For a flat roof with an average size of.. let’s say 600 square feet, you can expect to pay around $3,000 to $7,000 for a simple, single-story installation. Keep in mind that costs may increase if there are any access obstacles or construction in progress. Commercial properties and larger residential projects have more variable pricing, and can span between $5,000 and over $500,000. To get an accurate quote, it’s best to obtain a flat roof survey.