gravel mulch on the roof of a flat green roof. covers and protects layers

iron wheel machine drum lighting engine glass vehicle drums drink art tire room furniture metal ceramic gravel mulch roof flat green covers protects layers acclimatization air boiler building chimney circle closeup color conical construction design evaporation exhaust exhausted four garden gas gray heating hole hot hotel industrial installation instruction kitchen maintenance metal nozzle object outdoor outlet pebble pebbles pipe protection revision roof service shape silver stainless steel sweep sweeping temperature top tube turbo white laboratory