Roofing manufacturers' websites can be very useful sources of information, but you always have to remember that these websites were written by people who are trying to sell you something. Browsing through various manufacturer sites and seeing the available products can help you imagine new ways of accomplishing whatever it is that you're working on. Many contain extensive technical information, including roof specifications, material safety data sheets, and roof construction details. Relevant building code, energy code, and tax rebate information can also be found. Remember that these sites will typically have some degree of bias in favor of their own products.