Soil Mixes
The engineered soil mixes are what plants need to grow on green roofs, they function like soil in gardens on the ground. But they look and feel different. Lightweight Soil Mix used in Portland Ecoroof projects is made up of 75% mineral and 25% compost/organic. Most of the mineral is made up of pumice, which is available in the Pacific Northwest. Lightweight soil medium consists of coarser mineral aggregates and some organic matter.

Green roof medium vs. garden soil
Not fine textured, soft, and earthy
When wet the medium does not get sticky or muddy
Most of its particles are larger than sand, silt, and clay that make up garden soil
It looks and feels rockier than garden soils.
Getting the proper specifications for engineered soil mix can be very difficult with so many influential variables and its characteristics being so different from garden soil.

Soil Depth
The deeper the soil the better the soil is at moisture retention, stability for vegetation, and insulation for the building.

General qualities of growing medium
Good and consistent aeration and drainage
Good medium structure that will retain water so the plants can use it
Good at retaining nutrients and providing exchanges of cation capacity for uptake through plant roots
Resistant to decompositions and compression
Lightweight
Physical and chemical stability

Materials for growing mediums
expanded clay 40 lbs/sq ft dry, 54 lbs/sq ft saturated
expanded shale 44 lbs/sq ft dry, 58 lbs/sq ft saturated
expanded slate

All aggregates are heated in an extremely hot kiln which causes the material to expand into lightweight pop-corn forms that retain their strength and density

Pumice (a volcanic material) 40 lbs/sq ft dry, 47 lbs/sq ft saturated
Pumice is available in the Pacific Northwest as the source is in the form of volcanoes that make up the Cascade Mountain Range. It is durable, light, and easier to produce and extract.

Each of these materials listed above have sponge like forms with pores that hold water and organic matter, that allow water to infiltrate slowly.

Perlite (a volcanic material)
Styrofoam (unnatural material)
Sand 90 lbs/sq ft, 130 lbs/sq ft

Sand is more heavy and does not retain nutrients well, can be used sparingly
Expanded aggregates are more difficult to produce and when it is not locally available it is expensive to transport.

Compost/Organic Matter- use very little

This graphic breaks down each component that is on a green roof system and its mass. Photo courtesy of Ecoroof Handbook City of Portland.

Use of benchmarks and guidelines for the right specifications:
Selecting a growing medium blend can be done with existing guidelines, benchmarks, and testing from a German Landscape Research,Development, and Construction Society, known as FLL (Forschungsgesellschaft Landschaftsentwicklung Landschaftsbau) and in North America the ASTM international.
The FLL is a good starting point in providing information on specific growing mediums breaking it down by ganulometric distribution (particle sizes), levels of organic matter, and other properties. It provides a good range, which helps as variables in site conditions, regions, and design ranges.
A sample of a lightweight soil blend can be tested at only one facility in the US with FLL and ASTM test methods at Penn State University Agriculture Analytic Service Lab.

Soil Specifiers have FLL tested soil blends in the market. The engineered soil must factor in the region and the specific plant palette that a designer wants to grow.