French Drain Basics
A traditional french drain consists of a trench filled with rock or gravel. The bottom of the trench slopes at a grade of approximately 1 to 3 percent to lead water away from the yard or structure.
Most modern builders place a weed-blocking landscape fabric across the bottom and sides of the trench and lay a perforated drain pipe at the trench's bottom.
Advantages of Installation Below Frost Line
Year-round functionality is the most important benefit of placing a french drain below the frost line. Placed below the frost line, a french drain's drainage gravel or pipe remains clear of frozen obstructions and continues to drain regardless of freeze and thaw cycles.
Additionally, placing plastic drainage pipe below the frost line is particularly important. Subject to regular freezing and thawing, plastic pipes burst or crack.
Reasons for Above Frost Line Installation
Occasionally, installing a french drain below the frost line is impractical. For example, the presence of tough, rocky soils might make trenching below the frost line difficult or impossible.
Alternatively, if the frost line is exceptionally deep, you might lack the equipment or labor to complete the job. Traditional french drains are often located above the frost.
Without plastic drain pipes, traditional french drains do not contain components that become damaged during freeze and thaw cycles. However, if placed above the frost line, the french drain will not properly function during freezing temperatures.
French Drain Variations
Builders and landscapers add several features to basic french drains to increase the drains' functionality and improve the drains' appearance. Connections to household gutter systems are the most common functional variation of french drains.
Rather than dump on the ground, gutter downspouts plunge below the ground's surface to connect with a french drain's plastic drainage pipe. Directly fed to the french drain, a gutter's runoff no longer pools or floods near a structure's foundation.
To improve drain appearance and functionality, landscapers often lay sod over a french drain, particularly on golf courses and parks. Whereas the upper, gravel layer of traditional french drains remains exposed, landscapers can cover the gravel with landscaping fabric, a layer of sand and sod.