What are the pros and cons of drains that go below grade?

MMH Expert_Matt T EXPERT April 24, 2008

Downspouts often disappear into the ground adjacent to the building. On older houses, these extensions may be cast iron, clay tile, or asbestos cement piping. On modern houses, the downspout extensions below grade are often ABS or PVC plastic.

The advantages of these systems are

• there are no unsightly downspout extensions
• water will not be discharged onto the lawn, driveway or grade around the building

The disadvantages of drains going below grade include:

• It is difficult to see the early indications of nonperformance, since water can discharge below grade and collect, saturating the soil for some time before it is noticed.

• When pipes do get clogged or collapse below grade, repair or replacement is more expensive, often requiring excavation.

Discharge Into Floor Drain In many older homes, the downspout extensions extend down below ground beside the foundation, and then run horizontally under the basement floor, terminating in the vertical section of a floor drain pipe, just above the trap. The rain water goes through the floor drain trap out to the street through the combination sewer pipe. (In many old houses, there are not separate storm and sanitary sewers. All water is carried in a combination sewer.)

Debris can accumulate in the floor drain trap, clogging it. This can cause water to back up into the basement up through the floor drain during heavy rains. Rain water from the roof runs into the gutters, down the downspouts and to the floor drain. If the floor drain trap is clogged or broken, the water will back up through the floor drain, flooding the basement. Many people confuse this with a sewer backup.

One of the ways to identify this arrangement is by noting a draft (a cold draft in winter) coming back up through the floor drain. Check that the draft is not coming back up through the sewer system. This may happen if the trap has no water. This would cause a sewer odor. If there is a draft of fresh air, it is probably air coming down the downspout, under the basement floor, and into the floor drain above the trap. You may be able to see where the downspout extension discharges into the floor drain.

Discharge to Other Areas In some cases, downspouts drain into below ground pipes which eventually discharge onto the ground at a lower point. Look for this on sloped lots where water can flow by gravity to outlets in the side of hills. The downspout may drain into a dry well below ground (French drain), or into a storm sewer, which leads to the street. Storm sewers in newer developments often serve downspouts.

Clogging The causes of downspouts below grade becoming clogged or broken are debris accumulation, soil pressure (perhaps frost), tree roots entering the pipe through joints and deterioration of the pipe. The implications are leaky or flooded basements and subgrade areas. Foundation damage may also result.

There is not much that can be seen of these downspout extensions after the house is built, so it is difficult to detect problems. The only clues are usually moisture in the building. In some cases, the top of the downspout extension at grade can be seen to be cracked, broken or backing up.

Downspouts extending below grade do not have to be changed if there are no problems, but when problems are encountered, they are often expensive. One cost effective solution is to abandon clogged or collapsed pipes below grade, and provide above-grade downspout extensions. The section below grade is capped at grade level, abandoning the pipe below that. The downspout is cut off a foot or so above grade, and an elbow is provided with a 6- to 10-foot-section of nearly horizontal pipe. This is an inexpensive and effective solution to what is otherwise an expensive problem.