A concrete slab is a flat, reinforced concrete plate that serves as the floor or foundation of a building. For a garage, shed or workshop, the concrete slab often functions as both foundation and floor in one. The weight is distributed evenly across the entire ground surface.
When do you use a concrete slab?
| Application | Concrete slab suitable? |
|---|---|
| Garage floor | Yes — must support vehicle weight |
| Shed storage | Yes — stable and level |
| Workshop | Yes — flat, strong floor needed |
| Garden building | Optional — piers are often sufficient |
| Canopy | Optional — depends on use |
Build-up of a concrete slab
From bottom to top:
1. Undisturbed soil — The original, undisturbed ground layer
2. Sand bed — 10-15 cm of compacted sand for levelling and drainage
3. Membrane — Plastic sheet as a moisture barrier (prevents ground moisture from wicking into the concrete)
4. Reinforcement — Steel mesh or bars for tensile strength
5. Concrete — The actual slab, at least 10 cm thick
6. Finish — Power-floated (smooth), broom-textured, or coated
Dimensions and specifications
| Parameter | Guideline |
|---|---|
| Minimum thickness | 10 cm (garden building), 12-15 cm (garage) |
| Concrete grade | Minimum C20/25 |
| Reinforcement | Steel mesh dia. 8-150 (8 mm bars at 150 mm centres) |
| Cover | Minimum 3 cm (distance from reinforcement to underside of concrete) |
| Slope | 1-2% towards door or drain (for garage) |
Pouring a concrete slab yourself
Preparation
1. Set out — Mark the area with stakes and string line
2. Excavate — Dig out the formation level to the correct depth
3. Lay the sand bed — 10-15 cm of sand, compact layer by layer with a plate compactor
4. Lay the membrane — Overlap at least 30 cm, upstand along the formwork
5. Build formwork — Scaffold boards at the correct height, level, with slope
Reinforcement
6. Place reinforcement mesh — On spacers (3 cm cover), mesh overlap at least 20 cm
7. Extra reinforcement at edges — Along edges and at the door opening, add extra bars
Pouring
8. Order concrete — Calculate the volume (length x width x thickness) and order 5-10% extra
9. Pour in one go — Distribute the concrete evenly, vibrate with a poker vibrator
10. Finish — Screed with a straight edge, power-float for a smooth finish
11. Aftercare — Cover with sheeting or keep damp, allow at least 7 days to cure
Expansion joints
For slabs larger than 4×4 metres: install expansion joints to prevent uncontrolled cracking. This can be done by:
- Casting in an expansion profile beforehand
- Saw-cutting afterwards (within 24-48 hours of pouring)
Cost indication
| Item | Cost per m2 (indication) |
|---|---|
| Groundwork + sand bed | EUR 10-20 |
| Membrane + reinforcement | EUR 8-15 |
| Concrete (12 cm thick) | EUR 15-25 |
| Total (DIY) | EUR 35-60 per m2 |
| Total (contracted out) | EUR 60-100 per m2 |
Related terms
- Reinforcement
- Shallow foundation
- Strip foundation
- Expansion joint
- Slope (fall)