Living Roof (Earthen) Calculator
Soil, drainage, and structural load estimates for earthen (green) roof systems.
About this calculator
Roof Area and Soil Depth together set the volume of growing medium (their product converted from sq ft x inches into cubic yards), and that same soil volume, plus a separate Drainage Layer volume, rolls up into Saturated Load -- the dead load per sq ft your roof structure must support once the medium is fully waterlogged. Roof Slope % is collected on the form and displayed back in the calculation log, but it never actually enters the math: soil volume, drainage volume, saturated load, and membrane area all come out identical whether you enter the 2% code-minimum slope or the 50% maximum. Real living roofs do need adequate slope for drainage, so treat that field as a planning reminder rather than a driver of any number below it.
Membrane area is simply roof area plus a flat 20% overlap allowance. At the default 300 sq ft roof with 6 in of soil, saturated load comes out to 60 psf -- comparable to a light snow load -- which is why the calculator's own guidance is to verify that figure against your roof's actual structural design load before committing to an earthen roof retrofit.
Inputs
Results
Soil (cu yd)
5.56
How to Use This Calculator
- Enter Roof Area (sq ft) and the planned Soil Depth (inches) for your growing medium.
- Set Drainage Layer Depth (in) to size the gravel layer below the soil (Roof Slope % is for your own reference on drainage planning — it is not used in these calculations).
- Review Soil Needed (cu yd) and Drainage Gravel (cu yd) to plan material delivery.
- Check Saturated Load (psf) — verify this against your roof structural capacity before proceeding.
- Consult a structural engineer if the saturated load exceeds the roof's design load.
How the result changes with Roof area (sq ft)
| Roof area (sq ft) | Soil (cu yd) |
|---|---|
| 150 | 2.78 |
| 225 | 4.17 |
| 450 | 8.33 |
| 750 | 13.89 |
What each input means
- Roof area (sq ft)
- Total area of the living roof.
- Soil depth (in)
- Depth of growing medium (4-6 in extensive, 8-24 in intensive).
- Drainage layer (in)
- Thickness of gravel drainage layer below soil.
- Roof slope %
- Roof pitch as a percentage (2% minimum recommended).
What each result means
- Soil (cu yd)
- Volume of growing medium required.
- Drainage gravel (cu yd)
- Volume of gravel or drainage aggregate.
- Saturated load (psf)
- Dead load per sq ft when fully saturated with water.
- Membrane (sq ft)
- Waterproof membrane area including overlaps.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersRoof area (sq ft) = 300, Soil depth (in) = 6, Drainage layer (in) = 2, Roof slope % = 2 = 4 input(s) provided
- Calculate SoilSoil = round(soilVolCuFt / 27 * 100) / 1005.56 = 5.56
- Calculate Drainage gravelDrainage gravel = round(drainVolCuFt / 27 * 100) / 1001.85 = 1.85
- Calculate Saturated loadSaturated load = round(saturatedWeightLb / roofAreaSqFt * 100) / 10060 = 60
Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Does the Roof Slope % input change any of the calculated quantities?
No -- Roof Slope % is never referenced inside the calculation. Soil volume, drainage volume, saturated load, and membrane area are identical whether you enter 2% or 50%; the field exists for your own drainage planning, not as a driver of these outputs.
What actually drives the Saturated Load figure?
Saturated Load comes from Roof Area, Soil Depth, and Drainage Layer thickness together -- it's the combined weight of soil (at roughly 100 lb per cu ft saturated) and drainage gravel (about 60 lb per cu ft) divided back over the roof area, expressed as pounds per square foot.
How much soil does a typical extensive green roof need versus an intensive one?
The calculator's own guidance is 4-6 in of soil for an extensive (low-maintenance, lightweight) system versus 8-24 in for an intensive (garden-capable) system -- moving from the shallow to the deep end of that range roughly quadruples both soil volume and saturated structural load.
Why is checking saturated load against structural capacity so important?
Because a living roof's weight isn't fixed -- it swings from dry to fully saturated after rain, and the saturated figure (60 psf at this calculator's defaults) is comparable to a light snow load added on top of the roof's normal dead load, which many conventional roof structures were never designed to carry.
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