Water is what makes or breaks an underground building: drainage comes first, to keep water from ever reaching the structure, and a waterproof membrane comes second as the final barrier. Use several redundant barriers instead of trusting one, because earth above you means water above you.
Waterproofing and drainage are the two systems that keep groundwater and rain out of a buried building.
Drain First, Then Seal
The most effective waterproofing is stopping water getting to the structure at all. A perimeter drain at footing level carries water away, and a roof crowned to shed water keeps it from pooling. The membrane is the second line: if the drain fails or is overwhelmed, the membrane must hold. A building that depends on a membrane alone has a single point of failure.
The three parts of the subject are:
- Drains around the building: French Drains and Perimeter Drainage.
- Membranes, including improvised ones: Waterproof Membranes (Primitive and Salvage).
- Moisture inside the finished space: Interior Moisture Management.
For general sealants, see Waterproofing.
What You Need
- A drainage outlet lower than the footing.
- Perforated pipe, gravel and filter fabric or a substitute.
- A membrane or several, with materials for overlapping and protecting them.
- A roof shaped to shed water.
Method: Layers In Order
Step 1: Set the outlet level first. The drain only works if its outlet is lower than its start.
Step 2: Lay the perimeter drain and cover it with gravel and filter.
Step 3: Apply the membrane layers to the roof and walls.
Step 4: Protect the membrane from sharp stones during backfilling.
Step 5: Shape the surface so water runs away; an earth roof is crowned with a slope of at least 2%.
Common Mistakes
- Relying on one barrier. Expect water intrusion and build redundancy.
- No drainage. Water pressure against walls and roof is relentless.
- Forgetting the roof surface. A flat earth roof pools water.
Safety
Hot tar and deep trenches
Tar and bitumen burn and catch fire when heated; heat them outdoors in small batches and keep water away from hot tar. Drain trenches deeper than about 1.2 m need sloped or shored sides.
What’s Next
- Learn next: French Drains and Perimeter Drainage
- Combine with: Structural Considerations — wet soil is heavier