Earthbag walls are kept dry by layers rather than a single membrane: gravel-only base courses against rising ground water, a drained foundation, plaster on every exposed face, and a roof that keeps rain off the wall. A sealed, impermeable skin is the wrong goal for an earth-filled wall, because water that gets behind it cannot escape.
Part of Earthbag Building
Waterproofing here means keeping water away from the earth fill and letting any that gets in dry out again.
A Layered Defence Beats A Single Skin
The fill is earth, so it softens and weakens if it stays wet. The parent article’s defences work at different levels:
| Layer | What it stops |
|---|---|
| Gravel-only first courses | Water rising from the ground |
| Drained foundation | Water standing under the wall |
| Plaster (lime or cement-lime outside) | Rain on the wall face |
| Roof overhang | Rain running down the plaster |
| Time to dry after brief flooding | Flood water in the fill |
The reasoning for not sealing the wall completely is a drying argument. Any barrier is imperfect, and water that enters behind a sealed face is trapped. A breathable plaster lets the wall dry both ways, which is why the parent article prescribes lime or cement-lime outside, and the straw bale material makes the same point about cement plaster causing hidden rot.
What You Need
- A gravel stem and drained trench; see Gravel-Filled Stem Courses.
- Exterior plaster; see Plaster Systems.
- A roof overhang and ground sloped away from the wall.
- For extra protection at the base: pitch or bitumen as a coating, see Pitch and Waterproofing, and general sealants in Waterproofing.
Method: Build The Layers, Then Inspect
Step 1: Complete the base layers (gravel courses, drain, outlet) before the wall rises.
Step 2: Plaster all exposed faces, with particular care at the base and around openings.
Step 3: Shape the ground so water runs away from the wall.
Step 4: Check after the first wet season for dampness at the base, soft plaster and blocked drain outlets, as set out in Moisture Management.
Step 5: Dry before repairing. After a flood, allow the wall to dry before plastering.
Common Mistakes
- Impermeable coating on a wet wall. It traps the moisture.
- Relying on plaster alone with no gravel base and no drain.
- Forgetting the roof overhang. A long dry face needs less repair.
Safety
Hot tar and pitch
If you coat the base with pitch or bitumen, heating it risks burns and fire, and the fumes are harmful. Heat it outdoors in small batches, never over an open flame near unplastered polypropylene bags, and keep water away from hot tar because it spatters.
What’s Next
- Learn next: Finishing and Protection — the full finishing sequence
- Combine with: UV Protection for Bags — the other threat to bags