After deciding to start a roofing business, you will need to go through all the proper government institutions to make your business legal. There are different rules and procedures depending on where you plan to operate your business. Make sure you are following proper protocol in your region. This means having necessary insurance, following tax codes, getting proper licenses and permits, etc. This applies even to a part-time roofing business.
Here are links to register a new roofing business in your area:
Start a New Roofing Business in The United States
Start a New Roofing Business in Canada
Start a New Roofing Business in The United Kingdom
More on starting a new roofing business here: How To Start a Roofing Business