The roof provides a waterproof barrier to the elements. Water will destroy the interior of your home if it is allowed in. Even a minor leak can cause rot in wood, corrosion of drywall and encourage the growth of molds. This is why a shingling job must be done correctly. The good news is, once done, asphalt shingles will last 15 to 20 years, so you won't have to do the job again for a long time.