Generally speaking I use the following rule of thumb, 1/2 yard per square. A 21 square roof with two layers would follow this formula: .5 x 21 x 2 = 21 yards. This .5 yards per square is on the high side and will leave you room for cut offs, packaging, felt, etc... If I were ordering a dumpster I would order a 20 yarder and would have room to spare for neighbor's donations.
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