Durability class
Durability class measures a species' natural resistance to decay when exposed to weather and ground contact. It runs from Class 1 (very durable) to Class 4 (non-durable), and applies to the heartwood. Sapwood of any species is treated as non-durable.
| Class | Fully protected | Outside, above ground | Outside, in ground |
|---|---|---|---|
| Class 1: Very durable | 50+ years | 40+ years | 25+ years |
| Class 2: Durable | 50+ years | 15–40 years | 15–25 years |
| Class 3: Moderately durable | 50+ years | 7–15 years | 5–15 years |
| Class 4: Non-durable | 50+ years | 0–7 years | Under 5 years |
Source: our Australian Hardwoods poster.
Examples from the poster
- Class 1: Merbau, Kwila, Red Ironbark, Tallowwood and Cypress Pine.
- Class 1–2: Jarrah and Turpentine.
- Class 2: Spotted Gum, Forest Red Gum and Karri.
- Class 2–3: Blackbutt and River Red Gum.
- Class 3: Brushbox and Tasmanian Oak.
Matching class to exposure
- Decking, external cladding and exposed structures: Class 1 or 2 species. Check each species' recommended treatment below: for example, the poster recommends oil finishes for Merbau but H3 CCA for external Spotted Gum.
- Flooring and internal framing: Class 2 or 3 species work well because they are protected from the weather.
- Ground contact (posts, poles, sleepers): a Class 1 species, or timber treated to the right hazard level. See our fence post guide.
F rating: stress grade
On the poster, the F rating is the stress grade: the timber's bending strength in megapascals (MPa). It is not a fire rating. Lower numbers are weaker and higher numbers are stronger.
| F rating | Bending stress | Typical timber | Best applications |
|---|---|---|---|
| F4 | ~4 MPa | Lower-grade pine, unseasoned | Non-structural, light framing, temporary structures |
| F5 | ~5 MPa | Radiata pine, unseasoned | Light framing, internal studs, non-load-bearing |
| F7 | ~7 MPa | Pine, unseasoned | Wall framing, light construction, internal structures |
| F8 | ~8 MPa | Seasoned pine | Floor joists, wall frames, roof trusses (residential) |
| F11 | ~11 MPa | Medium-strength timber | Structural framing, beams and posts (residential) |
| F14 | ~14 MPa | Unseasoned hardwoods | Decking, joists and bearers, outdoor structures |
| F17 | ~17 MPa | Unseasoned hardwoods | Heavy framing, large spans, commercial construction |
| F22 | ~22 MPa | Kiln-dried hardwoods | Heavy-duty structures, long-span beams |
| F27 | ~27 MPa | Kiln-dried hardwoods | High-load members, engineered applications, bridges |
| F34 | ~34 MPa | Premium kiln-dried hardwoods | Critical structural work, heavy engineering |
- F4–F8: common in residential construction, mostly pine.
- F11–F14: the transition zone where both softwoods and hardwoods are used. F14 or higher is recommended for decking, joists and bearers.
- F17–F27: mainly hardwood grades for structural work.
- F27–F34: premium grades for critical structural applications.
Species ranges on the poster include Merbau F11–F14, Spotted Gum F11–F17, Blackbutt F8–F14, Cypress Pine F7, Tasmanian Oak F7–F17 and Red Ironbark F17–F27. More on how grades are assigned is in our introduction to timber.
H rating: hazard level
The H rating is the hazard level: the exposure a timber can handle, from indoors to sea water. It sets the treatment level for treated timber, and the poster also lists a hazard range for each species.
| Level | Exposure | Main hazard |
|---|---|---|
| H1 | Indoors, fully protected | Lyctid borers |
| H2 | Indoors, protected from wetting | Borers and termites |
| H3 | Above ground, periodic moderate wetting | Decay, borers and termites |
| H4 | In ground, severe wetting | Decay, borers and termites |
| H5 | In ground or fresh water, extreme wetting | Decay, borers and termites |
| H6 | Marine, prolonged immersion in sea water | Marine borers and decay |
Examples of species values on the poster: Merbau, Tallowwood, Red Ironbark and Jarrah H4–H5; Cypress Pine, Spotted Gum and Blackbutt H3–H4; Tasmanian Oak and Brushbox H3.
BAL ratings (Bushfire Attack Level)
A BAL rating describes the bushfire attack a building site may face, based on distance to vegetation, vegetation type, slope and the fire danger index. Australian Standard AS 3959 sets six levels. Your site's BAL is assessed by a qualified bushfire consultant or certifier, usually during development approval; local councils keep maps of bushfire-prone areas.
| BAL | Risk | Material requirement |
|---|---|---|
| BAL-LOW | Very low | Standard materials; any species |
| BAL-12.5 | Low | Ember protection required |
| BAL-19 | Moderate | Non-combustible or bushfire-resistant timber |
| BAL-29 | High | Enhanced protection; bushfire-resistant timber only |
| BAL-40 | Very high | Limited combustible materials; only bushfire-resistant timber with extra protective treatments |
| BAL-FZ | Extreme (flame zone) | Non-combustible materials only; no timber for external applications |
Species BAL values
Each species on the poster carries its own BAL value. We use these species values:
- BAL-12.5: Tasmanian Oak, Karri.
- BAL-12.5 to BAL-19: Cypress Pine, Brushbox.
- BAL-19: Blackbutt, River Red Gum.
- BAL-19 to BAL-29: Spotted Gum, Forest Red Gum.
- BAL-29: Merbau, Tallowwood, Jarrah, Turpentine.
- BAL-29 to BAL-40: Red Ironbark, Kwila.
A BAL rating also depends on construction: subframe materials, fixings, ember protection and clearances all count. We can advise on species and details together.
Termite and ant resistance
This rating describes a species' natural resistance to subterranean termites and borers. It is separate from durability class, which measures decay. On the poster, Merbau, Kwila, Red Ironbark and Jarrah are rated excellent; Spotted Gum, Tallowwood and Turpentine good; Blackbutt and Brushbox moderate; and Cypress Pine naturally resistant.
"Termite resistant" doesn't mean no termite protection is needed.
- Termites attack from below, through subframes, joists or bearers that may be more vulnerable.
- Heartwood resists termites, but sapwood does not, so any sapwood is a way in.
- AS 3660.1 requires a termite management system in termite-prone areas, whatever the species: physical or chemical barriers, regular inspections, good drainage and clearances.
- Different termite species behave differently, so resistance to one may not mean resistance to all.
Treatments and hazard levels
| Treatment | Hazard classes | Notes |
|---|---|---|
| ACQ (alkaline copper quaternary) | H3–H5 | Arsenic-free; approved for playgrounds; can go in ground or fresh water; corrodes mild-steel fixings |
| CCA (copper chrome arsenate) | H2–H6 | Very long in-ground life; contains arsenic and needs special disposal; not for child-contact areas since 2005 |
| LOSP (light organic solvent preservative) | H2/H3 only | Must stay above ground; reseal cut ends; warranty conditional on painting |
Recommended treatment by species
Whether a species needs preservative treatment depends on the species, not just its durability class. Our poster gives a recommended treatment for each one:
- Oil finishes, minimal treatment: Merbau, Kwila, Red Ironbark.
- Minimal preservation, oil treatments: Tallowwood.
- Oil finishes, stains or sealers: Cypress Pine, Karri.
- Oil-based treatments for appearance: Jarrah.
- H3 CCA for external use, H4 for ground contact: Spotted Gum.
- CCA treatment for external use: Forest Red Gum.
- CCA treatment recommended: River Red Gum.
- CCA or LOSP treatment recommended: Blackbutt.
- H3 CCA treatment recommended: Brushbox.
- H3 CCA treatment essential: Sydney Blue Gum.
- LOSP treatment: Tasmanian Oak.
- CCA for ground contact: Turpentine.
For in-ground use, choose a Class 1 species or timber treated to H4, or H5 for extreme wetting or fresh water. Anything in sea water needs H6. Whatever the species, good construction details still matter: ventilation, drainage, clearance from the ground and moisture barriers.
How finishes change appearance
- CCA: a greenish tint that fades to grey.
- LOSP: little colour change.
- Oil finishes: enrich the natural colour; reapply every 1–2 years. They protect appearance but are not preservative treatments.
- Stains and sealers: add colour and UV protection while showing the grain.
Fire performance of timber
Large timber members behave predictably in fire. The outside chars, and the char layer insulates the inside and slows further burning. Hardwoods char at about 0.6–1.0 mm per minute, so engineers can calculate the char rate and design for it. Steel, by comparison, loses about 50% of its strength at 550°C and can fail suddenly without fire protection.
Selecting the right species
- Application and exposure. Above or in ground, wet or dry, coastal or inland.
- Durability class. Match it to the exposure, using the table above.
- Stress grade. F14 or higher for decking, joists and bearers.
- Bushfire rating. Check your site's BAL and choose a species that meets it.
- Termite risk. Favour excellent or good termite resistance, as part of a management plan.
- Appearance. Colour and grain vary a lot between species.
- Lifecycle cost. A Class 1 species lasting 40+ years above ground can be better value than a cheaper Class 3 species that needs replacing sooner.
Examples
- Exposed external decking: Merbau or Red Ironbark, with oil finishes and minimal treatment; or Spotted Gum, treated with H3 CCA for external use as the poster recommends.
- Coastal decking above the water: Merbau, Red Ironbark or Tallowwood, with 316 stainless fixings. Timber in sea water needs H6.
- Bushfire-prone decking (BAL-29): Merbau, Tallowwood, Jarrah or Turpentine, with correct construction details.
- Residential flooring: Blackbutt, Spotted Gum, Brushbox or Tasmanian Oak, with a surface finish such as polyurethane or oil. For any external use, follow each species' recommended treatment above.
- External beams and posts: Red Ironbark or Tallowwood (oil finishes, minimal treatment), or Turpentine, which the poster recommends treating with CCA for ground contact.
Australian-grown species (Spotted Gum, Blackbutt, Red Ironbark, Jarrah) support local forestry. Read more in our sustainability and provenance guide, or see the species we supply in our Adelaide hardwood supply page.
Why ratings show ranges
Ranges such as H3–H4 or BAL-19 to BAL-29 appear because timber is a natural material. Properties vary with growing conditions, age, region and where in the tree the piece comes from, and different test samples give slightly different results. When you specify, treat the lower end of a range as the minimum to design for.
