Checklist for Avoiding Plastic Lumber Failures
Product selection
• Choose an appropriate product for your application. Not all plastic lumber is created equal. Just as you wouldn’t expect white pine to perform structurally like red oak, don’t expect unreinforced commingled plastic to perform like pure HDPE. Where structural strength isn’t an issue, such as in landscape ties or car stops, use inexpensive and local products. • Look for a product manufactured nearby. Plastic lumber is quite heavy, making transportation expensive and energy-consuming. A local manufacturer can work with you to develop the specs you need and show you existing installations of their product. The manufacturer should also be able to take back any off-cuts or remainders of the material, to be mixed back into their product. • Examine existing installations of a product you are considering and interview other users if possible. Check for signs of deterioration and fading (if that is a concern). • If your installation will see a lot of direct sunlight, test a sample in the sun to make sure that it won’t overheat to the point of discomfort for users, weaken, warp, or fade unacceptably. Some material is foamed to the point of being much less dense than standard recycled plastic, and it should feel less hot to the touch. • Consider your climate. One hundred percent plastic products have their best physical properties around 0°F, while wood-plastic composites are quite brittle at very low temperatures and more dependable in warmer conditions, according to Alan Robbins of The Plastic Lumber Company. • Get a performance guarantee from the manufacturer. Keep in mind, however, that the industry is still highly volatile, and the company may not be around in a few years to honor a warranty. • Highly foamed or hollow-extruded profiles have much better strength-to-weight ratios than solid plastic materials. Solid plastic members are prone to sagging when used for unsupported spans. • Plastic lumber is most cost-effective in large dimensions, where wood is most expensive.Design
• If your installation has any structural requirements at all, have your plans and the proposed material reviewed by a structural engineer who has experience with thermoplastics. Be especially careful about combining plastic lumber with non-plastic materials in a structure, as differing rates of thermal expansion can cause problems. • As much as possible, stick to black or dark brown material. Plastic waste of mixed colors produces dark materials naturally. Sorting the waste by color, or getting only uncolored material and adding pigments, adds significantly to the cost—doubles it, according to Bruce Honig of Recycling Polymer Associates. • Design for expansion and contraction. Except for the products with large amounts of wood or glass fiber, coefficients of expansion tend to be very high (about 1/4” per 8’ length). Avoid using pieces longer than 10’ or 12’ and incorporate gaps for expansion.Installation
• Join members with through-bolts or screws, not nails. Repeated expansion and contraction is likely to loosen nails. No commonly available adhesives work well with plastic lumber. • Use carbide-tipped tools to work the plastic, as both plastic itself and the contaminants in it can be very hard on high-speed steel.
