem area.

What a wet basement usually means

A basement that gets wet in heavy rain but dries out within a day almost always has a surface water problem: downspouts dumping at the foundation, grade sloping toward the house, or a window well without a drain. Those get fixed outside, with extensions, regrading, and drainage, without opening the basement floor.

A basement that seeps for days after a storm, or stays damp through spring, is usually groundwater. The water table is high, and hydrostatic pressure pushes water through the floor joint. That calls for a footing drain outside the foundation, or an interior drain and sump inside. The footing drain is the better fix when it can be dug, because it stops water before it reaches the wall.

Water that appears in the same spot every time, on the uphill side of the house, generally means a slope is delivering water faster than the soil can take it. An interceptor French drain across the slope, above the house, catches it before it arrives. On lots below the Ramapos in Suffern and Sloatsburg this is the most common fix.

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