Marking out a concrete slab is the first physical step on-site — and if you get it wrong, everything from levels to reinforcement can go sideways. Whether it’s a garage base, an extension, or a patio, proper set-out means you’re working to the right size, square, and height from day one. This guide walks through the exact process used by professionals to mark out a slab area using pegs, string lines, and laser levels. No guesswork. No costly errors. Just clean lines and perfect prep.
Start by confirming the exact size of the slab you're building — including thickness, width, length, and any edge details like thickenings or curves. Use your plans or building regs drawings to identify the finished footprint. Then decide exactly where it will sit on the site. If the slab abuts another building or structure, ensure you account for wall gaps, damp-proof zones, or drainage runs. Measure from fixed features like fence lines or walls to get an accurate reference point. Mark this “starting corner” with a peg — every other dimension comes from this point.
Use wooden pegs or steel pins to mark the slab’s corners. Hammer them in securely so they won’t move during setup. Measure diagonals to ensure the layout is square — a common builder’s trick is the 3-4-5 triangle method (a form of Pythagorean triangle) to create perfect 90° corners. Pegs should sit outside your actual slab area, leaving room for formwork. You’re creating a grid to build around, not into. Don’t be tempted to rely on just visual guesswork — squareness is crucial, especially for slabs adjoining walls or where modular materials will sit on top (e.g. sheds, garages, patio paving).
Tie builder’s line between each peg to outline the slab’s perimeter. Keep the string taut and level. It acts as both a visual guide and a level reference for excavating, compacting, and eventually pouring concrete. If your slab has internal steps, thickenings, or rebar beams, use additional lines to mark those too. For complex layouts, different colour string or marking spray helps track zones. Check again for square corners and matching diagonals — if the string lines are slightly off, adjust the pegs, not the layout. A single cm out now can become a major headache once the concrete’s in place.
Decide where your Finished Floor Level (FFL) will sit — usually this aligns with adjoining structures, thresholds, or paving. Use a laser level or spirit level on a staff to mark the FFL on corner pegs. Mark this level visibly with spray paint or a nail notch. All other site layers (hardcore, insulation, mesh, and concrete) are worked down from this level. Use a tape measure to calculate depths for each layer from the FFL — e.g. 100mm concrete, 50mm insulation, 150mm hardcore. This ensures your final slab height is perfect, and there are no nasty surprises with step levels or damp bridging.
Before digging starts, re-check all dimensions, diagonals, and levels. Make sure your slab is square, the layout matches plans, and there’s nothing in the way — including manholes, tree roots, or pipes. If you’re building close to boundaries, double-check your measurements meet planning or build-over agreements. Mark any service lines or danger zones with high-vis spray. Time spent now will save you hours of rework later. Once the slab’s marked out, signed off, and square — you’re ready to break ground.
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