You're done with the siding job, but something's not right. The problem? It isn't laying flat to the wall. And if you think the customer's going to notice and say something, you're probably right. A properly installed siding job, whether vinyl or fiber cement, should lay flat and look great for years to come.

Length of Pieces a Factor Keith Russell, owner of Siding by Russell in Cherry Grove Beach, S.C., says that pieces cut too long can be one cause of warping. Another? That installers sometimes fail to take seasonality into account. In winter, for instance, pieces need a bigger tolerance (distance from trim and corners) than in the summer months. That's because cold causes vinyl to contract. Vinyl siding installed in summer months requires less tolerance, since the material expands with warm weather. Always maintain the tolerance recommended by the manufacturer, which is typically ¼-inch from the trim and corners. Russell and others say that overlap on siding should be at least one inch.

Another factor in expansion-contraction is color. “The darker the color, the more prone it is to expansion,” says Ralph Seano, owner of Lifetime Remodeling Systems in Portland, Ore. “A white panel will have virtually no expansion. A dark brown panel will have a good half-inch.” Grayback, he says — low-cost recycled vinyl panels with a coat of new vinyl over the old — is particularly prone to expansion-contraction. “It's old vinyl, re-done, and it's never the same.”

Nailing Too Tight Seano also points out that probably the biggest cause of warping and waving is that siding is nailed too tight. Combined with expansion-contraction, it's asking for trouble He suggests leaving an 1/8-inch space between nail head and vinyl and placing the nail in the center of a 1-inch nail slot, “so that it can move ½ inch in either direction.” Others recommend turning down the air pressure, using a depth of drive adjustment, or shimming the siding.

Another reason for warping, Seano says, is thin vinyl. “The thinner the product, the more the expansion and the more prone to waving. So you want to go with nothing less than an .044 gauge, to eliminate as much of that as possible.”

Heat sources can occasionally wreak havoc with vinyl siding. Seano says that in the event of a heat source hot enough to cause warping — from, say, a stove or a heater — he installs 12 inches of plywood around the heat source or pipe, and paints it the color of the siding, bringing the siding up to the plywood. —Carl Sperry is a contractor who lives in South Carolina.