Treated timber is a staple in Australian construction, offering reliable protection against pests and decay when used correctly. But not all treatments are the same. Understanding the difference between H2 vs H3 treated timber is essential for choosing the right product for your job.
Whether you’re framing a house or building an outdoor structure, knowing when to use each treatment class can help you meet compliance standards, ensure long-term durability, and avoid costly mistakes.
What is Treated Timber?
Treated timber is timber that has been infused with chemical preservatives to protect it from decay, insect attack, and other environmental hazards. In Australia, the treatment level is classified using the H (hazard) rating system, which indicates the level of protection and the intended environment of use.
The most commonly used treatment classes in residential construction are H2 and H3.
H2:
H3:
What is H2 Treated Timber?
H2 treated timber is designed for internal, above-ground use in dry conditions. It is treated to resist termites, but not fungal decay or moisture exposure.
Common uses include:
- Wall frames
- Internal trusses
- Floor joists (in enclosed, dry areas)
H2 is widely used in structural framing applications, particularly in residential buildings across Australia. At ATS Building Products, H2 treated pine is available in structural grades like MGP10, making it suitable for framing requirements under AS 1684.
What is H3 Treated Timber?
H3 treated timber is designed for external, above-ground use, and is treated to protect against both termites and fungal decay. It can withstand occasional wetting, making it ideal for exposed outdoor applications.
Common uses include:
- Pergolas
- Decking joists and bearers
- Fence rails
- Weather-exposed battens
While H3 offers more protection than H2, it’s important to note that it is not suitable for in-ground use, that requires H4 or higher.
Comparison Table: H2 vs H3 Treated Timber
| Feature | H2 Treated Timber | H3 Treated Timber |
|---|---|---|
| Intended Use | Internal, above ground (dry) | External, above ground (weather-exposed) |
| Protection Against | Termites only | Termites and fungal decay |
| Moisture Resistance | Low | Moderate |
| Typical Applications | Wall frames, internal trusses | Decking, pergolas, fence rails |
| Timber Grades Used | MGP10, structural pine | Treated pine |
When to Use H2 or H3 Timber
Use H2 timber when building internal frames or structures that will remain dry and protected from the elements. It’s the go-to for framing houses in suburban builds and works seamlessly with other common construction timber types.
Use H3 timber when building external structures like decks, pergolas or screening that are exposed to occasional moisture or rain. H3 offers an additional layer of protection, helping the timber withstand fungal attack and weathering.
If you’re ever unsure, it’s safer to choose a higher treatment class than to under-specify. For example, using H3 timber in place of H2 is generally acceptable in most cases, but not the other way around.
ATS’s Treated Timber Range
At ATS Building Products, we supply a broad range of construction timber products to suit both indoor and outdoor structural applications. Our range includes:
- H2 Treated Pine framing in structural grades like MGP10
- H3 Treated Pine for weather-exposed projects
- Structural Hardwood and other specialist timber for load-bearing applications
With locations in Sydney, Brisbane, Melbourne, and Adelaide, we stock construction timber suitable for framing, decking, and general structural use. All products meet relevant Australian Standards and are sourced from trusted brands and mills.
Frequently Asked Questions
Depending on the level of exposure and type of paint used, outdoor treated timber should generally be repainted every 2–5 years for maximum protection.
It depends on the application. While H3 is treated to resist decay, sealing can further prolong its life, especially in exposed areas.
Priming is strongly recommended. Primers help paint adhere better and act as an additional moisture barrier.
Yes, H3 timber can be painted. It’s recommended to use a primer suitable for treated wood followed by a durable exterior paint to help extend the lifespan of the timber.
H2 timber used in outdoor environments is likely to fail over time due to fungal decay or moisture damage.
It depends on the application. While H3 is treated to resist decay, sealing can further prolong its life, especially in exposed areas.
Yes, H3 timber can be used indoors. It’s often used in transitional areas or where moisture exposure is unpredictable, such as bathrooms or kitchens.
No. While painting may provide some surface protection, H2 timber is not treated for fungal resistance or moisture exposure and should not be used outdoors.
Understanding the difference between H2 and H3 treated timber can save time, money, and prevent structural issues down the track. When in doubt, matching the right treatment class to your environment is the safest way to ensure long-term performance.
For expert advice or to place an order, contact your nearest ATS Building Products branch or visit our Construction Timber page.