1 The cost of a slate roof can be anywhere from around $10 to $40 per square foot installed and as much as $75 per square foot or more. 2  For a 1,500 square foot roof, that works out to approximately $15,000 to $112,500. 3  Synthetic roofing slate costs roughly $6 to $12 per square foot installed. The total cost for a 1,500 square foot rough might be $9,000 to $18,000. 2  Some homes may not be able to support a heavy slate roof without structural reinforcement. 3  The cost of additional framework could cost roughly $2,000 to $12,000 or more.