How to Buy Plastic Lumber

Close-up of a deck surface, made of plastic wood planks
Close-up of a deck surface, made of plastic wood planks (Image: sutichak/iStock/Getty Images)

Plastic lumber offers a durable alternative to natural lumber, especially in environments where outdoor materials are hard to maintain, such as marshy areas. Because it's smooth-surfaced and available in colors, plastic lumber can reduce construction and finishing work. Plastic lumber also addresses concerns about environmental sustainability because it is made from recycled plastic consumer goods. Not all plastic lumber is the same, however. Exploring the source and life cycle of plastic lumber varieties will help you find the right lumber for your job, .

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Plastic Lumber Sources

Different recycled plastics used in making plastic lumber have different environmental effects. The Institute for Local Self Reliance's Healthy Building Network rates polyethylene, which is used in high-density products such as milk jugs and low-density items including plastic bags, as doing the least environmental damage in its manufacture and useful life. All petroleum-based plastics require substantial chemical action during production, but polyethylene dissipates few toxic chemicals when it's in use. Benzene, a known human carcinogen, is used in the manufacture of polystyrene, familiar as packing "peanuts" and plastic-foam cups. Byproducts of polystyrene manufacture and use include possible carcinogens styrene and 1,3 butadiene. Polyvinyl chloride plastic, or PVC, is made from carcinogenic vinyl chloride and leaches toxic phthalates and dioxin as it ages.

Recycling Past and Future

Manufacturers can describe plastic lumber as recycled even when the amount of post-consumer plastic is as low as 10 percent. Ask about the percentage of recycled plastic in the product you select; for the lowest environmental impact, choose plastic lumber with 30 percent to 50 percent recycled content. Also find out whether the lumber contains a single variety or multiple varieties of plastic. Mixing plastic with sawdust, wood flour or fiberglass enhances strength and durability, which can improve performance but complicate recycling of aged lumber because of technological difficulties in separating its components.

Lumber Strength

Although plastic lumber is often heavier than natural wood of the same size, it may display greater flexibility than comparable wood timbers. When building a flight of stairs, for example, lumber must be rigid enough to provide a stable footing. Rails that might be leaned on should be rigid enough to bear weight. Determine whether you need larger dimensions of plastic lumber for stability or should substitute structural-grade plastic lumber for dimensional lumber. Other factors affecting plastic lumber are the percentage of sawdust and the presence of ultra-violet light filters. Plastic lumber with a high wood content or without UV filters is subject to fading and weathering.

Size and Spacing

While plastic lumber dimensions correspond with those of wood lumber, you may need to adjust the amount of lumber to allow for different performance. While wood both expands and contracts over time, plastic only expands. Allowing for expansion, deck builders suggest spacing joists closer together with plastic than with wood to reduce excess flexibility. Exact measurement is essential since even thin pieces of plastic lumber must be cut with power tools; minor measuring errors are harder to correct when working with plastic lumber rather than wood. Plastic lumber has no grain, making it unreceptive to planing or sanding. While you may be able to correct a short cut in a piece of wood by nailing a patch piece into the gap, pieces of plastic lumber must be joined with screws or bolts for a secure hold. Provide for possible errors by ordering extra lumber when you plan your project.

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