When you think about it, it's rather odd to cover a building with an organic substance whose nature it is to decay, burn, be eaten, and in general return to the soil whence it came. Yet that's what we Americans have been doing ever since we began building in this land of abundant wood. And so the straight lines of clapboards and the sensual texture of shingles have become part of our collective idea of home, as have the rituals of scraping, painting, and residing. Since the late 1980s, however, a product has been gaining acceptance that gives the look of wood but has few if any of its problems. Called fiber-cement, or cementitious, siding, it's a combination of Portland cement, ground sand, cellulose (wood) fiber, and sometimes clay. Mixed with water and cured in an autoclave, these ingredients yield planks, panels, and shingles that will not rot, burn, or serve as lunch for insects. It's not to be confused with some of its troubled predecessors, such as asbestos-cement siding (which wore like iron but was pulled from the market when asbestos was declared a carcinogen) or the hardboard, oriented-strand-board and synthetic stucco sidings that were targeted by various lawsuits over the past few years. The clapboards in Billerica, "hung" blind-nailed in staggered 12-foot lengths, look so much like wood that most visitors have to be told what they're looking at. The shingle panels on the garage are less eye-fooling, because the spaces between them are far more regular than one would find between cedar shingles. These panels are, however, a new product, and the company expects to address the spacing issue in later iterations. We're using clapboards on the main house in Billerica, shingle panels on the garage, and corner boards and window trim made of built-up planks on both buildings. Silva Brothers carpenters, a traditional lot, find fault in the heavy weight of the product, and in the dust it kicks up when cut with a circular saw. Not only is it messier than sawdust, it "contains silica, which has been known to cause lung damage," as the packaging states. Masks help, as do the dust-collecting attachments that have been developed for saws. Nearly dust-free pneumatic or electric shears can also be used. Those complaints aside, Tom Silva says, "the stuff is a great alternative to cedar or redwood." Far more stable than wood, it's said to hold paint three to five times as long, and it is warranteed for 50 years. The final attention-getter: price. Priced in the Boston area, factory-primed planking goes for about 40 cents a lineal foot; western red cedar, also pre-primed, is anywhere from 75 cents to a dollar per foot, depending on the grade. Nationally, fiber-cement siding accounts for about 5 percent of the siding market, but that number is expected to grow quickly. So quickly, in fact, that the companies that make it are opening at least three new plants over the next few years. As for Dick Silva, painting, scraping, and residing are three things he'll be doing a lot less of.