Reduced level (RL)
A reduced level (RL) is the height of a point relative to a benchmark (typically AHD or a TBM). FFL, ridge, soffit, and slab tops all read as RLs on drawings.
Ask Chalkline about this →A reduced level (RL) is the height of a surveyed point expressed as metres above (or below) an agreed benchmark. The benchmark is typically the Australian Height Datum or a temporary benchmark (TBM) set up for the site. RL 32.450 means the point is 32.450 m above the benchmark.
Where RLs appear on a job. Every height-sensitive element on a residential build is called up as an RL on the drawings:
- FFL (finished floor level), the top of the slab or top of finished flooring.
- Slab top and slab soffit, the upper and lower faces of a suspended slab.
- Ceiling level and soffit RL for eaves.
- Ridge RL for setting roof pitch and overall height envelope.
- Top of footing, top of bearer, top of plate, used for framing height checks.
- Existing surface RLs on the contour survey, used to balance cut and fill.
Working to RLs on site. The surveyor sets a TBM near the building footprint (usually a steel pin in concrete or a stable stake) and notes its RL. From that one mark, the builder transfers heights using a dumpy level, laser level, or rotating laser to set formwork, slab edges, FFL stamps, and bracket heights. Every task that needs a vertical dimension in the right place reads from the same TBM.
Two ways to get it wrong. First, working from the wrong benchmark: a TBM set on a temporary stake that gets bumped or removed mid-job invalidates every height set off it. Always re-check the TBM RL at the start of each new stage. Second, mixing up RL with a relative height: “ridge RL 36.500” is an absolute height, not 36.500 m above ground.
Also known as: RL, spot height, level.
Category: Surveying / set-out / heights.
Related
- Site set-out and survey
- Conventional reinforced slab
- Earthworks: cut and fill
- Slab-on-ground construction
- AHD (Australian Height Datum)
See also
Last updated: 2026-05-14. Verified: 2026-05-14.