Nothing is more disheartening than spending thousands on a new tile system, only to see water still ponding. Perhaps the main tile collecting water from laterals wasn’t sized big enough.

Tom Nugent and his son, Thomas, Elnora, Ind., say making sure the main tile line is large enough is a priority. Tom knows that if the main can’t carry water away fast enough, water backs up in laterals. That defeats the purpose of investing in narrower tile spacing, he notes.

“There are two parts to understanding how to size drainage systems,” says Ehsan Ghane, Extension drainage specialist at Michigan State University. “One is drainage intensity. That refers to how water moves through soil into tile lines, or laterals, measured in inches moved per day.

“The second part is water movement through the drainage tile network, referred to as the drainage coefficient. The drainage coefficient should be equal to or greater than drainage intensity to get best performance.”

Practical tiling examples

If the main isn’t sized large enough, water doesn’t flow through the system efficiently, Ghane says. The bigger the diameter of the main line, the higher the cost of installing the system. If the main collector is bigger than necessary, you spend extra money, but it is better to be oversized than undersized, Ghane says. A larger main allows for a safety factor.

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Here are examples illustrating his point:

Undersized system. Based on soil type, suppose drainage intensity is 0.5 inch per day. That means a half-inch of water can move through the soil to tile lines each day.

Field A: The main is sized at a drainage coefficient of 0.5 inch per day, matching the drainage rate within the soil. This system should carry water away from laterals at the same rate as water moves through the soil.

Field B: The main is sized at a drainage coefficient of 0.375 inch per day. Even though this is a commonly used sized, in this case, it is too small. “Water moves through the soil and reaches tile lines faster than the main line can carry water away,” Ghane explains. “The system is inefficient, and soils would not dry out as fast as they should. This system would underperform.”

Oversized system. Assume drainage intensity is 0.3 inch per day.

Field A: The main is sized at a drainage coefficient of 0.5 inch per day. It carries water away from laterals faster than water moves through the soil.

Field B: The main is sized at a drainage coefficient of 0.4 inch per day. Water still moves through the soil slower than the main carries it away, but it is closer to the correct size. “The tile sized to remove 0.4 inch would be a good investment, allowing for future expansion by adding more laterals,” Ghane says.

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Help for design

So, how do you determine the proper size and spacing for lateral lines in a pattern-tile system? And how do you figure drainage coefficient so the main line is sized correctly?

MSU hosts a drainage workshop, teaching how to determine if your existing main collector is sized properly. Registration is open. MSU also has a computer-based tool for making these determinations, Ghane says. Find a tutorial and other helpful videos online as well.