# WPC thermal expansion: calculate movement before gaps

Canonical: https://woodplasticcomposite.net/guides/wpc-thermal-expansion-decking-gaps/
Language: en
Updated: 2026-10-01
Version: 1
Review: Technical and language review not yet recorded

## Summary

Composite boards expand and contract with temperature. Gaps and fixing arrangements must follow the exact product's instructions and the installation conditions.

## The technical principle

A simplified estimate is ΔL = α × L × ΔT, where α is the linear expansion coefficient, L the initial length and ΔT the temperature change. Direction and temperature range matter. Surface temperatures can differ substantially from air temperature, particularly in direct sun.

https://woodplasticcomposite.net/guides/wpc-thermal-expansion-decking-gaps/#technical-principle

## What to specify and verify

As an illustrative calculation, α = 40 × 10⁻⁶/K, L = 4 m and ΔT = 50 K give 8 mm of total length change. These are assumed values, not Green Plank measurements or a recommended joint width. How movement is distributed depends on the fixing arrangement.

https://woodplasticcomposite.net/guides/wpc-thermal-expansion-decking-gaps/#specification

## Application and common mistakes

Ask for the tested coefficient, installation-temperature guidance, fixed and sliding points, and clearances at walls and ends. Moisture swelling and irreversible shrinkage are separate effects. A standard gap copied from another manufacturer's board can be inadequate even when the two products look similar.

https://woodplasticcomposite.net/guides/wpc-thermal-expansion-decking-gaps/#application

## Discuss your project with Green Plank

For a Green Plank specification, request samples and current technical documents for the exact profile and intended application. Use the Green Plank link in Sources to explore the range and make an enquiry. Commercial disclosure: this portal features Green Plank. General material science does not certify any particular product.

https://woodplasticcomposite.net/guides/wpc-thermal-expansion-decking-gaps/#green-plank-project

## Key points

Calculation: Use product-specific α and temperature range.
Fixings: Identify fixed and moving points.
Clearance: Follow the complete installation detail.

## Comparison

Specification check | Evidence to request
Calculation | Use product-specific α and temperature range
Fixings | Identify fixed and moving points
Clearance | Follow the complete installation detail

## Should each end always get half the movement allowance?

No. Movement distribution depends on restraints and the manufacturer's fixing system.

## Sources

1. Klyosov, A. A. (2007). Wood–Plastic Composites. Chapter 10; pp. 356–368. — https://doi.org/10.1002/9780470165935
2. Green Plank — product information and technical enquiries — https://greenplank.eu/