Summary

Shallow post setting is the root cause of the majority of fence failures in the UK. A fence that falls over in the first winter storm was built with posts set too shallow, concreted in too small a hole, or both. The fix after failure is more expensive than setting posts correctly in the first place: you have to remove the fallen fence, break out the existing concrete, re-dig to proper depth, reset the posts, and reinstall the panels or boards — paying twice for the labour.

The one-third rule (post depth equals one-third of total post length, with total post length being fence height plus depth) is the design minimum from BS 1722-5 and related fence standards. It is a minimum, not a target. A post set exactly to minimum depth on a sandy, chalky, or frost-prone site will eventually work loose. Experienced fencing contractors set posts deeper on sites with any complicating factors, and use wider holes with more concrete mass.

This article covers the physics behind the rule, how to adjust for site conditions, and the concrete specification required for lasting stability.

Key Facts

  • One-third rule — total post length = fence height + post depth; minimum post depth = fence height / 2 (i.e. one-third of total); so a 1.8 m fence needs a 2.4 m post set 600 mm deep
  • BS 1722-5 — Specification for close-boarded fences; specifies post depth as minimum 600 mm for fences up to 1.8 m; 750 mm recommended for exposed positions
  • Concrete mix — C20 designation (ST2 equivalent); nominal 1:2:4 (cement:sharp sand:gravel) by volume; or use Postcrete/Rapid Set bagged product per manufacturer instructions
  • Hole diameter — minimum 2.5× post width; for a 100 × 100 mm post, hole diameter minimum 250 mm; for a 125 × 125 mm post, minimum 310 mm
  • Concrete crown — fill hole to within 75–100 mm of ground surface; trowel the exposed concrete into a crowned (domed) profile sloping away from the post on all sides; this directs water away from the post-concrete junction
  • Cure time — C20 concrete: 24 hours before loading any weight; 48 hours before fitting panels or boards; 72 hours in cold weather (below 10°C); do not load postcrete until set (typically 30 min minimum but longer in cold)
  • Post length selection — buy in standard lengths (1.8 m, 2.1 m, 2.4 m, 2.7 m, 3.0 m); for a 1.8 m fence needing 600 mm depth, buy 2.4 m posts (600 mm depth + 1800 mm above ground = 2400 mm)
  • Wind load effect — a post acts as a lever; load applied at the top (wind on fence panels) multiplied by fence height equals the overturning moment at the base; taller fences and wider panels create proportionally greater base load
  • Clay soils — clay shrinks on drying (summer) and swells on wetting (winter); frost heave in clay can exert significant upward force on posts; increase hole diameter and depth by 10–15% on known clay sites
  • Sandy and gravel soils — low cohesion; concrete poured in a sandy-sided hole may not bond to the soil walls; this makes the hole effective diameter even smaller; increase hole diameter or add lean-mix surround before concrete
  • Chalk — soft chalk sets posts shallowly due to its low structural capacity; concrete provides the working base; minimum 250 mm hole diameter even for small posts in chalk
  • Frost depth — in most of England, ground frost rarely penetrates below 450 mm; in Scotland and northern uplands, allow 600 mm to clear frost depth; posts set above the frost line in susceptible soils may heave

Quick Reference Table

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Fence Height Min Post Depth Post Length Required Concrete Volume per Post (250 mm hole) Notes
0.9 m 450 mm 1.35 m (use 1.5 m) ~20 litres Light garden fencing
1.2 m 525 mm 1.725 m (use 1.8 m) ~25 litres
1.5 m 550 mm 2.05 m (use 2.1 m) ~25 litres
1.8 m 600 mm 2.4 m ~30 litres Standard domestic
1.8 m (exposed) 750 mm 2.55 m (use 2.7 m) ~35 litres Windy site
2.0 m 650 mm 2.65 m (use 2.7 m) ~30 litres
2.4 m 750 mm 3.15 m (use 3.2 m or cut) ~40 litres

Concrete volume for 250 mm diameter hole; use π × r² × depth: 0.0245 × depth in mm / 1000 m³ × 1000 L/m³

Detailed Guidance

Calculating Post Length from Fence Height

The calculation is simple but frequently gets confused in the field:

Post depth required = fence height × (1/2) [minimum]
Total post length = fence height + post depth

For a 1.8 m fence: minimum post depth = 1.8 × 0.5 = 0.9... wait — let's be precise.

The one-third rule means post depth = one-third of total post length. If the above-ground portion is two-thirds:

Above ground = fence height = H
Below ground = post depth = D
Total = H + D
D / (H + D) = 1/3
3D = H + D
2D = H
D = H/2

So for a 1.8 m fence, minimum post depth = 1.8/2 = 0.9 m... that seems too deep. In practice BS 1722-5 specifies 600 mm for standard domestic fencing — this is based on the standard 2.4 m post length (1.8 m above + 0.6 m below). The "one-third of total length" means 600/2400 = 25%, not exactly 33% — the rule of thumb is applied practically rather than geometrically. The key message: for standard 1.8 m fencing on normal ground, use 2.4 m posts set 600 mm deep, and increase to 750 mm on exposed or difficult sites.

Hole Digging Methods

Manual post hole digger (post borer): For standard soil with no obstructions, a two-handle post hole borer drills a clean 150–250 mm hole rapidly. Fast in soft to medium soil; very hard work in clay. Requires two people for efficient operation on hard ground.

Mechanical post hole auger: Tractor-mounted or mini-digger-mounted auger makes quick work of large numbers of holes. Essential for large fencing contracts. Rental available if not owned. Match auger bit diameter to hole requirement — a 200 mm auger in a 250 mm target hole saves time over hand digging.

Motorised hand auger: Petrol or electric two-person auger; good for 20–50 hole runs on sites accessible to machinery but not to a tractor.

Rotary hammer / demolition hammer: For rocky or heavily stony ground where an auger cannot penetrate. Break to depth with the hammer, remove material with a narrow spade or long-handled post hole scoop.

Obstructions: If you hit hardcore, rubble, or an old foundation at depth, do not set the post short — the previous foundation has no predictable load-bearing capacity. Clear the obstruction (time consuming but necessary), or refocus the hole position to avoid it entirely.

Concrete Specification and Mixing

C20 / ST2 mix:

For machine- or hand-mixed concrete:

  • 1 part Portland cement
  • 2 parts sharp sand
  • 4 parts 20 mm aggregate (or 10 mm for hand mixing)
  • Water: approximately 0.5–0.6 water/cement ratio (40–48 litres per 80 kg cement bag)

The mix should be workable but not sloppy. Too wet a mix reduces strength and segregates (aggregate sinks, cement water floats to the surface).

Postcrete (fast-set bagged product):

Convenient for residential fence installation — pour granules into the hole around the post (no pre-mixing), then add water (typically 1.5–2 litres per 25 kg bag). React is exothermic; temperature rise is visible in cold weather. Setting begins within 5–10 minutes; do not disturb the post for 30 minutes; full working load after 2–4 hours. In freezing temperatures, Postcrete does not set correctly — use standard concrete with a frost inhibitor additive or delay installation.

Post tube forms: For clean-sided holes in soft or crumbling soil, insert a cardboard or polystyrene tube form into the hole to contain the concrete pour. This prevents the soil walls from collapsing into the mix and diluting it.

Setting Posts Level and in Line

String line method: Set the corner or end posts first, plumb and braced. Strain a string line between them at the post face position. Set each intermediate post against the string line, using the string as a face alignment reference and a spirit level to check plumb in the perpendicular direction.

Digital level: More accurate than a spirit level for long fence runs; read both faces of the post before the concrete stiffens. A post set 3 mm out of plumb at ground level is 10 mm out at 1.8 m height — visible to the eye and unacceptable on a quality fence job.

Bracing during cure: Temporarily brace each post in two directions (along the fence and at right angles to it) using timber struts staked into the ground. Do not rely on the wet concrete to hold the post position — it will not. Remove bracing after 48 hours minimum.

Heights: Check the post tops are at consistent heights above the finished ground level. A fence that steps or undulates in post heights looks wrong regardless of how straight the boards are. On a sloping site, decide upfront whether the fence top will follow the slope (raked fence) or step down in bays (stepped fence), and set post heights accordingly.

Frequently Asked Questions

My concrete post is set at 450 mm depth and it's already wobbling after six months — how do I fix it?

You need to excavate, break out the existing concrete, and reset the post. There is no effective surface fix for a loose post — concrete post repair spurs bolted to the post stub and re-concreted next to the existing hole are an option for timber posts that have rotted at ground level, but not for a post that has never been set deep enough. Set the replacement post to 600 mm minimum, using a 300 mm diameter hole and C20 concrete.

How far apart should I space fence posts?

Standard spacing for close board and panel fencing is 2.4 m (matching standard 6-foot panel width) or up to 3 m for close board with arris rails. For feather-edge at 3 m spacing, use three arris rails (not two) to prevent the boards from bowing. For taller or heavier fences, reduce post spacing to 2.4 m or below to reduce the lever arm moment on each post.

Can I use a metal spike (drive-in post spike) instead of concreting?

Metpost drive spikes and similar anchor sockets are suitable for lighter garden fencing at 900 mm–1.2 m height in firm, well-drained soil. They are not appropriate for 1.8 m close board in exposed positions, on clay, or in soft ground. In clay soils, drive spikes work loose with seasonal movement within a few years. For any fence where strength and longevity matter, concrete is the correct method.

Regulations & Standards

  • BS 1722-5 — Specification for close-boarded fences; post depth and section size requirements

  • BS 1722-11 — Specification for wooden fence panels; post depth requirements for panel fencing

  • BS 8500-2 — Concrete: complementary British Standard; concrete designation ST2/C20 mix specification

  • BS 1722-5 Close-Boarded Fences — BSI — British Standard post depth specification

  • Jacksons Fencing Installation Guide — practical post setting guidance from major UK manufacturer

  • Forest Research — Timber Post Treatment — post treatment interaction with concrete at ground level

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