Excavating for a concrete slab isn’t just about digging a hole — it’s about setting up your project for success. The right depth, fall, and soil prep make sure your slab doesn’t crack, sink, or fail over time. Whether you’re laying a base for a garage, extension, shed, or driveway, getting the groundwork right is critical. This guide walks through the full excavation process — from setting out levels to digging safely and accurately — the same way it’s done on professional sites.
Everything is based off your Finished Floor Level (FFL). You’ll need to work backwards and calculate the total excavation depth. For example, a standard domestic slab might need: 100mm concrete, 50mm insulation, and 150mm compacted hardcore = 300mm total dig. Deeper slabs for garages or driveways may go down 350–400mm or more. Account for any slope you’re building in — slab designs often include a fall for drainage. Use string lines and a laser level to mark the FFL on pegs or profiles around the site. Measure your dig depths down from that, not from uneven ground level.
Before digging, remove all topsoil, turf, organic matter, and debris from the site. This surface layer is unstable and holds moisture — not suitable to build on. Strip the area slightly beyond the slab’s footprint to allow space for edge formwork, insulation, and any external drainage channels. You should be exposing firm, dry subsoil. If the area is soft or waterlogged, consider using a geotextile membrane before laying hardcore. This step ensures you’re not compacting onto organic mush or mud — the kind of stuff that causes slab movement later down the line.
Use a digger or spades to excavate to the calculated depth. Keep checking your depth from the string line or laser reference. If the slab includes deeper edge beams or thickened pads (common for structural loads), dig those zones accordingly. Keep sides neat and square — over-excavation means more hardcore later. For sloped sites, you may need stepped levels or bulk excavation. Remove spoil as you go — don’t pile it near the dig where it could collapse back in. A solid excavation = consistent slab performance and no uneven loading on your finish.
After digging, compact the base of the excavation. This stops later movement and provides a stable platform for your sub-base. Use a plate compactor or roller depending on access. A few passes in multiple directions are key — you want minimal deflection underfoot. If the soil is soft or saturated, remove another layer and replace with crushed stone or Type 1 hardcore. You can also lay a woven geotextile membrane to stabilise the base. Subgrade compaction is often skipped by DIYers — but pros know it’s the first line of defence against sinking slabs and cracking.
Before moving on to laying your sub-base or hardcore, re-check everything. Use a laser level and staff or string/tape from your FFL pegs. Make sure your dig depth is uniform, your edges are square, and you’ve not over-dug anywhere. If you're installing insulation or DPM later, now’s the time to check those layer depths as well. A little time spent here saves major issues later. If something’s wrong, fix it now — the slab will only be as good as what’s underneath it.
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