Depth by job
| Job | Depth in ground | Hole | Source |
|---|---|---|---|
| Timber fence up to 1.2 m | about 450 mm | ≥300 mm, 100 mm gravel | Timber Queensland TDS 20 |
| Timber fence 1.2–1.8 m | about 600 mm | ≥300 mm, 100 mm gravel | Timber Queensland TDS 20 |
| Steel sheet fence (e.g. Colorbond) | 100–900 mm by wind and soil | 200 mm | Lysaght installation guide |
| Fence post, 1.8 m above ground (NZ) | 600 mm | about 3 × post width | Dricon |
| Rotary clothesline | 600 mm | 250 mm, 150 mm gravel | Installation guides |
| Pergola or carport post | as designed | as designed | AS 1684 / NZS 3604 and your plans |
Lysaght's table shows how much the answer moves for steel fences. In a typical Sydney suburb on stiff clay, a standard fence needs a 200 mm wide footing 400 mm deep. The same fence in an exposed wind region, or on soft clay or loose sand, can need 900 mm. Lysaght also asks for at least 40 mm of concrete under the post, and 20 MPa concrete as a minimum.
About the "one third" rule
Many websites say a third of the post should be in the ground. We could not find that rule in any Australian or New Zealand standard or manufacturer guide. It also means different things depending on what you take a third of. For a 1.8 m fence, 600 mm is a third of the height above ground, but only a quarter of the 2.4 m post. Dricon's guidance for New Zealand says "at least a quarter of the post in the ground". The Timber Queensland depths work out close to a third of the fence height. Use the table above, then add depth for poor soil or wind.
How soil changes the depth
- Rock. In Lysaght's tables, footings in rock can be as shallow as 100–300 mm.
- Stiff clay is the case Lysaght's worked example uses: 400 mm deep in a typical Sydney suburb. Medium-dense sand needs a little less.
- Soft clay, loose sand and fill need the deepest footings, up to 900 mm for steel fences. Never set footings in loose, uncompacted fill.
- Sand that drains fast can suck the water out of wet concrete. Cement Australia suggests lining the hole with plastic.
- Reactive clay shrinks and swells with the seasons. Houses on it are built to AS 2870 site classes, from M up to H1, H2 and E. We found no Australian guidance on fence or pergola footings in reactive clay specifically. If your house sits on an H or E class site, ask a local fencer or engineer what depth works in your area.
None of the Australian or New Zealand guides we checked set a frost depth for fence footings. Cold weather matters more when pouring: Cement Australia says not to pour its concrete mix below 5 °C, or its rapid set below 10 °C.
H4 or H5, or keep timber out of the ground
Treated timber carries a hazard class under AS 1604 in Australia and NZS 3640 in New Zealand. Both H4 and H5 are rated for ground contact. H4 is for a "severe decay" hazard and H5 for "very severe decay".
- Fence posts: H4 at least (Timber Queensland; NZS 3640 lists fence posts under H4).
- Pergola, deck and house piles buried in the ground: H5, with 100 mm of gravel under the post and termite protection (Timber Queensland; NZS 3640 lists house piles and poles under H5).
- Better still for pergolas: Timber Queensland prefers hot-dip galvanised stirrups set in the concrete, holding the timber 75 mm clear of it. The post stays out of the wet, and the gap lets you see any termite activity.
- Aluminium posts should not go straight into cement-based mixes unless they are primed or sealed. Both Cement Australia and Boral say so.
Hole, gravel and concrete
Timber Queensland asks for holes at least 300 mm across for fence posts. Dricon's rule of thumb is about three times the post width. Put 100 mm of coarse gravel in the base so water drains away from the end grain, then stand the post on it. Concrete fills the rest of the hole around the post. A 100 × 100 mm post in a 300 mm hole 600 mm deep takes about 30 litres, a little over three 20 kg bags. The post hole concrete calculator does the arithmetic for any size and brand of bag.
Rapid set mixes are poured dry into the hole after the water. They harden in about 15 minutes, and Australian and New Zealand makers say the braces can come off after about an hour. They set too fast to mix in a barrow. Standard rapid set is sold for non-structural uses, so for pergolas use the concrete your plans specify. Don't add extra water to any mix: "too much water ruins good concrete", as Cement Australia's data sheets put it.
Finishing so posts don't rot
Most timber posts rot at the ground line, where water sits against the timber. Two finishes work:
- Crowned at ground level. Trowel the concrete so it slopes away from the post on every side, as Cement Australia's instructions show.
- Stopped 100 mm below and covered with soil. Dricon's method, which lets lawn or garden grow right up to the post.
What doesn't work is a flat or dished top that collects water against the post. Every guide we checked warns against it.
Questions
How deep should a 1.8 m fence post be?
About 600 mm in the ground for a timber fence, in a hole at least 300 mm across on 100 mm of gravel, so buy 2.4 m posts. Steel sheet fences vary with wind and soil: from 400 mm on stiff clay in a sheltered suburb up to 900 mm in soft ground or exposed areas.
Should fence posts be H4 or H5?
H4 is the minimum for fence posts in the ground. Use H5 for posts that carry a structure, such as pergolas and decks, if they are buried, or set them on galvanised stirrups instead.
Do you put gravel under fence posts?
For timber posts, yes: about 100 mm of coarse gravel, so water drains away from the end of the post instead of sitting in the concrete.
Sources
- Timber Queensland, Technical Data Sheet 20: Residential timber fences (2006) and TDS 19: Pergolas and carports – depths, gravel, H5 and stirrups.
- Lysaght (BlueScope), Fence and gate installation guide, non-cyclonic (January 2026).
- Dricon (Firth), RapidSet technical data sheet – hole size, embedment, soil cap.
- Cement Australia, How to set a fence post and Rapid Set product data sheet; Boral, Rapid Set No-Mix data sheet.
- Timber Preservers Association of Australia, AS 1604 explained; BRANZ, Builder's Mate 8A: Treated timber (NZS 3640 uses).
- CSIRO, BTF 18: Foundation maintenance and footing performance – AS 2870 site classes.
Last reviewed 23 September 2026 · Report an error