How to Build a Concrete Footing

What you need

A spade and shovel, a tape measure, string lines and pegs, a spirit level, a wheelbarrow, bagged concrete or a ready-mix delivery, gloves, eye protection and boots. For a load-bearing footing you will also need the structural engineer's drawings — the footing size and depth come from them.

Step 1 — Set out and dig

Mark the footing positions with pegs and string lines, and check the diagonals are equal so the layout is square. Dig to the depth on the drawings — never shallower. The bottom of the trench must be level and firm: loose soil at the bottom is dug out, not compacted back in.

Step 2 — Work out the concrete

Multiply the footing's length, width and depth for the volume of one footing, then multiply by the number of footings and add 10% wastage. The concrete footing calculator does this and converts the volume into bags, using the standard yield of an 80 lb bag (0.60 ft³). Order the concrete only after the trench is dug and measured — a trench always holds more than the drawing says.

Step 3 — Pour and level

Pour the concrete in one go, working it into the corners with a shovel so there are no air pockets. Level the top with a straight edge and check it with a spirit level. Do not add water to make it easier to work — water weakens the mix.

Step 4 — Cure before you build on it

Keep the concrete damp for at least a few days — cover it with plastic sheeting in hot or windy weather so it does not dry out and crack. Do not build on a footing until it has cured to the strength the design requires; for most small footings that means waiting the time your engineer or the concrete supplier states.

Worked example

Four footings, each 24 × 24 in and 12 in deep. One footing is 4 ft³, so four are 16 ft³ (0.45 m³). Add 10% wastage: 17.6 ft³ (0.5 m³). At 0.60 ft³ per 80 lb bag, that is 30 bags. Those are exactly the numbers the concrete footing calculator gives you.

Common mistakes

• Guessing the size. Footing size and depth come from the ground conditions and the load. A footing that is too small settles; one that is too shallow heaves with frost.
• Adding water on site. It makes the mix easier to pour and much weaker when it sets.
• Building on green concrete. A footing that has not cured will crack under load. Wait the full curing time.

Safety

Footings are structural work: the size, depth and reinforcement must follow a structural engineer's design and your local building code. Never guess them, and never build a load-bearing footing without approval where your area requires it. Wet concrete burns skin and eyes — wear gloves, eye protection and boots, and wash splashes off immediately. Deep trenches can collapse: never work in an unsupported trench deeper than you can climb out of, and get a professional in.

Try these calculators

• Concrete Footing Calculator — volume and bags for any number of footings.
• Concrete Calculator — volume and bags for a slab or pour.
• Concrete Volume Calculator — volume for rectangular and round shapes.