You've decided to improve a part of your home. And you've decided not to do it yourself. Obviously, you will need to hire someone to do it for you.

This is when you need to know more about the inner workings of home improvement contracting. Deep within our vault of contracting knowledge, there are many secrets consumers should know.

Here's a formula to save you some money and heartache:

If you're shopping for a new sweater, you know about lots of factors to consider: color, size, fabric, neckline, weave, pattern, appearance and price. After all, you're good at sweater shopping because you have done it for years.

But if you're in the market for, say, a new roof, chances are it's not something you have purchased before.

So you're going to have to very quickly learn as much about roofs as you've learned over the years about sweaters.

Don't call a contractor before you do. Why?

That's why you should spend a lot of time on the Internet, reading magazines and talking to friends before you call a contractor, even if the temptation to bring in an expert early on is strong.

Learn about all the materials out there, their strengths and weakness and, importantly, their price.

Doing this puts all the bidders on an even playing field, and makes clear what you want, and your price range.

And, here's the big payoff: If you have a good sense of the product price, the main variable will be the installation cost.

Some contractors count on you not knowing about your various material choices and costs so that they can charge you an exorbitant installation price.

You can rule that our right there, with your knowledge of the product, and a written document that explains what you know.

So the lesson: When you know the product price and all about the product, suddenly the daunting task of a major home improvement can feel a lot more like buying a sweater.