Drain lines are underground drainage systems designed to collect excess water from within the soil and drain it away. They may be used to protect a home from water damage or to drain a wet garden. Today’s drain lines are made of flexible plastic tubing with two rows of holes on the bottom to let water enter the drain. Not too long ago, drain tiles made of terra cotta were commonplace, and still drain many yards.
Another type of drain, called a French drain, is a ditch filled with rubble or drain rock. The term is often applied to any type of drain line.
It’s important to note that drain lines only function when the soil is saturated. To test whether a drain line will solve your drainage problems, dig a hole in the area you want to drain. If water seeps from the soil into the hole, a drain line will help you. However, it will not help if your problem is heavy soil that only accepts water slowly, or compacted soil.
Drain lines can be open to the surface of the soil. If so, they are visible as a gravel path or border. Open drains catch surface runoff as well as water moving through the soil. Drains that are covered with soil are not visible in your garden, and anything may be planted over them. Covered drains, however only collect water that has already entered the soil; they don’t collect surface runoff.
Installing a Drain Line
To plan a drain line, first determine where the water will be taken once it leaves the line. Usually it will go into a storm sewer, street gutter, or drainage ditch. The point at which the water leaves the drain line is called the outfall. The elevation of the outfall determines the construction of the rest of the system.
Since water moves through the line by gravity, there must be a fall from the top end to the bottom end of the line. There must be at least a 1-foot drop for every 100 feet of line, or 1/8 inch per foot, or the drain line will eventually fill up with silt and stop functioning. To find out how deep the line can be placed, start with the elevation of the outfall and measure backwards.
Plan to use a herringbone pattern, keeping the lines from 10 to 20 feet apart. They should be farther apart in heavy soil, where water moves sideways to the drain line more readily, and closer together in sandy soil.
If there is only one low area in the yard, a single line under it will probably be sufficient.
Start digging the ditch at the outfall, and work back into the yard. Maintain a steady upward slope from the outfall (minimum of a 1-foot rise per 100 feet of ditch, or 1/8 inch per foot) without dips, regardless of the contour of the surface.
Laying the Drain Line
The most commonly-used drain lines are made of a flexible, corrugated plastic hose. Light and simple to install, this hose will make all but the sharpest bends without the need of an angle joint. It comes in 3- and 4-inch diameters. The 4-inch hose offers the advantage of being less likely to clog if a piece of debris or a small animal gets stuck in it.
Other drain lines are made of rigid PVC or styrene pipe. Rigid pipe has the advantage that the joints can be glued, and the pipe can span short, low spots in the trench without bending. With rigid pipe there is also a better chance of routing out a blockage without tearing up the pipe.
An effective drain system must be surrounded by an envelope of rock that protects the lines, keeps soil from entering them, and increases their capacity by conducting water. Use either smooth river rock or crushed rock—the former drains faster, but the latter gives greater mechanical strength.
Whichever is used, backfill the ditch with drain rock to a minimum depth of 4 inches over the line. In heavy soils backfill the trench with rock as high as possible.
To prevent the envelope of drain rock from silting up, enclose it with soil filter cloth. Place the cloth in the ditch first, then spread 2 inches of sand as a bed for the line, then place the drain line, then cover it with drain rock and wrap the filter cloth over the rock.
Lay perforated drain lines with the holes facing downward. This prevents soil from dropping into the line. It also lowers the water table to the bottom of the line, rather than to the level of the top, because water will rise into the line, keeping the water table at the level of the holes.
Install a clean-out—an access hole—at the highest point of the system. Place additional clean-outs every 100 feet or so in the drain line. Try to avoid using 90 degree bends, so that there is enough space to push a garden hose or sewer snake through the pipe for cleaning.
Now take the soil that was removed from the ditch and shovel it back in, backfilling the ditch to ground level. Before the ditch is quite full, run water into it to settle the soil; this will keep it from settling later on, after it has been planted over.
Catching Surface Drainage
A variation on this drain line is to fill the ditch with drain rock up to the surface. This type of drain will allow surface water to enter the drain quickly, and will catch water that runs off a slope before it gets to the garden. However, the drain is visible on the surface.
To keep soil out of the rock on the surface, place a couple of header boards on either side of the ditch, extending them 1 or 2 inches above ground level.
Use an envelope of soil filter cloth, but cover the cloth with a final layer of gravel, since it will break down if exposed to direct sun.
The exposed gravel may be used as a path or mowing strip.