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4.2 Foundation concrete

Strip and pad footings that carry walls, posts or a small building such as a shed, garage or garden wall.

Cross-section of a strip footing: the footing concrete at the bottom of a trench, as deep as the wall is thick and twice as wide, with a block wall starting on it. The bottom sits at the local frost line. local frost line 400 mm 2 Wall or blocks 200 mm thick 1 Footing concrete 200 mm deep · 16–32 mm stone Undisturbed soil
Cross-section of a strip footing under a 200 mm wall. The underside of the concrete sits at or below the local frost line.

At a glance

Difficulty
3 of 4
Time (estimate)
1 day to dig, 1 day to cast 10 m
Tools
Spade or mini excavator, string line and pegs, carpenter's level, mixer, wheelbarrow, tamper
Parts
  1. 1  Footing concrete
  2. 2  Wall or blocks on top

What you need per meter of footing, 400 × 200 mm

MaterialSizeTypeAmount
Sand 0–4 mm Washed 57 kg
Stone 16–32 mm Crushed or rounded 105 kg
or 8–16 mm Where there is rebar or a narrow pad
Cement – Common cement (EN 197-1) 18 kg

Why 8–16 or 16–32?

16–32 mm

For plain footings 200 mm and deeper with no rebar. The largest stone may be a quarter of the thinnest part, and bigger stone needs less mixing water.

8–16 mm

Where the footing has rebar, or for small pads. The stone must pass between the bars. See 4.4 for the rule.

Step by step

  1. 1

    Mark out and check what is allowed.

    Set string lines on pegs for both edges of the trench. A house, extension or garage needs a permit and a design from an engineer. This page covers the concrete.

    String lines on pegs
  2. 2

    Dig the trench down to firm soil and below the frost line.

    The local frost line ranges from nothing in frost-free climates to more than 2 m in the coldest places, so check the value where you live. Without frost, FAO gives about 30 cm for light buildings and up to 1 m for heavy ones.

    The trench with a depth rod
    The trench with a depth rod Illustration
  3. 3

    Set level pegs for the top of the concrete.

    Drive pegs into the trench bottom along its length and level them to the top of the footing: as deep as the wall is thick, and at least 150 mm.

    Leveling the pegs
  4. 4

    Mix 1 part cement, 3 parts sand, 5 parts stone and fill.

    Fill in layers no deeper than 150 mm, and tamp each one into the corners so no air is trapped.

    Tamping the footing
  5. 5

    Keep wet for at least 7 days.

    Keep it wet or covered so it stays moist while it gains strength.

    Footing kept wet
Specifications
PartSieve sizeThicknessHow
Footing16–32 mm or 8–16 mmas deep as the wall is thick, at least 150 mm1 : 3 : 5. Twice as wide as the wall. Tamped in layers.

Depth

The underside of the footing sits below the local frost line, so ice in the ground cannot lift it. The depth depends mainly on how cold the winters are, and on the soil, its moisture and the snow cover. ISO 13793 covers how to design foundations against frost heave. Heated buildings warm the ground below them.

Thickness and width

FAO: as deep as the wall is thick and twice as wide. At least 150 mm thick (East Suffolk Council). For a building, take the size from its design. The example on this page is for a 200 mm wall: 400 × 200 mm.

Mix

FAO suggests 1 : 3 : 5 by volume for footings and puts strip footings in its lowest grades, C7 and C10.

Pad footings

A pad under a post or pillar uses the same mix and the same depth rule. For fence and pergola posts set straight in concrete, see 4.3.

Sources

  1. FAO Farm Structures, ch. 5: Footings: footing as deep as the wall is thick and twice as wide, 1 : 3 : 5 mix, 30 cm deep for light buildings in warm climates, up to 1 m for heavy ones
  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. 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
  4. 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
  5. ISO 13793 Thermal design of foundations to avoid frost heave (ISO)
  6. USDA NRCS: Cold weather concreting: concrete is not placed on frozen ground; thaw it under insulation first
  7. East Suffolk Council building control: Foundations: footing at least 150 mm thick
  8. Penn State University: aggregate and strength: mixes with larger stone need less mixing water
  9. American Concrete Institute FAQ: consolidation: rodding for flowable mixes, tamping for stiff ones, vibration internal or external
  10. Mile High Flood District: curing specification (based on ACI 308.1): cure at least 7 days, forms left in place count as curing
  11. EN 1997-1 Eurocode 7: geotechnical design – Part 1: general rules (EVS)
  12. EN 12620 Aggregates for concrete (DIN)
  13. 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)
  14. EN 197-1 Cement: composition, specifications and conformity criteria for common cements (EVS)

How the amounts are worked out: FAO's table of sand, stone and cement per m³ of concrete, times the volume. Loose stone is counted at 1.55 t/m³ and moist sand at 1.4 t/m³, inside FAO's ranges of 1.5–1.65 and 1.3–1.5 t/m³. Amounts include no waste; FAO allows 5–10 % for waste and spill.

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