Article: Topsoil (soil from the garden) is not used for green roofs because it's usually too heavy and tends to become too compacted.  Use a mixture of organic and inorganic materials. Focus on keeping the elements lightweight but water retaining.    {"smallUrl":"https:\/\/www.wikihow.com\/images\/thumb\/c\/c1\/Build-a-Green-Roof-Step-6Bullet1-Version-2.jpg\/v4-460px-Build-a-Green-Roof-Step-6Bullet1-Version-2.jpg","bigUrl":"\/images\/thumb\/c\/c1\/Build-a-Green-Roof-Step-6Bullet1-Version-2.jpg\/aid688104-v4-728px-Build-a-Green-Roof-Step-6Bullet1-Version-2.jpg","smallWidth":460,"smallHeight":345,"bigWidth":"728","bigHeight":"546","licensing":"<div class=\"mw-parser-output\"><p>License: <a rel=\"nofollow\" class=\"external text\" href=\"https:\/\/creativecommons.org\/licenses\/by-nc-sa\/3.0\/\">Creative Commons<\/a><br>\n<\/p><p><br \/>\n<\/p><\/div>"}  Roof growing mediums are usually fabricated from variances of primarily expanded shale, perlite, coconut husk, and/or peat moss.    {"smallUrl":"https:\/\/www.wikihow.com\/images\/thumb\/2\/2c\/Build-a-Green-Roof-Step-6Bullet2-Version-2.jpg\/v4-460px-Build-a-Green-Roof-Step-6Bullet2-Version-2.jpg","bigUrl":"\/images\/thumb\/2\/2c\/Build-a-Green-Roof-Step-6Bullet2-Version-2.jpg\/aid688104-v4-728px-Build-a-Green-Roof-Step-6Bullet2-Version-2.jpg","smallWidth":460,"smallHeight":345,"bigWidth":"728","bigHeight":"546","licensing":"<div class=\"mw-parser-output\"><p>License: <a rel=\"nofollow\" class=\"external text\" href=\"https:\/\/creativecommons.org\/licenses\/by-nc-sa\/3.0\/\">Creative Commons<\/a><br>\n<\/p><p><br \/>\n<\/p><\/div>"}

What is a summary?
Use a lightweight growing medium.