Fence Post Installation Depth: How Deep to Set Posts for Different Heights and Soil Types

Quick Answer: UK domestic fence posts should be set to a minimum depth equal to one-third of the above-ground height — 600mm for a 1.8m fence, 750mm for a 2.0m fence, 900mm for a 2.4m fence — with the hole 250–300mm diameter and the post bedded in 1:2:4 concrete mix or postcrete to BS 8500. Clay and waterlogged soils require deeper holes (add 100–150mm); rocky or compacted ground accepts shallower depths. The base of each hole should have 50mm pea-shingle for drainage. Insufficient depth causes posts to lean within 2–3 years; insufficient hole diameter prevents the concrete envelope developing enough mass to resist wind overturning.

Summary

Post depth is the single biggest factor in fence longevity, and the most commonly under-specified item in a fencing quote. A fence is in effect a sail standing on cantilevered posts. Wind load multiplied by the height gives a moment that the post foundation must resist. If the foundation is too shallow or too narrow, the post will rotate in the ground — pulling out of plumb within a few years and eventually failing.

The traditional "one-third rule" — set the post one-third of the above-ground height into the ground — is a good starting point for solid, dry ground and standard residential fence heights. It needs adjustment for clay, waterlogged conditions, exposed sites, or where the fence is solid versus open. This article covers the calculations, the practical decisions per soil type, and the concrete mix specifications.

For the post material decision (timber vs concrete vs metal), see timber close board fencing and concrete post gravel boards. For repair of leaning or rotten posts, see fence repair replacing posts.

Key Facts

Quick Reference Table

Spending too long on quotes? squote turns a 2-minute voice recording into a professional quote.

Try squote free →
Above-Ground Height Minimum Depth (Standard) Depth in Clay Depth in Exposed Location Hole Diameter
0.9m (3ft) 450mm 550mm 500mm 200mm
1.2m (4ft) 500mm 600mm 600mm 250mm
1.5m (5ft) 600mm 700mm 700mm 250mm
1.8m (6ft) 600mm 750mm 750mm 300mm
2.0m (6.5ft) 750mm 900mm 900mm 300mm
2.4m (8ft) 900mm 1050mm 1100mm 350mm
Post Material × Height Hole Volume (m³) Concrete (kg or bags postcrete)
75mm × 75mm timber, 0.9m fence 0.015 m³ 25 kg / 1 bag
100mm × 100mm timber, 1.8m fence 0.042 m³ 80 kg / 4 bags
Slotted concrete post, 1.8m fence 0.042 m³ 80 kg / 4 bags
100mm × 100mm timber, 2.0m fence 0.053 m³ 100 kg / 5 bags
125mm × 125mm timber, 2.4m fence 0.108 m³ 180 kg / 9 bags
Steel post (50×50 RHS), 1.8m 0.042 m³ 80 kg / 4 bags

Detailed Guidance

Why depth matters — the cantilever physics

A fence post in the ground is a cantilever beam loaded laterally by wind on the panel. The bending moment at the ground line is wind force × height. Below ground, the surrounding soil and the concrete envelope resist that moment through passive earth pressure.

If the embedded depth is too shallow:

If the embedded depth is adequate:

The one-third rule is a crude but reliable approximation for typical residential exposures. For exposed coastal sites, hilltop properties, or solid fences over 2m, increase by 25–50%.

Adjusting for soil type

Clay — high plasticity, swells when wet, shrinks when dry; provides good passive resistance when undisturbed but moves seasonally:

Sandy / gravelly soil — quick-draining, less plastic; provides reasonable passive resistance but limited cohesion:

Chalk / rocky — naturally stable; difficult to excavate but excellent foundation:

Made ground / fill — unpredictable; old building rubble, garden soil over fill, or recent landscaping:

Waterlogged sites — perched water table; poor drainage:

Concrete vs postcrete

1:2:4 wet-mix concrete:

Postcrete (rapid-set bagged concrete):

For new-build or larger commercial fencing projects, wet-mix is more economical. For domestic single-day installs, postcrete is the practical choice.

Concrete cover and reinforcement

The concrete envelope must provide at least 75mm cover around all sides of the post. Hole diameter of 250mm for a 100mm post gives 75mm cover — the minimum. For 125mm posts, 300mm diameter is the minimum.

For posts over 2m above ground or in poor ground, consider:

Setting posts plumb

Standard procedure:

  1. Mark post position with stakes and string-line
  2. Dig hole to depth + 50mm for drainage base
  3. Place 50mm pea-shingle in base; tamp
  4. Position post; check height (top of post should match adjacent posts ±5mm)
  5. Brace with two timber struts at right angles to each other
  6. Plumb on two faces with spirit level
  7. Pour concrete in 100mm lifts, tamping each lift to remove air
  8. Re-check plumb after every lift; adjust struts if necessary
  9. Finish concrete top with a slope away from the post (water shedding)
  10. Leave 24 hours minimum before fitting panels (wet-mix); 30 minutes for postcrete

For long runs, set end posts first; run a string-line between them at top-of-post height; then set intermediate posts to the string.

Stepping vs raking on a slope

For ground gradient up to 1:10 (10%):

For gradients above 1:10:

Quick estimation of materials

For a 30m run of 6ft fence with 100×100mm timber posts:

Frequently Asked Questions

Can I use ballast and cement instead of pre-mixed concrete?

Yes — mix on site at 1:5 ballast:cement ratio (where ballast is pre-mixed sharp sand and 20mm aggregate). This gives roughly equivalent strength to a 1:2:4 separate mix. Easier to source for small jobs.

Do I need to leave concrete to cure before fitting rails?

For wet-mix, yes — 24 hours minimum. For postcrete, 30 minutes is sufficient for normal loading. In cold weather (<4°C), extend cure times by 50% or use rapid-set products.

Can I drive posts in instead of digging holes?

For light agricultural fencing on soft ground — yes, driven steel or timber posts work. For residential close board or panel fencing — no; driven posts don't provide enough lateral resistance for solid panels in wind.

What about Metpost ground-spikes?

Galvanised steel ground-spikes (Metpost, Securit) drive into soft ground and accept a square timber post in a socket. They are convenient but provide much less lateral resistance than concrete-set posts. Use for amenity fencing or temporary work; avoid for residential boundary fences.

My fence is leaning after 3 years — what's the cause?

In order of likelihood:

  1. Post not deep enough (most common)
  2. Hole too small (insufficient concrete envelope)
  3. Post rotted at ground line (timber post in clay or saturated soil)
  4. Soil saturated and movement (clay shrinkage/swelling)

Diagnosis: dig out one suspect post. If the timber is sound but the concrete envelope is small and the soil disturbed, depth was inadequate. If the post is rotten at the ground line, the timber failed.

Can I use a post auger / digger instead of hand-digging?

Yes — 250mm or 300mm auger bits are standard for fence posts. Mini-digger with auger attachment makes a 30m fence install a half-day rather than two days. Be careful of buried services; CAT scan first.

Regulations & Standards