22nd September, 2022

Does Plastic Lumber Expand and Contract Like Wood Lumber?

Plastic lumber is a unique alternative material with the look of regular wood but with very different structural characteristics. Does plastic lumber expand and contract as wood lumber does under extreme temperatures and environmental stress?

Yes, Plastic Lumber Expands and Contracts (a Little)

Like typical lumber, dramatic changes in the temperature can affect plastic wood material—causing it to expand and contract to some degree. However, the expansion and contraction in synthetic lumber is less prominent than in organic lumber.

In dramatic temperature changes, typical wood can expand or contract by as much as a quarter of an inch, while plastic lumber can change up to an eighth of an inch. Plastic wood also expands and contracts on the ends of boards, while regular lumber will fluctuate perpendicular to the grain of the wood.

Installing Plastic Lumber with Expansion in Mind

When installing plastic lumber, these fluctuations—however minor they may seem—are worth considering. If you don’t account for the expansion and contraction of the material, the structural plastic lumber could experience pressure that damages its structural integrity and hardware.

If you’re installing plastic lumber yourself, ensure that you take potential expansions and contractions into account by leaving gaps on the butt ends of the boards and oversizing the drill holes.

Pro Tip: If installing the plastic lumber material on an excessively hot or cold day, give the lumber some time to adjust to the temperature before installation.

Why Does Regular Wood Expand More Than Plastic Wood?

Why isn’t plastic wood as affected by temperature changes as regular wood? It all comes down to moisture and its absorption within the structural material.

Natural wood loves moisture—it’s how it grows and thrives—so it’ll absorb as much moisture from the air as possible. Plastic lumber, however, utilizes synthetic materials resistant to moisture and water vapor. The less moisture absorbed, the less the material expands or contracts.

Understanding how plastic lumber and wood lumber expand and contract due to temperature is an essential part of preparing to install decking. Even in a moderate climate, give your structural materials enough room to expand and contract if needed—otherwise, warping could significantly damage the board’s structural integrity.