100×50 GOS Hardwood F17 H4

Can I use F17 H4 hardwood for fence posts in the ground?
Yes. H4 treatment is specifically rated for in-ground applications, making it the correct specification for structural fence posts that go into or near the soil. Using a lower treatment level like H3 for in-ground posts will significantly reduce their service life, as H3 is only rated for above-ground use. Learn more about timber treatments.

What does GOS mean in timber?
GOS stands for Green Off Saw, which means the timber hasn't been air or kiln dried before being treated (it's fresh off the saw from being cut). This keeps the moisture content higher than kiln dried timber, which has a practical benefit on site. GOS timber is easier to bend and straighten, so any bowing in a piece can be corrected during installation before it's fixed in place.

What is an F17 structural grade and why does it matter?
The F grade refers to the structural stress rating of the timber (how much load it can carry before becoming structurally compromised). F17 has the load-bearing capacity for demanding structural applications like posts, bearers and framing.

Can I use GOS Hardwood F17 H4 for a pergola or deck substructure?
Yes. It's well suited to pergola posts, deck bearers and joists where structural strength and ground contact or near-ground exposure are involved. The F17 rating gives you the load-bearing performance needed for structural framing, while the H4 treatment handles the conditions at ground level.

Do I need to treat cut ends after cutting GOS Hardwood H4 on site?
Yes, you should always apply an appropriate H4 end-grain sealant or timber preservative to any cut faces before installation. The treatment is concentrated in the outer layers of the timber, so fresh cuts expose less-treated timber at the cut face.

Is GOS hardwood weaker than kiln dried hardwood?
No, GOS timber is structurally sound and the F17 stress grade applies regardless of moisture content. The higher moisture content of GOS timber simply means it hasn't been dried before treatment, which makes it more flexible and easier to work with on site. It will naturally lose moisture over time once installed.

Can GOS Hardwood F17 H4 be used for retaining walls?
Yes. H4 treated structural hardwood is commonly used for retaining wall posts and framing where the timber is in direct contact with soil. For higher retaining walls, engineering approval may be required depending on your local council requirements, so it's worth checking before you build.

How long will H4 treated hardwood last in the ground?
Service life depends on soil conditions, drainage and the specific application, but H4 treated hardwood in typical structural applications is generally expected to last well in excess of 25 years under normal conditions. Well-drained sites with good airflow around the timber will extend service life, while constantly wet or waterlogged conditions are harder on any timber regardless of treatment level.