It is funny how Tim Gentry just wrote a blog on this subject, and the same question came up probably 3 times this week in my phone conversations with design professionals.
Design Opportunity - We want to use your product, but we really need to stay within the budget. How much waste do I need to order? I can't afford 20% waste like I need with other steep slope roofing options.
Design Solution - Unlike our natural slate counterpart, when ordering our plastic slate roof tiles you need minimal waste. According to Tim, you need no waste for a hip roof and one extra square of material per 100 lineal feet of valley on a gable roof. If using our multi-width products, as you reach the eaves, you can always find a combination of two or three pieces so you can avoid cutting when you get to the end of a row.
Relief! Crisis averted through the power of blogging. Though I have heard of people using their extra material on dog houses, children's play houses, and gazebos.