Setting out is the process of transferring the dimensions on the structural drawings to the site, and for slabs it happens at two critical moments: before the formwork starts and after the reinforcement is tied. The purpose is to make sure every slab aligns vertically and horizontally with the grid lines in the issued drawings. The same logic runs through all setting out and setting procedures in construction projects: each element is located, leveled, and checked against a fixed reference before concrete is placed.
Setting Out a Slab: Two Stages, One Purpose
Slab setting out is done in two passes. The first pass fixes the formwork position, and the second verifies the work after the steel is fixed. The second pass covers beam setting out, slab alignment, slab leveling, and column setting out, and both the consultant and the contractor attend the checks. Errors caught after concreting are the expensive kind, because concrete cannot be moved back into line.
Stage One: Setting Out Before Formwork
Before any formwork goes up, the slab position is marked from the building plan. The setting out of the building plan on the ground fixes the outline, the grid lines, and the beam positions that the formwork crew works from. Beam bottoms are laid first, and the slab formwork is then fixed to them, so an error in the first pass carries directly into the second.
Stage Two: Post-Reinforcement Checks
Once the reinforcement in the slab and beams is tied, the setting out team returns to check four things:
- Beam depth, width, and location against the grid lines
- Slab horizontal alignment and vertical alignment
- Slab level after the steel is in place
- Column starter positions and verticality
Each check is made against the grid lines or the level datum, never against an adjacent pour, because errors accumulate when one floor is used as the reference for the next. The results are recorded on a setting out sheet that becomes part of the site quality records.
Grid Line Offsets and Reference Marks
Grid lines usually run through the middle of the beams, so the setting out team marks offsets on the slab formwork for convenience. All alignment checks are made against these offset lines. The offset is typically 100 to 300 mm from the face of the beam, and it stays constant across the floor so the checks stay repeatable from one bay to the next.
Below-Grade Setting Out: Excavation and Basement Work
For slabs built over a basement or a below-grade floor, setting out starts at the excavation stage rather than at the formwork. The methods of basement excavation and basement construction set the outer limits that the slab grid must fit inside, and an error at this stage carries through the whole building height.
Benchmarks, Datums, and Reference Lines
A benchmark or temporary datum is established before excavation starts and is kept outside the working area so plant traffic does not disturb it. All slab levels, beam bottoms, and column positions are set from this single reference. Site engineers check the datum against a nearby permanent bench mark at the start of each week, because a disturbed datum produces a building that steps at every floor.
Excavation Tolerances and Basement Construction
Excavation limits are set out with offset pegs so the machine operator does not disturb the reference lines. Tolerances at excavation level are looser than at slab level, commonly plus or minus 50 mm on plan, but the setting out grid has to survive the entire basement sequence: excavation, dewatering, blinding, and wall construction. Re-establishing a lost grid line costs days, so the pegs are duplicated at a safe distance from the edge of the cut.
Foundation Setting Out: Procedure, Safety, and Dewatering
Where the slab bears on spread footings or a raft, foundation setting out sits between excavation and formwork. The excavation for foundation procedure, setting out, safety measures, and dewatering should be planned as one package, because all four affect the position and the condition of the slab support.
Setting Out Foundation Trenches and Footings
A typical foundation setting out sequence is:
- Set out the trench lines from the grid and mark the corners with pegs
- Check the diagonal dimensions, which must be equal for a rectangle
- Transfer the levels to the trench bottom for blinding and footing thickness
- Set the forms and re-check the position before concrete is placed
The diagonal check catches most rectangular errors, because a parallelogram with equal sides but unequal diagonals is out of square. On a raft, the same checks are made against the full grid rather than individual footings, and the grid corners are re-verified before the blinding pour.
Dewatering and Safety Measures
When the water table sits above the formation level, dewatering keeps the excavation dry so setting out marks and level instruments stay accurate. Safety measures protect the setting out points themselves: guard rails around open trenches, protected pegs, and a clear exclusion zone for plant. A peg that survives the excavation is worth more than one re-established after a machine has run over it.
| Check | Reference | Typical tolerance |
|---|---|---|
| Grid line spacing | Grid line to grid line | Plus or minus 10 mm |
| Beam width and location | Drawing dimensions | Plus or minus 10 mm |
| Slab level | Level datum | Plus or minus 10 mm |
| Column verticality | Plumb from grid | 1 in 500, max 10 mm |
| Wall face position | Grid line offset | Plus or minus 10 mm |
Beam and Slab Alignment: Grid Lines and Service Sleeves
After the reinforcement is tied, the beam depth and width are checked against the drawings and the beam location is compared with the grid lines. The slab gets the same treatment: horizontal alignment against the offset marks and vertical alignment against the datum. Setting out does not stop at the concrete edges, because sleeves and openings for pipes have to be positioned before the pour. The methods of setting water distribution system layout show how supply networks are laid out so their penetrations land where the drawings show them.
Beam Depth, Width, and Location Checks
Beam depth is measured from the underside of the formwork to the top of the slab, and beam width is checked against the drawing at several points along the span. The location check compares the beam faces with the grid lines using the offsets marked on the slab. A beam that finishes 10 mm wide of position throws the slab edge and the wall above it off at every floor.
Grid Line Offsets and Service Sleeves
Marking Sleeves and Openings
Sleeves for water supply, drainage, and conduits are marked on the slab formwork at the same time as the grid offsets. Each sleeve position is checked twice: once when it is marked and again after the steel is tied, because bars get moved to make room for the sleeve. A misplaced sleeve costs more to fix after the pour than any other setting out error, since it means breaking out finished concrete.
Slab Leveling: Falls, Grades, and Drainage Setting Out
Slab level is checked after the reinforcement is tied, and it can be taken from the top of the slab or from the bottom formwork. Checking from the bottom is more common, because the formwork position controls the final level. The same level control extends to the pipes that drain the building, where the methods of setting line and grade in the construction of sewer and sanitary systems keep flow lines at the correct gradient.
Checking Slab Level from Top or Bottom
The level check compares the slab surface with the datum at regular points across the floor, typically every 3 to 5 meters and always at the corners. The acceptable deviation for a finished slab level is usually plus or minus 10 mm. When the check is taken from the top, the reading has to allow for the slab thickness and the cover to the top steel, which is why the bottom formwork method is preferred on large pours.
Setting Falls and Grades for Drains
Falls for floor drains and sanitary pipes are set with the same instruments used for slab levels. A floor drain needs a fall of about 1 in 60 in the screed, and a house drain runs at 1 in 60 to 1 in 100 depending on the pipe diameter. The grade is set once and checked at the outlet and at the invert, because a drain that runs flat collects solids and blocks.
Line and Grade Instruments
The standard tool kit for slab setting out is an automatic level with a staff for levels, a total station for grid coordinates, and a steel tape or laser distance meter for offsets. Laser levels speed up the slab level check across large floors, and a rotating laser on a tripod can cover a whole pour in one setup. Instruments are checked against the datum at the start of every shift.
Column Setting Out and Vertical Alignment
Column reinforcement that continues beyond the floor level is checked to make sure it is vertical and located where the drawings show it. Column positions are checked with the same offsets marked on the slab, and the verticality is checked with a plumb bob or a level.
Column Starters and Verticality Checks
Starter bars are set into the slab before the pour, so the check happens before concreting rather than after. The bar cage is measured at the top and the bottom, and the verticality is compared over the full height of the starter. A starter that leans 20 mm at the base grows into a column that leans at every floor above it.
Correcting Misaligned Reinforcement
When a starter bar is out of position, the reinforcement is adjusted before the concrete hardens. Steel chains and lever bars are the standard tools, and the bars are bent back to the correct position in small increments so the steel does not fracture at the bend. If the bar is more than about 50 mm out, the engineer is consulted before any bending, because the structural cover and the splice length may both be affected.
Setting out errors that survive to the pour are paid for by the contractor in extra plaster, extra formwork, and rework that cannot be claimed from the owner, since payment for plaster is based on area and not on the material needed to fix misalignment. The discipline pays off most on long-span floors, where the design principles, construction methods, and long-term performance of post-tensioned concrete slabs depend on tendon positions and edge details that are only as accurate as the setting out that placed them.