Setting Out, Measuring and Marking for Plumbing Installation
Domain focus
This domain tests setting out pipe routes, levels, sleeves and supports from fixed references. It covers measuring, marking, squaring, gradient calculation and transfer of datum before fabrication.
Core rules
- Establish a datum and centreline from a permanent reference, not from unfinished floor or loose materials.
- Use a spirit level, water level or laser; check long runs for line and level at intervals.
- Square a room with the 3-4-5 method or equal diagonals before marking partitions and risers.
- Mark pipe centres, bracket spacing, sleeve positions and invert levels; add allowance for insulation and fittings.
- Gradient fall = length ÷ gradient denominator. Keep fall continuous and avoid back-fall.
- Record checks; protect marks from trade damage.
Worked example
An 8 m drain at 1:60 needs fall = 8000 ÷ 60 = 133 mm. If the upstream invert is 100.200 m, the downstream invert is 100.200 − 0.133 = 100.067 m. Set the laser or string line to that fall, then check the pipe invert, not the pipe crown.
Traps
Measuring from blockwork before plaster; ignoring wall thickness; using an approximate fall instead of calculating; marking bracket centres without checking fitting sockets; failing to check square; confusing invert with obvert.