Professional roofers' training varies by state, but they typically need to serve an apprenticeship of at least two years before applying for a roofing license. Always hire a professional with specific experience installing slate roofing. No other material goes on the same as slate tiles. It’s easy to crack the stone when installing it, leading to leaks and damage later on. Local roofing contractors should also have liability insurance and a business license. Call the issuers of these credentials to ensure they’re valid.