Natural Durability Ratings

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TBC

Overview

Natural durability describes the inherent resistance of timber species’ heartwood to decay and insect attack.It’s a key factor in determining whether timber can be used untreated in exposed or semi-exposed applications.

Durability ratings apply only to the heartwood of a species. Sapwood is regarded as non-durable, regardless of species, and must be preservative treated if it is to be used in exposed conditions.

Durability ratings guide specifiers in selecting timber that will perform reliably across cladding, decking, framing, joinery, and in-ground applications.

Durability Classes in AS 5604

AS 5604 defines four durability classes, each with a service life expectation under two conditions: above-ground and in-ground use.

Durability Class	How durable?	Above-Ground Life Expectancy	In-Ground Life Expectancy	Example Species
Class 1	Very durable	> 40 years	> 25 years	Spotted Gum, Ironbark, Tallowwood
Class 2	Durable	15-40 years	15-25 years	Blackbutt, Jarrah, Silvertop Ash
Class 3	Moderately durable	7-15 years	5-15 years	Vic Ash, Tasmanian Oak
Class 4	Non-durable	0-7 years	< 5 years	Radiata Pine, Slash Pine. Sapwood requires preservative treatment to meet hazard class requirements.

These figures assume good detailing and maintenance. Poor ventilation, trapped moisture, or concealed joints can accelerate decay in even the most durable species.

Above-Ground vs In-Ground Use

Above-ground framing more than 150 mm clear of the ground should be Class 1 or 2, or H3 treated.
Close-to-ground decks (<400 mm clearance) require Class 1 hardwood or preservative treatment to H4.
In-ground applications (posts, stumps, retaining walls) require Class 1 hardwoods or H5 preservative treatment.

Limitations of Natural Durability

Durability classes are based on test averages and may not reflect performance in extreme microclimates.
Only the heartwood is durable. Sapwood, even in Class 1 species, must be treated.
Natural durability does not protect against weathering (surface checking, greying). Finishes and detailing are still required.

Heartwood vs Sapwood

Durability ratings apply only to heartwood. The sapwood of all species is considered non-durable and must be treated if exposed.

Feature	Heartwood	Sapwood
Durability	Naturally durable (Class 1-4)	Not durable without treatment
Treatability	Resistant to chemical penetration	Easily impregnated with preservative
Colour	Usually darker and more resinous	Often lighter and more uniform

Figure 1: Heartwood (centre) contains natural extractives that resist decay. Sapwood (outer band) requires treatment.

Design Implications:

Class 1 species may still fail if made up of mostly sapwood.
Always check treatment penetration when using treated timber in exposed applications.
End-grain sealing is essential to stop moisture wicking through sapwood, especially in decking and joinery.

Matching Durability to Application

Durability must match the exposure, maintenance access, and risk of failure for each timber component. Not every application demands a Class 1 species.

Application	Minimum Recommendation
External Cladding	Class 2 heartwood or H3-treated softwood
Decking Boards	Class 1-2 hardwood or treated softwood (H3)
Window Joinery	Class 2-3 hardwood or LOSP-treated pine
In-Ground Posts	Class 1 or H4-H5 treated pine
Interior Lining/Furniture	Any class if protected from moisture

Figure 2: Guide for selecting appropriate timber durability for common applications
Source: Based on Table in TDG 05

Common Durable Australian Species

Species	Durability Class	Region	Notes
Spotted Gum	1	NSW, QLD	Very dense, excellent durability
Ironbark (Red/Grey)	1	NSW, QLD	Extremely durable, hard to work
Tallowwood	1	NSW, QLD	Self-lubricating; finish-friendly
Blackbutt	2	NSW, QLD	Readily available, BAL compliant
Jarrah	2	WA	Termite resistant
Silvertop Ash	2	VIC, NSW	Moderate hardness, Class 2 only
Vic Ash / Tas Oak	3	VIC, TAS	Must be sealed; interior use
Radiata Pine	4	National plantations	Requires H-class treatment

For a searchable guide, visit the WoodSolutions Species Database.

Key Caveats

Durability ≠ Termite Resistance
Use AS 1604 hazard classes (H1-H6) to address termite risk specifically.
AS 5604 ≠ EN 350 ≠ AWPA
Do not compare “Class 1” ratings across standards from different countries.
Durability is context-dependent
Even Class 1 timber can fail without proper detailing (e.g. poor drainage or no ventilation).

Summary

Natural durability refers to heartwood only. Sapwood must be treated.
AS 5604 defines Classes 1-4, with life expectancies based on exposure.
Use Class 2+ for exposed applications, or treatment for lower classes.
Match durability to application, not higher by default.
Species choice should consider availability, cost, finishing, and sustainability.
Always check local detailing and exposure conditions. They often matter more than the species rating.