What is TR26 Timber?
When purchasing roof trusses or Posi-Joists, you'll notice the timber is marked with 'TR26'. In this blog we'll go through what it is and why it's unique.
What is TR26 timber?
TR26 ('TR' standing for 'trussed rafter') timber is a specific grade of timber designed for roof trusses, engineered joists and metal web joists. It's machine graded to ensure it's of the highest quality, kiln-dried to around 20% or less, and made almost exclusively from Scandinavian spruce or pine. If you were to assign it a strength grade, it would be somewhere around C27, albeit more stringently controlled.
How is it graded?
Each section of timber is machine tested for stiffness and requires a minimum bending strength of 26-27 N/mm² (hence the '26' part of the name), along with a density of approx. 420-470kg/m³ and a MOE (modus of elasticity) of 10,000 N/mm².
Comparatively, the type of timber you might find at your local builders merchant will be C24 or below, will be visually graded rather than machine graded, will typically be less consistent in its moisture content and size, and will be used for more general purposes.
| Feature | TR26 | C24 | C20 | C16 |
| Grading | Machine | Visual | Visual | Visual |
| Strength | Equivalent C27 | C24 | C20 | C16 |
| Consistency | Very high | Moderate | Moderate | Varies |
| Usage | Trusses, Posi-Joists | Structural framing | Light structural | General framing |
| Bending strength | ~26–27 N/mm² | 24 N/mm² | 20 N/mm² | 16 N/mm² |
| MOE | ~10,000 N/mm² | ~9,300 N/mm² | ~8,000 N/mm² | ~7,000 N/mm² |
In short, TR26 is simply a higher quality and more precise timber that is essential for structural elements. It gives us, and you, the confidence that your trusses/joists will be of the highest quality available.