Interactive lesson

Foundations, step by step

Interactive lesson: trial pit, a true 3-4-5 right angle, concrete that holds, pads with post bases, strip foundation, slab on grade and water kept away from the foundation.

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This lesson is general guidance, not professional training. Local regulations and expert advice always take precedence. Read our disclaimer.

The whole lesson as text

1. Ground and depth

The foundation is the only part of a building you can't repair without pulling it down.

Trial pit — Dig a pit where the foundation will go and look at the layers. You are looking for sound ground, with no fill, rubble or water.
Topsoil always comes off — Topsoil has no bearing capacity: strip the whole layer. Keep the topsoil separate and use it in the garden, and don't mix subsoil with it.
Below frost depth — A foundation goes below frost depth: in lowland UK and Ireland at least 450 mm (18 in), in shrinkable clay often 0.9–1 m; in the US and Canada as the local code says, up to 1.2–1.5 m in the north.
Flat bottom — The bottom of the dig must be flat, level and undisturbed. If you have dug too deep, don't put soil back: fill with concrete or compacted gravel.

Why: In the country the same three mistakes come up again and again: the foundation is too shallow so frost lifts it, it sits on fill so it settles, or there is no drainage. A trial pit shows you the ground before you spend money on concrete.

Ask experienced local builders or building control how deep foundations go where you are.
A light lean-to on pads can go a little shallower on free-draining gravel, but in the UK still at least 450 mm, and in cold regions no less than the code frost depth.

2. Setting out

An error of 2 cm at one corner becomes 10 cm at the other end.

First corner — Fix the direction and the first corner, then drive pegs for the corner and the first side.
The 3-4-5 rule — From the corner peg measure 3 m along one side and 4 m along the other. When the distance between those two points is exactly 5 m, the angle is square.
Equal diagonals — Finally measure both diagonals: they must be equal. For a 4 × 6 m building the diagonal is 7.21 m.
Profile boards and height — Set up profile boards 1–1.5 m outside the building: two pegs and a horizontal board at each corner. Bring all boards to the same height with a water level and stretch string lines between nails.

Why: A building that isn't rectangular can't get a proper roof or floor. Corner pegs disappear as soon as digging starts, so profile boards hold the line and the height.

For bigger buildings use 6-8-10 instead of 3-4-5. In feet the rule works the same.
Keep the top of the foundation at least 150–300 mm (6–12 in) above the surrounding ground.