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Hello @vbrett
Welcome to the Bunnings Workshop community. It's wonderful to have you join us and thanks for sharing your question about protecting H3 pine for garden boxing.
H3 treatment provides protection to timber (such as pine) against borers and termites, moisture and the weather, thus making it suitable for above ground outdoor use. It is not rated for direct soil contact, and I don't recommend using it when building a garden box.
Instead, I suggest using 200 x 50mm Sleeper H4 Sienna MicroPro 2.4m. It uses a copper-based treatment, and still provides protection even if the timber is indirect contact with the soil. It's suitable for use in vegetable beds and Children’s play areas.
If you need further assistance, please let us know.
Eric
Hi @vbrett,
Typically, you would use H4 timber and not H3.2 timber when it's in contact with soil, such as in the case of a garden bed. The treatment on the H4 sleepers is arsenic-based, so that's something to be aware of if your plans are to have vegetables in the garden bed.
H3.2 timber is suitable for outside above ground use, periodic wetting but where the timber can dry out; some leaching. So, if you are using it in contact with soil that inhibits its ability to dry out, its lifespan will be reduced. Even with the addition of Metallex, it's not the appropriate product to use, so it's hard to give you a definitive answer on the lifespan. H3.2 timber used in contact with the ground/soil might last a few years before starting to decay, so with the addition of Metallex and isolation from the soil with plastic, you might see ten years+.
Please let me know if you have any questions.
Mitchell
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