Bet you've driven past a house with siding bent like dented oilcans and thanked your lucky stars it wasn't your job. But it takes more than luck to avoid the callbacks that come with warping and waviness.

Bang in the Center When Joe Iuvara sees that problem in vinyl siding, he knows that the installers didn't nail the panels in properly. They have to be nailed in the center of the hem, not too tightly. “You need to be able to slide the panel back and forth,” says the owner of Iuvara Siding and Windows in Haddon Heights, N.J. “That will tell you that it will expand and contract, and it's not too tight.” To be specific, the nail head should be between 1/8 inch and ¼ inch from the siding, says Ralph Streano, the owner of Lifetime Remodeling Systems in Portland, Ore. It's important to attach both siding and accessories to the studs too, Iuvara says. And do it by hand, Streano adds.

What Lies Beneath Also critical is what's underneath the siding. Gary Schoengold, president of National Energy Conservation Corp. in Olney, Md., likes to see ½-inch insulation, making sure that it's all level.

Another tip: Start out with a thicker product than some contractors use. “The thinner the siding, the more it's prone to expand and get wavy,” Streano says. In addition, darker-colored vinyl tends to expand more than lighter-colored products. He also has reservations about foam-backed siding, which he has found susceptible in a few cases to bowing in extreme heat. He theorizes that's because the siding can't move independently from the backing.

Fiber-cement siding doesn't warp, but it is flexible enough to distort if the studs to which it is being attached are bowed. To prevent a wavy appearance, Phil Birner of Amazing Siding in Houston says his installers strip the walls back to the studs and try to correct the problem at its source. Alternatively, they apply rigid foam behind the siding to help flatten the wall.

Streano likes to go with a 1¾ inch overlap on fiber cement, ½ inch more than some manufacturers suggest. “The additional ½ inch allows the installer to set the nail 1½ inches from the top of the board instead of ½ inch to ¾ inch, which gives the panel more support and allows it to lie much flatter.”

“A lot of contractors who swap out siding want the install done quickly,” Schoengold says. They're the ones who are more likely to get the callbacks for warping.