An earth-covered roof over a living space is a stack of layers on a structural deck, typically deck, tar coating, plastic sheeting, gravel drainage layer, filter fabric and soil, with the surface crowned so water sheds to the edges at a slope of at least 2%. The structure below must carry the soil when it is saturated, which adds about 25% to its weight, plus any people or snow on top.
The earth-covered roof puts a living room under a layer of soil, gaining insulation and fire resistance at the price of load and leak risk.
The Layers Each Do One Job
The parent article gives the sequence from the structure up:
| Layer | Job |
|---|---|
| Structural roof deck | Carries everything above |
| Tar coating | First waterproofing layer |
| Plastic sheeting | Second waterproofing layer, a different material |
| Gravel drainage layer | Carries water to the edges |
| Filter fabric | Stops soil washing into the gravel |
| Soil | Insulation, weight, planting medium |
The roof surface is crowned, higher in the middle, with a slope of at least 2% so water sheds to the edges and does not pool. Details of the membranes are in Waterproof Membranes (Primitive and Salvage), and of the structure in Roof Structures for Earth Loading.
Roots can push through some membranes over time, so a membrane that is also a root barrier, or a layer of clean gravel between roots and membrane, protects it. An earth cover does not burn, which protects the building against wildfire.
What You Need
- A roof structure rated for the saturated soil load plus margin; see Earth Load Calculations.
- Membranes, drainage layer and filter fabric.
- Edge restraint to hold the soil at the roof’s perimeter.
- Outlets for the drainage layer at the eaves.
Method: Build From The Deck Up
Step 1: Build and prop-check the deck.
Step 2: Crown the surface so the finished slope is at least 2% toward the edges.
Step 3: Lay the membranes and test for leaks before covering.
Step 4: Lay the drainage layer, then filter fabric.
Step 5: Place the soil in even lifts, working from the edges to the middle so the load is applied evenly, and plant to hold the surface.
Step 6: Inspect after the first heavy rain from below, looking for damp spots.
Common Mistakes
- Flat roof. Water pools and finds a way through.
- Soil sized for dry weight. Saturated soil adds about 25%.
- Edge washouts. The soil slumps at the eaves; restrain it.
- Standing loads on the roof. Stacked material or vehicles are live loads.
Safety
Earth roofs fail without warning
A collapsed earth roof buries the people beneath. Do not remove props before the deck is proven, keep everyone out from under the roof while soil is being placed, do not park vehicles or pile spoil on it, and plan for the saturated load, not the dry one.
What’s Next
- Learn next: Storm and Fallout Shelters
- Combine with: South-Facing Glazing