Fence Post Depth Guide: Minimum Depths, Concrete Mix Ratios and Soil Type Adjustments

Quick Answer: UK practice for domestic fence posts is to bury one-third of total post length, with a minimum of 600 mm in firm ground for a 1.8 m fence. Concrete the post in with a 1:2:4 mix (cement:sharp sand:20 mm aggregate) or proprietary post-fix dry mix, leaving the concrete domed above ground to shed water. In sandy or wet clay soils, increase depth by 150–300 mm and widen the post hole to 300 mm minimum. BS 1722-1 (general requirements for fences) and BS 1722-7 (chestnut paling) cover the specification baselines.

Summary

Fence post depth is the single biggest determinant of a fence's life. Get it right and a 1.8 m close-board fence will last 15+ years against UK wind. Get it wrong and the same fence is leaning into the neighbour's garden inside a season. The rule of thumb — bury one-third of the post — works in firm ground, but UK soils are rarely that obliging. London clay shrinks 30 mm in dry summers; sandy Kent loam can't grip a post enough to resist wind load; waterlogged Essex marshland rots timber from the bottom up.

The job is easy to underprice. A fence with twenty posts, all dug to 600 mm, concreted, plumbed and braced, takes one person a full day even in clean ground. In stony or root-bound ground it can take two days. The labour to put a post in properly is more than the cost of the post itself. Quotes that allow only £20 per post for the labour are loss leaders.

The other side is post material. Pressure-treated softwood (Use Class 4 to BS 8417) gives 15 years; concrete spur posts give 30+; metal post anchors with timber post above ground last as long as the timber. Material choice should match the customer's budget and how often they want to repaint or repair. Get this right at quote stage so the customer doesn't expect a 50-year fence for a £20 post.

Key Facts

Quick Reference Table

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Fence height Total post length Buried depth (firm ground) Buried depth (sandy/wet) Hole width Concrete (50 kg bags)
0.9 m (low) 1.5 m 450 mm 600 mm 250 × 250 mm 1
1.2 m (medium) 1.8 m 600 mm 750 mm 250 × 250 mm 1–2
1.5 m (medium-high) 2.1 m 600 mm 750 mm 250 × 250 mm 2
1.8 m (standard high) 2.4 m 700 mm 900 mm 300 × 300 mm 2–3
2.0 m (high) 2.7 m 800 mm 1000 mm 300 × 300 mm 3
2.4 m (max domestic) 3.0 m 900 mm 1100 mm 300 × 300 mm 3–4
Soil type Adjustment
Firm clay Standard depth
Sandy / loose +150 mm depth, +50 mm hole width
Wet clay +150 mm depth, drainage gravel essential
Made ground / fill +300 mm depth, consider concrete spur
Heavily compacted hardstanding Concrete spur or core-drilled installation

Detailed Guidance

Why One-Third Depth?

The buried portion of the post is what resists overturning when the wind blows. Wind pressure on a 1.8 m fence panel acts at roughly the half-height (0.9 m above ground). The buried section provides a moment arm in the opposite direction, anchored by the soil and the concrete.

For a typical close-board fence on a moderately exposed site, the wind pressure can hit 0.5 kN/m² (500 N/m²). On a 1.83 m × 1.83 m panel (3.35 m²) that's 1.67 kN trying to push the fence over — applied at roughly 0.9 m above ground. The buried 600 mm of post needs to resist that with the lever arm and the soil resistance against the concrete plug.

The maths gives a buried-third rule for firm ground; in soft ground, increase to 40% buried. For exposed coastal sites with severe wind exposure (Zones 4–5 of the BS 5534 wind zone system applies similarly), a half-buried post is sometimes warranted.

Hole Geometry

The post hole isn't just a slot for the post — it's a foundation. Critical dimensions:

Concrete vs Postcrete

Concrete (1:2:4 mix):

Postcrete / Postmix (proprietary):

Tamping — concrete must be tamped down with a stick to drive air bubbles out. Voids in the concrete plug make a weak post.

Doming — finish concrete 30–50 mm above ground, sloped away from post. Standing water at the post base is the leading cause of post rot.

Soil Type Adjustments

Sandy / loose: Sand grips poorly. Increase depth by 150 mm and hole width by 50 mm. Tamp the concrete deliberately to drive concrete into the surrounding sand. Consider a wider (300 mm) plug and a slightly stiffer concrete mix.

Wet clay: Clay holds the post but waterlogs the post end. Drainage gravel is essential — 100 mm of 20 mm aggregate at the bottom of the hole, before concrete. Without drainage, the post end sits in standing water and rots in 3–5 years.

Stony / chalky: Difficult to dig but usually grips well once the post is in. Use a SDS digger or post-hole spade. Watch for the hole opening up wider than intended at the top — fill with concrete to seal.

Made ground / hardcore fill: Unreliable foundation. Either dig deeper to firm ground below, or use a concrete spur post bolted to a timber post above ground.

Heavily compacted (driveways, footpaths): Core-drill if the hardstanding is to remain. Diamond-core 200 mm diameter to required depth, drop in spur post or anchor sleeve, concrete in.

Spur Posts and Repair Solutions

A concrete spur post is a 75 × 75 mm × 1.5 m precast concrete post buried 600 mm and concreted in the ground. The above-ground portion has bolt holes for fixing a timber post. The advantages:

Spurs add £20–£35 per post in materials but can extend a fence's life by 20+ years.

Drive-In Anchors and Bolt-Down Anchors

Drive-In (Met-Post and similar): 750 mm steel anchor with cup at top. Hammered into firm ground with a driving tool. Acceptable for low fences (≤1.5 m) in firm undisturbed ground. Do not use for fences over 1.5 m or in soft ground — the anchor leans over time as the soil compacts around it.

Bolt-down: Steel base plate with cup on top, bolted to a concrete pad. Requires a 300 × 300 × 300 mm concrete pad cast in advance. Suitable for fences on existing slabs (e.g. patio edges) where digging isn't an option.

Concrete Curing and Bracing

While concrete cures (typical 24–48 hours for setting, 28 days for full strength):

A post knocked out of plumb during curing can't be straightened later — you'd have to dig it out and start again.

Fence Treatment and Post Material

UK posts in ground must be treated to BS 8417 Use Class 4 (HC4). This means:

Untreated or under-treated posts (Use Class 3 or below) will rot at the ground line in 3–7 years even if the rest of the post is sound.

Frequently Asked Questions

Why does my old fence post snap at ground level?

Ground line is where the post is permanently damp from below and aerated from above — the perfect rot environment. The buried part stays wet but oxygen-poor (slower rot); the above-ground part dries out (less rot). The boundary line concentrates moisture + oxygen + microbial activity. Posts almost always fail at this point.

Should I use Postcrete or proper concrete?

For one or two posts, Postcrete saves time. For a full fence (10+ posts), bagged ballast-and-cement is cheaper in materials and gives a stronger long-term hold. The labour to mix concrete is fixed regardless of how many posts; the material savings scale.

Can I put a fence post on top of paving?

Only with a bolt-down anchor on a structural base — typically a 300 mm × 300 mm × 300 mm concrete pad cast in or under the paving. A standard slab is not strong enough to hold a wind-loaded fence post.

Do I need planning permission for a fence?

In England, planning is required for:

See fencing regulations and planning rules for the full list.

How deep for a 6-foot fence post?

A 6-foot (1.83 m) fence panel uses an 8-foot (2.4 m) post buried 700 mm in firm ground or 900 mm in soft ground. The "1/3 buried" rule gives 800 mm; rounding to 700 mm is acceptable in firm ground but not in poorly draining soils.

Regulations & Standards