Salvaged EPDM pond liner is the best membrane for a buried roof, followed by heavy polyethylene sheeting of 6 mil or thicker in several layers and tar or bitumen with fabric embedded in it. Primitive options are a 5-10 cm layer of bentonite clay, overlapping birch bark with tar between layers, or 15-20 cm of rammed clay, and they work best stacked as a layered system.
A membrane is the sealed layer that lies between the structure and the soil; layered membranes cover for each other’s weaknesses.
Each Membrane Fails In A Different Way
No single membrane is perfect, which is why the parent article recommends layers:
| Membrane | Strength | Weakness |
|---|---|---|
| EPDM rubber pond liner | Flexible, durable; the gold standard | Needs sourcing from pond supply or roofing salvage |
| Polyethylene sheeting (6 mil+) | Common and effective short-term | Degrades over decades underground; use several layers |
| Tar or bitumen with embedded fabric | Time-tested; tar waterproofing dates to Mesopotamia | Needs heating and application; hazardous to handle |
| Bentonite clay, 5-10 cm | Swells when wet and seals | Needs deposits of natural bentonite, and soil cover above |
| Birch bark, overlapped with tar | Traditional in Scandinavian and Russian building | Labour-intensive |
| Rammed clay, 15-20 cm | Pure, well-compacted clay | Cracks with seasonal movement |
A typical earth-sheltered roof stacks several: structural roof deck, tar coating, plastic sheeting, gravel drainage layer, filter fabric, soil. For the chemistry of pitch and tar see Pitch and Waterproofing.
What You Need
- Membrane materials from the list above.
- Tools for lapping and sealing joints: knives, brushes, tar pots.
- Protection for the membrane: a layer of fine soil, sand or boards between it and the backfill.
Method: Lay, Overlap, Protect
Step 1: Prepare the surface. It must be clean, smooth and free of sharp points that could puncture the membrane.
Step 2: Apply the first layer and overlap seams generously, sealing them as the material requires.
Step 3: Apply a second, different layer over the first so a flaw in one is covered by the other.
Step 4: Add the drainage layer and filter above; see French Drains and Perimeter Drainage.
Step 5: Backfill by hand near the membrane and use fine, stone-free soil for the first lift.
Step 6: Check the result by running water over the surface before the soil goes on, if you can.
Common Mistakes
- Punctures during backfill. A single sharp stone defeats a membrane.
- Leaving seams open.
- Rammed clay as the only layer. It cracks as the seasons move it.
Safety
Hot tar burns and ignites
Tar and bitumen are heated to be applied: they cause deep burns, catch fire and give off harmful fumes. Heat them outdoors in small batches, wear gloves, boots and eye protection, keep water and damp tools away from hot tar because it spatters, and keep a smothering lid and sand at hand. Never heat tar near polypropylene bags or other plastics. See Burn Classification for burn first aid.
What’s Next
- Learn next: Interior Moisture Management — the inside of the barrier
- Combine with: Earth-Covered Roof Living Spaces — the roof system as a whole