French Drain & Drainage Trench Slope Calculator
Calculate French drain downhill slope fall, starting and ending trench depth, washed drainage gravel tons, non-woven geotextile fabric, and perforated pipe length.
Calculated Results
Frequently Asked Questions
What is the minimum recommended slope for a French drain?
The minimum recommended slope for gravity drainage is 1% (1 foot of drop for every 100 feet of run, or approximately 1/8 inch per linear foot). A 2% pitch is preferred in heavy rainfall areas to prevent silt buildup.
Should the holes in perforated French drain pipe face up or down?
The holes must always face DOWN toward the trench bottom. Groundwater enters through gravel from below as the water table rises; placing holes downward allows water to enter and flow away immediately before filling the gravel bed.
What type of filter fabric is required for French drains?
Always use non-woven geotextile drainage fabric (4 oz needle-punched). Never use woven weed barrier fabric, as woven plastics clog quickly with fine silt particles, rendering the drain useless within months.
Why is round washed rock better than angular crushed gravel?
Smooth washed round river rock (3/4-inch to 1.5-inch) creates larger void spaces for rapid water velocity and is much easier to shovel into filter fabric envelopes than sharp angular stone.