How Much Stone to Add to My French Drains?

Considerations

• Trenches need not be slanted at an extreme angle. A fall of one inch per foot or less is sufficient to draw water away from the wet area. Trenches average 6 to 8 inches wide and 2 feet deep, but local conditions can affect construction. A high water table would make a 2-foot deep trench impossible; it would fill with water and drain continually. Instead, a network of shallower trenches might be the answer. Deeper trenches might be needed for very long drains.

Gravel in French Drains

• Crushed stone that is approximately 1 inch in diameter lines the trench and supports a 4-inch pipe that is perforated on its top and sides. In sandy or loamy soil, line the trench with landscape cloth to keep dirt out of the gravel and out of the pipe.

Computing Volume and Weight

• Stone merchants will help homeowners figure how much gravel they need for their trenches, but the homeowner can estimate volume and weight, too. Gravel is measured by the cubic yard or ton.

  Multiply the length, width and depth of the trench, measured in feet or percentages of feet, to find the total volume of the trench. To find the cubic yards of gravel needed to fill the trench, divide the volume of the trench by 27, the number of cubic feet in a cubic yard. To find the tons of gravel needed, multiply the volume of the trench by a factor of 1.7.

  Don't subtract the volume of the pipe from the volume of the trench because you want the gravel to crest over the trench and allow for settling.

Alternatives

• Digging a proper trench is hard work, even with a trencher or back-hoe. Filling the finished trench with gravel weighing a ton or more can be back-breaking labor. One new, alternative method of building French drains uses no gravel at all. Drain pipes sit in the trench surrounded by poly aggregate material enclosed in a permeable bag. The pre-fabricated drain is then buried with soil. The drain requires no assembly and is lightweight, allowing builders to carry several at once without any special equipment.