What kind of fallacy is this where one appeals to spare of the moment invented general principle? Is it still called argument from authority?

Shingles make your roof water proof. So not really sure what you or the contractor is talking about. There of course has to be properly installed flashing around vents,chimneys, etc. Unless it's not a house and it's a flat roof then it's hot tar and roofing paper mostly, sometimes a type of asphalt. So I have no idea where he gets 4 steps from. Unless your roof is simply rotting all the way to its cross members. And there's no fallacies here to discuss, are you sure you meant to put that here? If he's talking about actually building the roof from scratch for you and you're refusing steps then building codes would reject the construction. You really aren't being specific enough, I'm thinking there might be a language barrier. Roof from scratch would be, on a house is only support beams, plywood or decking, tar paper, then shingles. You can't not do all these steps and pass any building codes I've ever known.