GLAREUM.NET›Chapter 4›Concrete

4.3 Setting posts in concrete

Fence, gate, pergola and deck posts set in a hole in the ground.

Cross-section of a post set in concrete: a gravel base at the bottom of the hole, the post standing on it, and concrete around the post up to a sloped top at ground level. 0.9 m 4 Post 100 × 100 mm 3 Sloped top sheds water 2 Concrete 1:2:4 · at least 0.9 m 1 Gravel base 8–16 mm · tamped
Cross-section for a 100 mm post set 0.9 m deep.

At a glance

Difficulty
1 of 4
Time (estimate)
1 hour per post
Tools
Post-hole spade or auger, spirit level, two braces and screws, bucket or mixer, tamping rod
Parts
  1. 1  Gravel base
  2. 2  Concrete

What you need per post, 100 mm post in a 300 mm hole, 0.9 m deep

MaterialSizeTypeAmount
Gravel base 8–16 mm Washed about 1 bucket
Sand 0–4 mm Washed 34 kg
Stone 8–16 mm Crushed or rounded 75 kg
or 4–8 mm For narrow augered holes
Cement – Common cement (EN 197-1) 16 kg

Why 4–8 or 8–16?

4–8 mm

For narrow augered holes where the gap around the post is under 80 mm. The stone must stay under a fifth of that gap.

8–16 mm

For wider holes. A 300 mm hole around a 100 mm post leaves a 100 mm gap, which takes 8–16 mm stone easily.

Step by step

  1. 1

    Dig the hole at least 0.9 m deep.

    Go deeper in sandy or wet soil, and add room for the gravel base. Make the hole wide enough to leave a good collar of concrete all round the post.

    The empty hole
  2. 2

    Put a layer of 8–16 mm stone in the bottom and tamp it.

    The post stands on it, not on the soil, and water can drain away.

    Gravel in the hole
  3. 3

    Stand the post and brace it plumb.

    Check two adjacent faces with a spirit level. Screw a brace to each face and peg it into the ground.

    Post with braces
  4. 4

    Mix 1 part cement, 2 parts sand, 4 parts stone and fill the hole.

    Fill in layers and rod each one to drive out air. Slope the top away from the post so water runs off.

    Filling around the post
  5. 5

    Check plumb again, then leave it braced.

    Small corrections are possible while the concrete is soft. Leave the braces on.

    Spirit level on the post
  6. 6

    Keep wet for at least 7 days, then hang the panels.

    Keep the top wet or covered while the concrete gains strength.

    Finished post with panel
Specifications
PartSieve sizeThicknessHow
1 Gravel base8–16 mm–Tamped.
2 Concrete8–16 mmat least 0.9 m1 : 2 : 4. Rodded. Top sloped.

Depth

Fence posts at least 0.9 m (3 ft) deep, and deeper in sandy or wet soil (USDA NRCS fence specification). Posts that must not move in frost go below the local frost line.

Hole width and gravel base

Make the hole wide enough for a solid collar of concrete all round the post. Put a tamped layer of clean stone in the bottom so the post does not stand on soil and water drains away.

Stone size

The largest stone must be no more than one fifth of the narrowest gap it has to pass through (ACI 318). With a 300 mm hole and a 100 mm post the gap is 100 mm, so stone up to 20 mm fits.

Mix

Use the same 1 : 2 : 4 as the floor in 4.1.

Sources

  1. USDA NRCS fence specification 382 (Texas): posts at least 3 ft (0.9 m) deep, deeper in sandy or wet soil; slope the concrete top so water runs off
  2. FAO Farm Structures, ch. 3: Concrete: mixes and amounts per m³, stone ≤ ¼ of the thinnest part, sand silt test, bulk densities, layers ≤ 150 mm, keep wet at least 7 days, forms off after 3 days
  3. ACI 318-19 §26.4.2.1 Largest stone ≤ ⅕ of form width, ⅓ of slab depth, ¾ of bar gap (American Concrete Institute FAQ)
  4. US HUD / NAHB Research Center: Frost-Protected Shallow Foundations, Phase II: frost depth set mainly by winter air temperatures (air-freezing index), with soil type, soil moisture, snow and ground cover; the frost line rises near a heated building
  5. National Research Council Canada, CBD-26: Ground freezing and frost heaving: growing ice lenses lift the soil, often 100–200 mm; foundations of unheated buildings not taken below frost level are often damaged
  6. American Concrete Institute FAQ: consolidation: rodding for flowable mixes, tamping for stiff ones, vibration internal or external
  7. Mile High Flood District: curing specification (based on ACI 308.1): cure at least 7 days, forms left in place count as curing
  8. EN 12620 Aggregates for concrete (DIN)
  9. EN 206 Concrete: specification, performance, production and conformity (EVS; withdrawn from 1 October 2026 and replaced by EN 206-1:2026 and EN 206-2:2026)

Amounts are for one hole 300 mm wide: about a bucket of gravel and 0.055 m³ of concrete around a 100 mm post set 0.9 m deep. They use FAO's table for 1 : 2 : 4 per m³ and 1.55 t/m³ for loose stone. They include no waste.

Section 4.3 Find these materials near you →

GLAREUM.NET · Aggregate Handbook