Bad slab levels lead to more problems than anything else in concrete work — uneven floors, drainage issues, bridging damp-proof layers, or expensive grinding later. Before you pour, you need to set accurate height references for your slab: where it starts, where it ends, and what thickness you’re maintaining. Whether you’re using a laser level or string line, this guide walks through how to set the right levels for a concrete pour — accurately, efficiently, and without guesswork.
The FFL is the most important level reference on your project — it’s the height the top of your concrete slab will sit at. If your slab adjoins a building, doorway, or existing floor, this is usually fixed already. Mark the FFL on a nearby wall, post, or datum peg. From this single point, you’ll measure all the layers below: insulation, DPM, hardcore, and concrete depth. Everything is worked backwards from FFL — not up from ground level.
Using a laser level is the cleanest method. Set it up on a tripod and rotate it to cast a level plane across the working area. Mark the laser height on walls or profile boards. If you’re using a string line, stretch it tight across the slab area and check with a spirit level — not ideal for large slabs, but works on small pours. Once your reference is set, you can measure **down** from it to mark your actual pour height (e.g. 150mm from FFL if you have hardcore + insulation layers).
Use a measuring stick or staff to mark the pour height on all your perimeter pegs or formwork boards. Spray, notch, or pencil-mark it clearly. This top line is the guide for your screed rails or slab depth — it has to be consistent. If using insulation or mesh below the concrete, subtract that thickness and mark accordingly. The goal is that **all top edges** match across the entire area so your slab is level once poured and floated.
Now adjust your formwork boards or screed rails to match the marks. Pack low areas with wedges, or plane/sand down high spots. If using rails for screeding, make sure they’re aligned and match your intended slab depth. Use a straightedge or laser to cross-check each section. This is your working surface during the pour — if your formwork’s wrong, your slab will be too. For drainage slabs (e.g. patios, garages), now’s the time to introduce falls (usually 1:60 to 1:80).
For large slabs, use temporary screed pegs or small piles of dry mix to establish internal levels during the pour. These are set to slab height using your laser and act as internal points to drag your screed across. Space them out every 1.5–2m. Once your screed reaches that level across each section, you know your slab depth is consistent. These are removed or filled as you go, leaving a flat finish.
Use your laser or string + tape to measure from the top level reference down to the base of the slab — it should be the same depth (e.g. 100mm) across the entire site. Check multiple locations: edges, corners, centre. If there’s more than ±10mm variation, adjust now. If the base dips too far, top it with sand or concrete. If it’s too high, remove material and recompact. A consistent slab thickness = even strength and a better finish.
Just before the pour, do a final sweep. Are your top-of-slab marks still visible? Are any rails, boards, or markers out of place? If using a laser, keep it running during the pour so you can check as you go — especially on hot days or big slabs where timing matters. You only get one chance to pour level. Once set, it’s either flat — or it’s not.
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