Overview
ISO 19821:2017 - "Determination of span rating for natural fibre-reinforced plastic composite (NFC) deck boards" - defines procedures to establish a span rating for NFC deck boards and stair treads (when manufactured with the same materials and processes). The standard specifies test methods, acceptance considerations and reporting needed to support code recognition of NFC deck boards for exterior applications where combustible construction is permitted.
Key Topics
- Scope and definitions
- NFC = natural fibre‑reinforced plastic composite (also called WPC historically).
- Span rating = maximum centre‑to‑centre support spacing for a specified end use.
- Test specimens & sampling
- Use specimens with original product thickness/width.
- Sample across production runs to reflect variability.
- Conditioning
- Standard conditioning per ISO 291 (23 °C ±2 °C, 50 % ±5 % RH) for minimum 72 h; water‑soaked specimens tested within 30 min of removal.
- Flexural testing
- Deck boards/stair treads are tested in flexure to determine flexural strength (MOR) and flexural stiffness (MOE or EI) in accordance with ISO 16616.
- Temperature & moisture effects
- Verify allowable span/load at service temperatures: −30 °C ±2 °C and 55 °C ±2 °C; minimum 10 specimens per temperature.
- Moisture conditioning and testing procedures to determine performance changes.
- Durability and performance adjustments
- UV/weatherproofing (ISO 4892‑2 xenon‑arc exposure), freeze‑thaw, biodeterioration (fungal decay, termites), creep‑recovery, creep‑rupture, mechanical fastener holding, and slip resistance - each with test methods and acceptance criteria to adjust span rating where appropriate.
- Span rating determination
- Procedures to calculate nominal span rating and include two‑span test adjustments (Annex B).
- Reporting
- Required test report elements: test results, product description and installation instructions.
Applications
- Manufacturers of NFC deck boards and stair treads use ISO 19821 to:
- Establish certified span ratings for product datasheets.
- Design and validate boards for outdoor decking, stairs and landscaping.
- Test laboratories and compliance bodies use it to:
- Perform standardized durability and structural testing.
- Specifiers, architects, contractors and building code officials use span ratings to:
- Select appropriate NFC decking for exterior projects where combustible construction is allowed and to ensure safe support spacing and fastening practices.
Related standards
Keywords: ISO 19821, span rating, NFC deck boards, natural fibre‑reinforced plastic composite, deck testing, flexural strength, creep, UV resistance, freeze‑thaw, mechanical fastener holding.