At a glance
Creep is the increase in deformation under sustained load. Short-term bending data alone cannot justify support spacing for long-duration service.
Key points
- Load: Magnitude, position and duration.
- Climate: Relevant service temperature.
- Design: Verified supports and load path.
The technical principle #
Thermoplastic composites are time-dependent materials. Load duration, temperature, moisture and stress level can change deflection. A board that appears satisfactory immediately after loading may sag later. Permanent planters and equipment are different load cases from a person briefly walking across a deck.
What to specify and verify #
Ask for creep evidence and application-specific support instructions. Heavy equipment or a hot tub needs an assessed load path through the supporting structure; do not assume decking boards spread the load safely. Check the entire system, including joists, connections and foundations.
Application and common mistakes #
For an ideal simply supported beam with a central point load, elastic deflection varies with span cubed. Increasing span by 20% gives about 1.73 times the initial elastic deflection under the same load and EI. This illustration is not a span approval and does not predict long-term creep.
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.
Project specification checklist
| Specification check | Evidence to request |
|---|---|
| Load | Magnitude, position and duration |
| Climate | Relevant service temperature |
| Design | Verified supports and load path |
Common questions
Can extra clips solve excessive creep?
Not necessarily. A connection change does not replace verification of the board and supporting structure.
Question permalink ↗Sources & further reading
- Klyosov, A. A. (2007). Wood–Plastic Composites. Chapter 7; pp. 291–302. ↗John Wiley & Sons · Book · 2026-10-01
- Green Plank — product information and technical enquiries ↗Green Plank · Web reference · 2026-10-01
Cite this page
J. A. Gill. “WPC creep and joist spacing under permanent loads” Version 1. Updated 2026-10-01. https://woodplasticcomposite.net/guides/wpc-creep-joist-spacing-permanent-loads/
Author profile
Author: J. A. Gill
WPC and natural-fibre composite specialist · Innovator, inventor, product designer and entrepreneur
Original educational article informed by Klyosov (2007). Modern project applications are editorial synthesis. This is not a product certificate or a substitute for project-specific design.
General material information is not product certification or project-specific engineering advice.
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