Concrete volume — calculate it, don't guess

A rectangular concrete pour — a strip footing, a pad footing or a slab — is geometrically just a box. The volume is length times width times height, calculated in metres, so the result comes out directly in cubic metres.

Example: a strip foundation of 6.0 m × 0.40 m × 0.60 m gives V = 6.0 · 0.40 · 0.60 = 1.44 m³. With 5 % waste you order 1.44 · 1.05 ≈ 1.51 m³.

The excavation is rarely exactly to dimension, a little is lost during pouring and spreading, and the concrete shrinks slightly. A small allowance ensures you are not left short of the last few litres in the middle of a pour, where a cold joint is hard to avoid.

• 01Several footing strips or pads: calculate each volume separately and add them together.
• 02The allowance is an estimate based on the substrate and workmanship — not a fixed percentage.
• 03Remember to subtract larger openings if they take up a noticeable amount of space.

The volume is the figure you order by. But on site, the carpenter's real task is formwork: the mould the concrete is cast into. A form that gives way or shifts during casting ruins both dimensions and budget, because wet concrete is heavy and presses outward hard.

• 01Mark the outer edges of the foundation according to the drawing's dimensions and module lines, and stretch a chalk line.
• 02Raise form sides of concrete formwork or boards, and brace them on the outside with diagonal braces and pegs, so they hold the dimensions under concrete pressure.
• 03Check the form's internal dimensions, that it is plumb and level, and that diagonals are equal (square).
• 04Place the reinforcement on spacer blocks so the prescribed cover is maintained all the way around.
• 05Cast in one, compress (vibrate/tamp) the concrete and shape the top edge to the marked level.
• 06De-form only when the concrete has reached sufficient strength — early de-forming on a cold day can damage the edges.

A foundation must be led down below the frost line, so frost in the soil cannot lift it (frost heave). Under a heated building the frost-free depth is normally calculated at approx. 0.9 m below ground level; under a freestanding/unheated foundation approx. 1.2 m. BR18 § 340 ff. (structures) places the overall requirement that load-bearing structures must not suffer harmful deformations — e.g. due to frost heave — while the frost-free foundation depth itself is dimensioned geotechnically according to DS/EN 1997 (Eurocode 7). The precise depth depends on soil and project.

To avoid deep excavation, a perimeter foundation can instead be conducted approx. 0.3–0.4 m down and supplemented with horizontal, rigid perimeter insulation (XPS), which shifts the frost line outward. The solution must be designed — it is not a free choice on site.