How to Build a Strainer Assembly

A fence is only as good as its strainer assemblies. You can use the best mesh, the heaviest pickets, and the finest wire on the market, but if your end assemblies are weak, the whole thing will eventually pull itself apart. Strainers are where all the tension in the fence terminates, so they need to be rock solid.

WHAT IS A STRAINER ASSEMBLY?

A strainer assembly is the anchor point at each end of a fence run, at every corner, and at every gate. It typically consists of a vertical post, a horizontal stay (strut), and a diagonal brace — sometimes called a "box assembly" when it includes two vertical posts with stays between them.

The most common configurations are:

  • Single strainer: One post with a stay and diagonal brace. Suitable for lighter fences and mid-run tensioning points.
  • Box strainer (H-brace): Two posts with a horizontal rail and diagonal wire brace. The standard for end assemblies and corners on any serious fence.
  • Double box: Two H-braces linked together. For very long runs, heavy mesh, or high-tension applications.

WHERE DO YOU NEED THEM?

  • At both ends of every fence run
  • At every corner (change of direction greater than about 15 degrees)
  • At every gate position (both sides)
  • Every 200-300m on long straight runs (to limit the wire tension load and make future re-straining easier)
  • At any significant change in terrain (top and bottom of steep hills)

MATERIALS

Posts: Use treated timber (CCA H4 or H5 rated for in-ground use). Minimum 150mm diameter for standard fencing, 200mm for end assemblies and gate posts. Length depends on how deep you're setting them — typically 2.4m posts set 600-700mm into the ground.

Stays: Treated timber rails, usually 100-125mm diameter and 2.4m long. These go horizontally between the two posts in a box strainer.

Brace wire: Use 2.5mm high tensile wire for your diagonal bracing. This goes from the top of one post to the base of the other, forming a triangle with the stay.

Concrete: Optional but recommended in soft soil. A bag or two of premix concrete per post hole makes a big difference to long-term stability.

STEP-BY-STEP: BUILDING A BOX STRAINER

1. DIG YOUR POST HOLES: Two holes, spaced 2.4 metres apart along the fence line. Each hole should be at least 600mm deep and about 300mm across. In rocky ground, a post hole borer on a tractor saves hours compared to manual digging.

2. SET THE FIRST POST: Drop the post in, check it's plumb (vertical) with a spirit level, and backfill with soil or concrete. Tamp the backfill firmly. If using concrete, let it set for at least 24 hours before applying any tension.

3. SET THE SECOND POST: Same process. The two posts should be exactly 2.4m apart at ground level, both plumb, and in line with the fence direction.

4. INSTALL THE HORIZONTAL STAY: The stay sits between the two posts, usually about 300mm below the top. Notch the posts to receive the stay, or use stay brackets. The stay needs to be a firm fit — it's a compression member, meaning the fence tension tries to push the two posts toward each other, and the stay resists that.

5. RUN THE DIAGONAL BRACE WIRE: This is crucial and it trips people up. The brace wire goes from the TOP of the far post (the one away from the fence) to the BOTTOM of the near post (the one closest to the fence). Think of it this way — the wire brace and the horizontal stay form a triangle that resists the fence pulling the top of the near post inward.

Run the wire diagonally, wrap it several times, and twist it tight with a toggle stick (a short piece of timber you rotate to tighten the wire loop). The brace wire should be singing tight.

6. REPEAT FOR THE OTHER SIDE: If this is a corner or an end assembly, you'll need the strainer to resist pull from the other fence direction too. Build accordingly.

COMMON MISTAKES

Brace wire going the wrong way: If you run the diagonal brace from the top of the near post to the bottom of the far post, the assembly actually works against you. It's the most common mistake in fencing and it makes the whole thing weaker, not stronger. Remember: brace wire goes from the top of the POST AWAY from the fence to the bottom of the post NEAREST the fence.

Posts too shallow: A strainer post at 400mm depth in soft soil will lean over within a year. Go 600mm minimum, deeper in sand or loose soil.

Untreated timber: Untreated posts rot. It might take 5-10 years, but they'll fail. Always use treated posts rated for in-ground contact.

Skipping the assembly and just using a heavy picket: Star pickets, even jumbo ones, are not strainer posts. They cannot handle the tension load of a properly strained fence. Use timber.

Building good strainer assemblies takes time, but it's time well spent. Every fence you build is held together by these assemblies, and doing them properly means your fence will perform for 20-30 years with minimal maintenance.

Need strainer posts, stays, or any fencing materials? We're at 76 Astill Drive, Orange, or call us on 0434 093 077.

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