Layout and Preparation
Layout and preparation tests whether you can turn a drawing into accurate lines, grades and a stable subgrade before any concrete arrives: benchmarks, squaring, slope, base compaction, vapour retarder and form setting.
Core rules
- Confirm the benchmark and the latest revision before you set anything; a wrong elevation is expensive to undo.
- Square corners with the 3-4-5 method or by comparing diagonals. For a rectangle of sides a and b, the diagonal is the square root of a squared plus b squared.
- Set grades with a laser level or transit. Screed pins and rails are commonly set on about 3 m centres so the straightedge does not ride the pins.
- Drainage fall is expressed as a percentage: 1 percent is 10 mm per metre. Slabs that fall to a drain or away from a building typically use 1 to 2 percent, per the drawings.
- Strip topsoil and organics, proof-roll the subgrade, and compact to the specified density in lifts. A soft spot found after the pour is a crack later.
- Vapour retarder: thickness, laps and sealing per specification; laps typically 150 mm and taped; protect it from punctures and from UV before the pour.
- Set forms to line and grade, stake and brace them, and mark joint and embed locations.
Worked example
A 12 m by 9 m pad falls 2 percent away from a building along the 9 m dimension. The high edge is set at 100.000 m. Total fall is 9 m times 0.02, which is 0.18 m, so the low edge sits at 99.820 m. Pins every 3 m read 99.940, 99.880 and 99.820. For square: the diagonal of a 12 m by 9 m rectangle is the square root of 144 plus 81, which is exactly 15.000 m, a 3-4-5 triangle scaled by three. If measured diagonals differ, the layout is out of square even when the sides measure correctly.
Traps
Reversing the slope so water ponds against the building. Confusing 2 percent with 2 mm per metre, a factor of ten. Puncturing or failing to seal the vapour retarder. Setting forms off one corner without checking diagonals, producing a rhombus instead of a rectangle.