How to secure fence posts with only 25-30cm of soil?

dippylegs

I’m looking to build a lightweight but more permanent fence with a simple gate around this garden area (running straight across from the left retaining wall to the right, dropped retaining wall). It’s mainly intended as a visual boundary for our dog rather than a heavy-duty security fence.

The issue is that beneath the grass there’s only about 25-30 cm of soil before hitting a very old concrete slab.

Would that depth be enough to set fence posts in concrete, or would the posts need to be fixed to the concrete slab underneath instead?

I’ve included a photo of the space and an example of the style of fence I’m aiming for. I’m hoping to reuse some existing Jack mesh panels/trellises (1800 x 1000 mm) by attaching them to timber posts.

Thanks so much!

MitchellMc

Welcome to the Bunnings Workshop community @dippylegs. It's wonderful to have you join us and many thanks for your question on creating a fence.

With only 25–30cm of soil before hitting the old slab, I’d avoid relying on buried timber posts alone, especially once you add a gate. That shallow depth usually isn’t enough to stop movement over time, even with concrete footings.

The strongest and most reliable option would be to fix the posts directly to the concrete slab underneath using bolt-down post supports. I’d definitely recommend this for at least the two gate posts:

• the hinge post
• the latch post

Those posts take the most load and movement, so anchoring them to the slab will keep the gate aligned and prevent wobble.

For the intermediate fence posts, you could potentially get away with shallow footings if the fence is lightweight, but 30cm is borderline for timber posts concreted into soil. Over time they may loosen or lean slightly.

Another good alternative for the non-gate sections would be driven steel fence posts, which work better in shallow ground conditions.

If you want a cleaner timber look, you could use bolt-down galvanised stirrups on the slab, attach timber posts into the stirrups and then fix your Jack mesh/trellis panels between them.

That’ll give you a much more permanent and rigid result while still keeping the project fairly simple and lightweight.

You might like to check out this excellent project: Fenced vegetable garden with raised beds by @Wheats.

Please let me know if you have any questions.

Mitchell