The materials, labor, and permit costs for a roofing or siding project at your home might make you shudder. But it's not just the anticipated expenses you have to consider. Roofing and siding cost more than planned about 40 percent of the time, at a median surcharge of $600 to $700, according to the Consumer Reports National Research Center's online survey of more than 17,000 Consumer Reports subscribers who altogether had recently done a total of nearly 30,000 home improvements.

(Read our June 2009 report to find the best roofing and siding, which includes ratings of dozens of products, information on types of shingles and siding, and buying advice; report and ratings available to subscribers.)

Any good contractor should inform you that structural damage means extra labor and materials. In fact, 17 percent of roof projects compensated for water damage alone, at a median overrun of $400. But the biggest cost boosts were contractor underbids and related costs—they added a median $1,000 in labor and $875 in materials to roofing projects and $1,000 in each to siding projects.

You might be able to avoid some surprises by doing a bit of homework before calling contractors. Among other tips, get at least three bids, with specifics down to the product name and number, and check licensing, insurance, and workers'-compensation insurance. The National Association of the Remodeling Industry also offers a list of questions to ask a prospective contractor along with other advice, including this remodeling FAQ.—Ed Perratore | e-mail | Twitter

Essential information: Learn how to keep your next home remodel from turning into a project runaway (available to subscribers) and uncover the hidden costs of home improvement.