An earth roof is carried by heavy timber beams, a masonry or earthbag arch or vault, or a corbelled stone roof for spans under 2.5 m. For timber, spans over 3 m need intermediate supports, and the parent article’s rule of thumb of a 25-30 cm log for a 3 m span should be checked by arithmetic and a load test, then sized with at least three times margin because earth roofs fail without warning.
The roof structure of an earth-sheltered building carries the soil, the water in it and any live load across the room to the walls.
Three Roof Forms Suit Three Spans
| Form | Span the parent article gives | Notes |
|---|---|---|
| Heavy timber | Over 3 m needs intermediate supports | Round logs or hewn beams across the short dimension |
| Arch or vault | Not limited by a stated span | Masonry, brick or earthbag in compression; side walls must be buttressed against outward thrust |
| Corbelled stone | Under 2.5 m | Overlapping courses closing inward; Irish clochan huts and Mycenaean tholos tombs |
An arch is the most durable form: it turns the soil weight into compression, which stone and earth handle well. The price is the sideways thrust at its base; see Buttressing and Reinforcement for the same problem in domes and Arch Construction for the geometry.
What You Need
- Round logs or beams of known species, straight and free of rot and large knots.
- Walls or posts to carry the ends; see Soil Bearing Capacity for the ground under them.
- A decking layer across the beams, then the waterproofing system.
- A way to test a sample beam: support a length of the same timber at its ends and add load in measured increments until it fails.
Method: Check A Log Roof Before Cutting Timber
Step 1: Get the soil pressure from Earth Load Calculations: 8.8-11.8 kPa dry for 0.6 m of cover, 25% more saturated.
Step 2: Fix the log spacing. Load per log is pressure times spacing times span.
Step 3: Find the bending moment for a simply supported log: M = w L² / 8.
Step 4: Find the section modulus of a round log: S = π d³ / 32.
Step 5: Compute the bending stress: σ = M / S.
Step 6: Compare it with the strength of your timber, found by test, and apply the three-times margin.
Worked Example
Inputs: span L = 3 m (the parent article’s example), log diameter d = 0.25 m, cover 0.6 m, log spacing 0.4 m (an assumed value). Roof weight of logs and decking, and live loads, are left out.
- Load per metre of log, dry, at 1,500 kg/m³: 8.83 kPa x 0.4 m = 3.53 kN/m. At 2,000 kg/m³: 11.77 x 0.4 = 4.71 kN/m. Saturated (x 1.25): 4.41 to 5.89 kN/m.
- Moment, M = w L² / 8 = w x 9 / 8: dry 3.97 to 5.30 kN·m; saturated 4.97 to 6.62 kN·m.
- Section modulus: S = π x 0.25³ / 32 = π x 0.015625 / 32 = 0.001534 m³.
- Stress, σ = M / S: dry 3.97 kN·m / 0.001534 m³ = 2.59 MPa up to 5.30 / 0.001534 = 3.45 MPa; saturated up to 6.62 / 0.001534 = 4.32 MPa. So the stress runs from 2.6 to 4.3 MPa across dry-low to saturated-high.
- For a 0.30 m log: S = π x 0.027 / 32 = 0.002651 m³, and the same loads give 1.5 to 2.5 MPa.
Applying the three-times margin to the 0.25 m log multiplies these by 3, giving about 7.8 to 13 MPa of demand. Whether a log can carry that depends on its species, knots and moisture, which are not known in advance; test sample logs to failure and use the test result. Closer spacing cuts the stress in proportion: halving the spacing to 0.2 m halves each figure. Shear, deflection, bearing at the ends and the weight of the roof itself are also excluded.
Common Mistakes
- Taking the rule of thumb as a design. The 25-30 cm figure holds only for the stated span and cover.
- Sizing for dry soil. Saturation adds about a quarter.
- Rotten or green timber. Wet earth keeps timber damp; use durable or treated wood.
- Ends bearing on loose fill. The load must reach firm walls.
Safety
Roof collapse is sudden and fatal
There is no warning before an earth roof fails. Do not remove temporary props until the full cover is on and tested, never stack spoil on a fresh roof, and keep people out from under it while it is loaded in stages. Test a sample beam with a margin before trusting a species.
What’s Next
- Learn next: Retaining Wall Design — the walls under the roof
- Combine with: Timber Framing — joining timber