An earth roof is cared for by watching its drainage and its load: inspect it from below after the first heavy rain for damp spots, clear the eaves outlets, repair erosion and bare spots, keep roots short and keep stacked loads and vehicles off it. A leak that saturates a 0.6 m soil cover adds about 25% to the load, 55 to 74 kN on a 5 m by 5 m roof, roughly the weight of 5.6 to 7.5 tonnes.
Part of Building Maintenance Schedules
Earth roof care is the inspection and upkeep of a soil-covered roof, its drainage layer, membrane and edges, and the loads it carries.
The Hazard Is Water Inside The Soil, Not Only Water Through The Roof
The maintenance guide’s earth-roof checks are erosion, bare spots and standing water, which points to poor drainage slope. The earth roof guide gives the construction: a deck, two waterproofing layers, a gravel drainage layer, filter fabric and soil, crowned to a slope of at least 2%, with outlets at the eaves and an edge restraint (Earth-Covered Roof Living Spaces). Roots can push through some membranes over time. The load on the structure is large and changes with the water in the soil, and that makes maintenance a structural matter.
Worked Example: What Water Adds
The earth load guide gives 0.6 m of soil at 1,500-2,000 kg/m³ as 8.8-11.8 kPa, 221-294 kN on a 5 m by 5 m roof, and about 25% more when saturated (Earth Load Calculations).
- Saturated: 221 × 1.25 = 276 kN to 294 × 1.25 = 368 kN.
- The extra from a leak or blocked drain: 276 − 221 = 55 kN to 368 − 294 = 74 kN. Dividing by g, 55,000 ÷ 9.81 = 5,600 kg and 74,000 ÷ 9.81 = 7,500 kg, so 5.6 to 7.5 tonnes. Force in kilonewtons is not mass; the tonnes are only the equivalent weight.
- Ponded water: 10 cm standing on the roof is 0.10 × 1,000 × 9.81 = 981 Pa, 0.98 kPa, which is 0.98 ÷ 11.8 = 8% to 0.98 ÷ 8.83 = 11% of the dry soil load.
- Erosion: losing 5 cm of the 60 cm cover reduces the load by 5 ÷ 60 = 8%, but also reduces the insulation and exposes the membrane to sun and frost.
So a roof that is safe when dry can be near its limit when the soil is soaked and the roof is ponded, and the same event that overloads it, a leak, also rots the timber that carries the load.
What You Need
- A flashlight or lamp, a probe and a tape measure
- A string line stretched between the walls under the deck at mid-span, to measure sag
- Sods or soil and a rake, and a shovel, for erosion repairs
- Access to the eaves outlets, with a rod to clear them
- A log with the sag measurement and the dates
Method: Inspect Below, Then Above
Step 1: After the first heavy rain, look from below for damp spots and drips on the deck, and trace each upslope.
Step 2: Measure the sag from the string to the deck at mid-span each season and log it. A figure that grows between two seasons is a warning.
Step 3: Inspect the eaves outlets and clear them; water that cannot leave stays in the drainage layer.
Step 4: From the edges, look for erosion, bare spots, slumped edge restraints and ponds, and repair with sods or soil, placed from the edges toward the middle in even lifts.
Step 5: Keep the surface to grass and shallow-rooted plants and remove shrubs and saplings, whose roots can push through membranes.
Step 6: Keep loads off it: no stacked material, no vehicles, no spoil piles.
Common Mistakes
- Walking on a saturated roof. It is at its heaviest and the timber beneath may be damp.
- Digging out the cover to find a leak without propping the deck first.
- Letting trees grow. Their roots and their weight are both loads.
- Ignoring a small sag. It is the first measurement of a failing deck.
Safety
Earth roofs fail without warning and bury the people beneath
A rotted log or a failed prop cannot be seen under the soil, and the inspector’s weight on a saturated roof can be the last load. Inspect from below first, keep everyone out from under a roof that is sagging or dripping, do not stack or park on it, and if a deck drops, empty the building and stay out. The corpus warns that a collapsed earth roof buries the people under it, and that the design load should be the saturated one (Roof Structures for Earth Loading).
What’s Next
- Learn next: Leak Detection & Emergency Patching — finding the entry point
- Combine with: Foundation Drainage Checks — the same water, at the base