What is the Janka Hardness Test?
The Janka hardness test is a standardised measure of the resistance of wood to denting and wear. The test, defined under ASTM F1573, measures the force in kilonewtons (kN) required to embed a steel ball 11.28 mm in diameter to half of its diameter into the surface of a timber sample. The result is the Janka hardness rating for that species.
The test was developed by Gabriel Janka, an Austrian forestry researcher working in the early twentieth century. It remains the most widely referenced measure of timber hardness worldwide, used by architects, furniture manufacturers, specifiers and procurement officers when selecting timber for applications where surface durability matters.
A higher Janka rating indicates a harder timber that resists denting, surface scratching and wear. A lower rating indicates a softer timber that may show wear more quickly in high-traffic or outdoor settings. For outdoor picnic tables and park furniture, Janka rating is one of the two most important technical specifications, alongside AS 5604 durability class.
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The Janka Scale: How to Read the Numbers
Janka ratings are expressed in kilonewtons (kN) in Australia and other metric countries. In North America, the same test results are sometimes expressed in pounds-force (lbf), where 1 kN equals approximately 225 lbf. When comparing international data, confirm which unit is being used.
To put the scale in context: Balsa wood, one of the softest commercial timbers, measures approximately 1.4 kN. Lignum Vitae, historically considered one of the world's hardest timbers, reaches approximately 14.5 kN. Most common Australian hardwoods cluster between 7.0 and 16.0 kN, placing them among the hardest timbers available for commercial use anywhere in the world.
For practical decision-making, the Janka rating tells you how well a timber surface will hold up against the physical stresses of everyday outdoor use: boots on table seats, dragging across concrete, the occasional impact from falling objects, and constant exposure to UV that can degrade softer surface fibres faster.
Janka Ratings for Australian Timber Species Used in Outdoor Furniture
The following table lists the key species relevant to outdoor furniture and picnic table manufacturing in Australia, including their botanical names, Janka ratings, AS 5604 durability class, outdoor suitability and relevant notes.
| Species | Botanical Name | Janka (kN) | AS 5604 Class | Outdoor Suitability | Notes |
|---|---|---|---|---|---|
| Ironbark | Eucalyptus sideroxylon | 14.0 kN | Class 1 | Excellent | Hardest common picnic table timber |
| Spotted Gum | Corymbia citriodora | 11.0 kN | Class 1 | Excellent | Most common Auscraft species |
| QLD Red Gum | Eucalyptus tereticornis | ~11.0 kN | Class 1 | Excellent | Rich red-brown colour |
| Jarrah | Eucalyptus marginata | 8.5 kN | Class 1 | Excellent | Deep red, WA species |
| Merbau | Intsia bijuga | 8.6 kN | Class 1 | Very Good | Warm golden-brown |
| Victorian Ash | Eucalyptus regnans | 6.5 kN | Class 3 | Not Recommended | Internal use only |
| Blackwood | Acacia melanoxylon | 4.1 kN | Class 3 | Not Recommended | Fine furniture only |
| Radiata Pine | Pinus radiata | 2.8 kN | Class 4 | Treated Only | Requires CCA treatment for outdoor use |
The practical minimum Janka rating for outdoor picnic tables in high-traffic settings is 8.0 kN. Below this threshold, surface denting and wear accelerates significantly in commercial use.
What Janka Rating Means for Picnic Tables
For outdoor picnic tables, the Janka rating has direct practical consequences that become visible over time. Tables installed in parks, school grounds, beer gardens and public spaces receive a level of physical stress that far exceeds anything a residential backyard setting produces.
A higher Janka rating means the surface resists denting from dropped tools, keys, bottles and cutlery. It means the edges of seats and table tops stay crisp rather than rounding off and splintering. It means the surface can be re-sanded and re-oiled many years into the life of the table without encountering structurally compromised timber beneath the finish.
Lower Janka softwoods, including treated pine, show surface wear within two to three years in outdoor commercial settings. The treated surface layer is cut through by everyday use, exposing untreated wood underneath. At this point, the timber is no longer protected by either the original treatment or the finish coat, and decay can begin.
Ready to order? Request a free quote from our West Gosford NSW workshop. We respond within one business day.
Choosing the Right Hardness for Your Application
Different applications have different hardness requirements. The following guidelines help match the right timber to the right setting based on Janka rating and expected traffic levels.
Residential backyard (low traffic): Merbau (8.6 kN) or Spotted Gum (11.0 kN) are both excellent choices. Either will provide decades of service with periodic oiling. The main decision is aesthetic: Merbau offers warm golden-brown tones, while Spotted Gum provides a more varied grain pattern.
School playground (high traffic, fully outdoor): Spotted Gum (11.0 kN) is the minimum recommended specification. The higher hardness means the surface stays smooth and splinter-free even under the sustained mechanical stress of children's everyday use. Most NSW and QLD school procurement specifications align with this threshold.
Commercial beer garden (constant use, chair dragging, bottle impacts): Ironbark (14.0 kN) or Spotted Gum (11.0 kN) are both appropriate. Ironbark's exceptional density means it will outlast almost any other commercial application without showing surface fatigue.
Council park (decades-long installation, minimal maintenance budget): Ironbark (14.0 kN) is the preferred specification. When a table must last 30 or more years in an exposed location with minimal maintenance, the hardest available Class 1 species provides the best total cost of ownership.
Auscraft's Premium Hardwood Picnic Table and Heavy Duty Commercial Grade are manufactured from Class 1 Australian hardwoods, selected for the application requirements of each order.
Auscraft's Timber Selection Process
At Auscraft Furniture's West Gosford NSW workshop, every piece of timber is sourced to AS 5604 Class 1 specification as a baseline requirement. No Class 2 or lower species are used in structural or surface components of our outdoor furniture range.
Beyond the standard classification, our timber selection process focuses on consistent grain orientation, absence of sapwood in structural sections, and moisture content within the acceptable range for the Central Coast's coastal climate. Sapwood, even in Class 1 species, carries lower natural durability than heartwood. All Auscraft furniture uses heartwood-selected timber throughout.
The result is outdoor furniture that will outlast most outdoor installations: picnic tables that remain structurally sound and visually attractive for 20 to 40 years in Australian outdoor conditions, with periodic oiling as the only maintenance requirement.
Ready to order? Request a free quote from our West Gosford NSW workshop. We respond within one business day.
Browse our full range at Auscraft picnic tables and contact us to discuss species selection for your project.
Frequently Asked Questions About Janka Hardness
What is a good Janka rating for outdoor picnic tables?
For outdoor picnic tables in Australia, a Janka rating of 8.0 kN or above is the practical minimum for above-ground outdoor use. For high-traffic commercial settings, 11.0 kN (Spotted Gum) or above is recommended. The Janka rating should always be cross-referenced with the AS 5604 durability class. A high Janka rating without Class 1 durability does not guarantee outdoor performance.
Is Ironbark the hardest Australian picnic table timber?
Ironbark (Eucalyptus sideroxylon) at 14.0 kN is among the hardest commercially available Australian timbers used in picnic table manufacturing. It is Class 1 rated under AS 5604 and is the preferred specification for the most demanding outdoor commercial applications, including council parks and high-traffic public spaces.
Does higher Janka mean better outdoor durability?
Not necessarily. Janka hardness measures resistance to denting and surface wear, but outdoor durability also depends on the timber's natural oil content, silica content and density, which determine how it resists moisture and biological decay. For a complete specification, cross-reference Janka rating with AS 5604 durability class. The best outdoor timbers, such as Spotted Gum and Ironbark, score highly on both measures.
What Janka rating does treated pine have?
Radiata pine, the most common species used in CCA-treated pine products in Australia, has a Janka rating of approximately 2.8 kN. This is considerably lower than Australian hardwoods used in quality outdoor furniture. Treated pine achieves outdoor durability through chemical preservatives rather than natural hardness or density, and the surface remains soft and prone to denting regardless of the treatment.
Which Australian timber has the highest Janka rating?
Among commercially available Australian species used in outdoor furniture, Ironbark (Eucalyptus sideroxylon) at 14.0 kN is typically the highest Janka-rated species. Some other eucalypt species, including Grey Ironbark, can reach similar or slightly higher ratings. On a global scale, Australian hardwoods as a group are among the hardest timbers available for commercial use.
