A French drain is just a gravel-filled trench with a perforated pipe that carries water away from where you do not want it. Done right, it is nearly maintenance-free.
Use angular rock, not pea gravel
Clean, angular 3/4 inch crushed drain rock is the standard. The sharp edges hold open voids so water keeps moving. Rounded pea gravel packs tight and chokes flow over time.
Wrap it in fabric
Line the trench with filter fabric before you add rock, then fold it over the top. The fabric keeps soil and silt out of the rock, which is the number one reason drains clog.
Slope and outlet
Give the trench a steady fall, about an inch every eight feet, toward a daylight outlet or drywell. Water only moves if you give it somewhere to go.
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