Downspout drainage options: where roof water should go

Why downspout water matters

One inch of rain on 1,000 square feet of roof is about 620 gallons of water. A typical house has four to six downspouts, so each one can put a few hundred gallons at a single corner of the foundation in one storm. No other spot on the lot gets that much water that fast.

Most of Rockland and Bergen sits on clay and glacial till. That soil takes water slowly, so roof water dumped at the foundation soaks into the loose backfill next to the wall and stays there. The backfill was dug and replaced when the house was built, so it is looser than the ground around it and holds water against the foundation.

Moving roof water away from the house is usually the cheapest drainage fix on a property. It should be done before anyone talks about French drains or regrading, because it often solves the problem on its own.

Extensions and splash blocks

A splash block is a concrete or plastic tray under the downspout outlet. It keeps the water from digging a hole in the mulch, but it only moves the water a foot or two. On a flat lot or a lot that slopes toward the house, a splash block alone does not do enough.

A downspout extension is a length of pipe or flexible corrugated tube that carries the water farther out on the surface. We like to see the outlet at least 6 to 10 feet from the foundation, on ground that falls away from the house. Extensions are cheap and easy to install, and they are a good test: if the wet spot in the basement goes away with a 10 foot extension, you know the downspout was the cause.

The problem with surface extensions is that they get stepped on, run over by the mower, and moved out of the way and never put back. Where the extension crosses a walkway or a lawn that gets mowed every week, burying the pipe is the permanent version of the same fix.

Buried solid pipe to a pop-up emitter or daylight

The standard permanent setup is an adapter at the bottom of the downspout, a buried solid pipe, and an outlet well away from the house. The pipe is solid, not perforated. A perforated pipe leaks roof water back into the soil along the way, which puts water next to the foundation again.

The outlet is either daylight or a pop-up emitter. Daylight means the pipe comes out of the ground at a lower point on the lot, such as a slope, a swale, or a wooded edge, and the water runs out onto the surface. A pop-up emitter is a round cap set flush with the lawn at the end of the pipe. When water fills the pipe, the cap lifts and water spills out. When the flow stops, the cap drops back down and the mower can go over it.

A pop-up emitter only works if the lawn around it falls away and the emitter sits lower than the downspout. If the emitter is on a flat spot or in a dip, the water comes out and puddles there. We check the grade from the downspout to the outlet with a laser before any trench is dug, because on a lot with no fall the answer may be a swale or a longer run to a lower point.

Material is a small part of the cost. For reference, 4 inch perforated pipe runs a couple of dollars a foot at supply houses. Most of the work is the trench, the fittings, and restoring the lawn. We give a written scope and fixed price after the site visit.

Pipe size, slope, and cleanouts

Most residential downspouts are 2 by 3 inch or 3 by 4 inch rectangular. Both connect to 4 inch round pipe with a reducer or adapter. We use 4 inch smooth-wall PVC for buried runs, not corrugated tube, because smooth pipe holds its slope, does not sag between rocks, and can be cleaned with a drain snake or a jetter.

The pipe needs a steady fall from the downspout to the outlet. The range we work to is 1/8 to 1/4 inch per foot, which is about 1 to 2 percent. At 1/8 inch per foot a 40 foot run drops 5 inches. At 1/4 inch per foot it drops 10 inches. Less than 1/8 inch per foot and the pipe holds water and collects grit. A pipe with a low spot in the middle holds water in that spot no matter how much fall the overall run has.

Where two or more downspouts join into one line, the main line should stay at 4 inches for most houses, and each branch should enter with a wye fitting pointed downstream, not a straight tee. We keep bends to long-sweep 45 degree fittings where we can, because tight 90 degree elbows are where leaves and shingle grit settle.

A cleanout is a capped fitting that brings the pipe to the surface so it can be flushed. We put one at the base of each downspout, usually with a leaf filter or a debris trap at the connection, and at long runs or sharp turns. Without a cleanout, a clogged pipe means digging up the lawn to find the blockage.

Dry wells: only where the soil can take it

A dry well is a buried chamber or a pit of stone that holds roof water and lets it soak into the ground. It is the answer when there is no lower spot on the lot to daylight the pipe and the town will not allow a street connection. It only works if the soil drains.

On much of Rockland and Bergen the soil is clay or dense till, and water moves through it slowly. A dry well in soil like that fills in the first half hour of a storm and then overflows or backs up the pipe. Before we recommend one, we dig a test hole at the depth the well would sit, fill it with water, and time how fast it drops. If it drops less than about an inch an hour, a dry well is the wrong choice for that spot.

Where a dry well does make sense, it goes at least 10 feet from the foundation, downhill from the house, and away from the septic system if there is one. It is sized for the roof area feeding it. A debris trap or a catch basin ahead of the well keeps leaves and grit from filling it in. We cover dry wells and the other outlet options in more detail in our yard drainage guide.

Freezing and winter problems

Frost in this area goes about 36 to 42 inches deep. Burying downspout pipe below that depth is not practical on most lots, and it is not necessary. A buried pipe with a steady fall drains out after each storm, so there is little water left in it to freeze. Pipes that freeze solid are almost always pipes with a low spot, a sag, or an outlet that is blocked.

Pop-up emitters are the weak point in winter. The riser under the emitter holds some water, and on a cold night that water can freeze the cap shut. Snowmelt then backs up the pipe and overflows at the downspout adapter. We set emitters on a small bed of stone and drill small weep holes at the bottom of the riser so the standing water drains out between storms.

Daylight outlets freeze less often than emitters because the pipe end is open. The outlet should be protected with a grate or an animal guard and kept clear of plowed snow. If a line does freeze, the downspout adapter should have an overflow or a way to disconnect so the water spills on the surface instead of backing up the gutter.

Where the water can go in New York and New Jersey

Getting roof water away from the house is only half the job. The other half is making sure it does not end up on the neighbor's property. In New York, the courts have long held that a landowner may improve and grade their land, but cannot collect surface water and cast it onto a neighbor's land with pipes, drains, or ditches. That rule comes from the Court of Appeals case Kossoff v. Rathgeb-Walsh, and later decisions have applied it to other artificial means as well. A buried downspout pipe that empties at the property line is exactly the kind of thing that rule covers.

New Jersey follows what is called the reasonable use rule, from the 1956 state Supreme Court case Armstrong v. Francis Corp. A landowner may make reasonable use of their land even if it changes the flow of surface water, but is liable when the interference with that flow is unreasonable. Concentrating roof water from several downspouts and aiming it at a neighbor's yard is likely to be judged unreasonable.

In practice, we keep every outlet on the owner's property, far enough from the line that the water spreads out and soaks in before it gets there. A connection to a curb, a street gutter, or a storm drain is a separate question for the town, and many towns require permission or do not allow it at all. We check with the town before we plan an outlet that leaves the lot. More on the work itself is on our drainage and excavation page.

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