A perimeter French drain is a gravel-filled trench around the building at footing level holding perforated pipe that falls at least 1% toward an outlet, with the gravel covered by filter fabric or burlap to keep soil out. The outlet must reach daylight or a dry well at a lower elevation, because a drain with nowhere to discharge only stores water.
A French drain collects groundwater around the footing and carries it away by gravity.
The Drain Lowers The Water That Would Otherwise Press On The Wall
Water pressing on a buried wall can be a larger load than the soil itself, as Earth Load Calculations shows. A drain at footing level keeps the groundwater from building up against the wall. For field drainage in farming, the same principles apply at a different scale; see Subsurface Drainage.
What You Need
- Perforated pipe (clay, plastic, or salvaged pipe drilled with holes).
- Clean gravel, without fines.
- Filter fabric: salvaged landscape fabric or burlap, to stop soil clogging the gravel.
- A level or water level to set the fall; see Levels and Plumb Bobs.
- An outlet at daylight or at a dry well at a lower elevation.
Method: Trench, Pipe, Gravel, Fabric
Step 1: Dig a trench around the structure at footing level. Slope its floor with a fall of at least 1% toward the outlet.
Step 2: Lay the perforated pipe in the bottom, bedded in gravel.
Step 3: Fill with gravel, then cover the top with filter fabric to prevent soil from clogging the gravel.
Step 4: Connect the outlet to daylight or a dry well at a lower elevation.
Step 5: Test it by pouring water in at the high end and watching it emerge at the outlet.
Worked Example
A fall of 1% is 1 cm of drop for every 100 cm of run.
- Inputs: 20 m of pipe run, 1% slope.
- Operation: drop = 20 m x 0.01 = 0.20 m.
- Result: the low end of the pipe is 0.20 m (20 cm) lower than the high end.
So the outlet must be at least 0.20 m below the pipe’s starting level at the footing, and the trench is correspondingly deeper at the low end. A longer drain needs a still lower outlet, which is why the outlet position is chosen before the trench is dug.
Common Mistakes
- No outlet, or an outlet that sits higher than the pipe.
- No filter. Silt clogs the gravel and the drain stops working.
- Roof water dumped into the drain without checking capacity.
- A pipe with no fall. Water stays in it.
Safety
Deep drain trenches collapse
A drain trench at footing level for a buried building is often deeper than about 1.2 m, especially at the low end where the fall adds depth. Slope the sides back or shore them, keep spoil at least 0.6 m from the edge, and never dig alone.
What’s Next
- Learn next: Interior Moisture Management — what remains inside
- Combine with: Waterproof Membranes (Primitive and Salvage) — the second line