Photo: Kolin Smith
A panel of polypropylene siding slips into a grooved "corner board" of the same material. Panels expand and contract with temperature changes and shouldn't be installed tight to trim pieces.
All Plastic Siding is Not the Same

Vinyl is a polymer formed during a chemical do-si-do between ethylene gas and chlorine, which produces a fine white powder called vinyl resin. When it's melted and mixed with different additives, the resulting compound can be as rigid as pipe, as supple as a shower curtain, or durable enough to survive the heavy foot traffic on a kitchen floor.

New, so-called virgin vinyl siding has a greater complement of the key additives that impart flexibility and resistance to UV degradation. Some manufacturers will tout their product as 100 percent virgin (along with a mention of its supposed superiority), but most siding is made with a core of remelted vinyl top-coated with virgin material.

Typically, vinyl siding is extruded through a die, but to produce the deepest patterns and crispest edges, panels must be molded from polypropylene, a more expensive plastic. Molded panels are typically no more than 4 feet long, while vinyl extrusions can be virtually any length.

Rap on a vinyl-sided wall with your knuckles, and it will flex and sound hollow. That's because, in most cases, only a relatively small area of a vinyl panel is actually resting against the sheathing.

A thin panel, or one without support, is more likely to sag over time. The thinnest siding that meets code is .035 inch thick. Premium siding can be .044 to .048 inch, and a few manufacturers sell .055-inch siding. The thicker sidings tend to be stiffer, and therefore more resistant to sagging, but stiffness depends on other characteristics as well.

Panels with a folded-over, doubled nailing hem and a relatively deep profile tend to be stiffer than others, as do those with narrow “clapboards”: The more bends the better. Although claims are made that thicker siding is also more impact resistant than thin siding, test results suggest that it has more to do with its chemical makeup, which, unfortunately, is not available to consumers who want to compare products.

Thinner, less-stiff sidings can also be sucked off a house when high winds blow. Reading the manufacturer's warranty should give you a good indication of the product's ability to handle heavy weather. Some even comply with the 146-miles-per-hour wind code in hurricane-prone Miami, Florida.

One siding, Wolverine Millennium, comes with a “won't-blow-off” warranty, and its literature states that it will withstand 180-mph winds, when nailed properly.
Photo: Kolin Smith
Siding is nailed loosely through horizontal slots in the hem at the top of each panel. Tightly nailed plastic siding can buckle on very hot days. The L-shaped clip under the nailing slot hooks into a channel in the butt of the panel above.
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