How to build a Hardiboard fence to reduce road noise?

Question

Due to the recent freeze, my clusia plants that were 12-15' tall have bit the dust.  We need sound abatement and I an reviewing hardi board.  W

e also are trying to figure out how to build a fence (approx 80') in our backyard to reduce road noise, etc.  Vinyl and wood I don't believe will help get any noise reduction.  Your thoughts?  I see that someone else has done this but I don't understand how they have framed each panel.

MitchellMc · Answer

Those 90 mm (4") steel posts are actually a good choice @rdiaz. Strength-wise you’ll be in a very safe position, especially for a solid, wind-loaded fence. As long as they’re properly set in concrete (around 600–800 mm deep) and spaced no more than about 1.8 m apart, they’ll perform well.

The fibre cement panel you’re looking at (the vertical groove, one-sheet style) is also suitable, but the key thing is that it must be fixed to proper framing between the posts—not directly to the posts themselves. You’ll need horizontal rails or a small framed section (like a wall) between each post so the panels are well supported and don’t flex or crack.

Your post choice is more than adequate, and the panel will work, just make sure you include proper intermediate framing and follow the fixing requirements, otherwise you won’t get the strength or finish you’re expecting.

Mitchell

rdiaz · Answer

Thanks Mitchell.  I was looking at the hardie siding that looks like vertical wood planks but it is one piece.

Here's what I thought for the posts, but I will take your advice.  4 in. x 8 ft. Powder Coated Black Steel with Top Plate for In-Ground Line Applications Metal Fence Post.

Board:  Hardie Panel HZ10 48 in. x 96 in. Primed Sierra 8 Fiber Cement Panel Siding

Thanks for the quick response!

MitchellMc · Answer

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

Sorry to hear about the clusias. That’s a big loss, especially at that height.

Using fibre cement (Hardi board) can actually be a pretty effective option for noise reduction. The key advantage is that it creates a solid, dense barrier, which helps block direct sound waves from the road. Compared to timber or vinyl fences (which often have gaps or are lighter), a properly built solid wall will perform noticeably better.

That said, it’s important to set expectations as sound doesn’t just stop at a wall. A lot of it will diffract (bend) over the top. Since your hedge was previously 12–15 ft tall, it would have been blocking more of that higher path. A standard fence height will still help with direct noise, but you’ll likely continue to hear some sound coming over the top. The taller and more continuous the barrier, the better the result.

In terms of construction, the setup is more like a wall than a typical fence. You’d be looking at:

* Posts set at regular intervals (timber or steel)
* Horizontal rails or noggins between the posts
* Then a framed structure between posts, similar to an exterior house wall
* The Hardi panels fixed onto that frame

Because these products are designed for cladding houses, the framing needs to follow similar principles (stud spacing, fixing points, expansion gaps, etc.). It’s definitely not just attaching sheets to fence rails like you would with timber palings.

If you have a specific Hardie product in mind, I can help point you to the exact installation guide and framing requirements, as they vary a bit depending on the panel type.

Also worth considering: sealing any gaps (including at the bottom) is important. Even small openings can let a surprising amount of noise through.

You might like to reinstate another hedge once you have the fence installed, which will help abate that sound clipping over the top of it.

Please let me know if you have any questions.

Mitchell