Setting out: right angle, diagonals and levelling

Decide on one fixed starting point (zero point) for the whole setting out — e.g. a corner, a wall line or an existing straight edge. All measurements are taken from here. By measuring cumulatively from the same point instead of laying folding rules end to end, you avoid small measuring errors building up over the length.

Always measure cumulatively (c/c, 2·c/c, 3·c/c …) from the zero point — never section by section.

Stretch a string line or snap a chalk line between two points so you have an unambiguous, straight reference line to work from. The line must be taut — a slack line sags and gives an inaccurate measurement. On long runs, check the line with a laser so that it has not been pulled out of true.

A right angle is set out with the classic 3-4-5 triangle: mark 3 units along one side and 4 units along the other — when the diagonal between the end points is exactly 5 units, the angle is square. This is Pythagoras' theorem, 3² + 4² = 5² (9 + 16 = 25). Use larger multiples (e.g. 60-80-100 cm or 3-4-5 m) on large setting-out jobs for greater accuracy.

On a rectangle — a foundation, a floor frame, a wall panel — you measure the two diagonals from corner to corner. If they are exactly the same length, the shape is square. If they differ, push the shape in the direction of the long diagonal until the two measurements are equal. The diagonal check is quick and catches a skew that the eye cannot see.

Equal diagonals = square. Different diagonals always reveal an out-of-square frame.

Horizontal and vertical surfaces are checked with a spirit level; over longer distances, a rotating laser or water level is more reliable than a long spirit level. Always check the spirit level by turning it 180° — if it reads differently, the vial is out of calibration. For vertical setting out, use a plumb line or the laser's plumb point.

Set out one continuous height reference (datum line) on all walls — typically a horizontal line 1.00 m above the finished floor — and measure all other heights from it. One common reference means that floors, frames and ceilings end up in the same plane throughout the room, wherever you stand and measure.

Set out all centre lines for rafters, joists, studs or battens cumulatively from the zero point. The number of members follows the fence-post principle: if the length is divided into n spaces, there are n+1 members when there is a member at both ends. Spaces = ⌊ length / cc ⌋, and number = spaces + 1.

Mark each centre line clearly and on the same side of the line all the way, so all components are placed consistently. Feel free to write the component number on it if that makes installation easier. Clear, consistent marking means several people can work from the same setting-out without guessing.

Check the last gap before you fix. If the spacing does not divide evenly into the run, spread the remainder evenly over several bays rather than leaving the last bay as a half — a single uneven spacing is noticeable and can cause problems with board formats. Use the «Number of rafters/battens» calculator to check the number and the actual even spacing.