By Topic Advanced

Concrete Slab for a House: Complete Pouring Guide

12 min read

The Slab: Your House’s Ground Floor

The concrete slab forms the ground floor of your house. It is the surface on which your partitions, floor finishes and furniture will rest. Two main techniques exist: the ground-bearing slab and the suspended floor over a crawl space. This guide walks you through both.

GROUND-BEARING SLAB — CROSS SECTION Compacted natural ground 20-30 cm 10-14 cm 12-15 cm 3-5 cm Sub-base (gravel) DPM membrane Insulation (EPS) Welded mesh ST25 Slab C25/30 Screed 1 2 3 4 5 6 Typical total thickness: 50 to 65 cm (from natural ground to floor finish) House interior
flowchart TD A{Which slab type?} -->|Assess the site| B{Flat and dry ground?} B -->|Yes| C{Stable soil
soil survey?} C -->|Yes| D[GROUND-BEARING SLAB] C -->|No| E[SUSPENDED FLOOR
CRAWL SPACE] B -->|No| E D -->|Tight budget?| F{Compare costs} F -->|Ground slab cheaper| D F -->|Crawl space safer| E style A fill:#0F4C81,stroke:#0F4C81,color:#fff style B fill:#FDFCF9,stroke:#C67A3C,color:#0F4C81 style C fill:#FDFCF9,stroke:#C67A3C,color:#0F4C81 style F fill:#FDFCF9,stroke:#C67A3C,color:#0F4C81 style D fill:#56C6A9,stroke:#56C6A9,color:#fff style E fill:#F58220,stroke:#F58220,color:#fff

Ground-Bearing Slab vs Suspended Floor

Criterion Ground-bearing slab Suspended floor (crawl space)
Principle Slab poured directly on the ground Slab supported on foundation walls
Cost Less expensive More expensive (foundation walls + floor)
Insulation Insulation under slab Insulation between beams or under slab
Moisture Risk if poor drainage Natural ventilation via the void
Service access Difficult (services embedded) Easy (services run through the void)
Site suitability Flat ground only Sloping or damp ground

💡 Tip — The choice between a ground-bearing slab and a crawl space depends primarily on the soil survey results and site constraints. In flood-risk zones or on clay soil, a crawl space is often preferable.

Building a Ground-Bearing Slab

Step 1: The Sub-Base (Hérisson)

The sub-base is a layer of free-draining material (gravel, crushed stone) spread over the stripped ground. It serves three purposes:

  • Preventing capillary moisture rise
  • Levelling the surface
  • Draining any water

Method:

  1. Strip the topsoil to a depth of 20 to 30 cm
  2. Compact the formation level with a plate compactor
  3. Spread 20 to 30 cm of Type 1 sub-base or crushed aggregate
  4. Compact in layers of 15 cm maximum
  5. Finish with a 5 cm layer of fine gravel (0/20) for levelling

Step 2: The DPM (Damp-Proof Membrane)

Lay a polyethylene membrane (DPM) of at least 200 microns over the entire sub-base. This membrane prevents moisture rising into the slab.

  • Allow 30 cm minimum overlaps between sheets
  • Turn the membrane up against perimeter walls by at least 15 cm
  • Avoid puncturing the membrane (be careful with reinforcement and service runs)

Step 3: Thermal Insulation

Under-slab insulation is mandatory to comply with current energy regulations. Common insulation materials include:

Insulation Typical thickness Lambda (λ) Compressive strength
Expanded polystyrene (EPS) 10 to 14 cm 0.032 to 0.038 Good
Extruded polystyrene (XPS) 8 to 12 cm 0.029 to 0.036 Very good
Polyurethane (PU) 8 to 10 cm 0.022 to 0.028 Very good

⚠️ Warning — The insulation must be laid on a flat and clean surface. Any unevenness in the sub-base will transfer to the slab and may create weak points or cracks.

Step 4: Welded Mesh Reinforcement

Welded mesh (or anti-crack mesh) limits shrinkage cracking in the concrete. It is not structural reinforcement (except in the case of a raft foundation).

Typical specifications:

  • ST25 mesh (150 × 150 mm grid, 7 mm wires) for a standard slab
  • Position the mesh in the upper third of the slab (not at the bottom)
  • Allow 30 cm minimum overlaps between panels
  • Use spacers to hold the mesh in position

Step 5: Movement Joints

For large slabs (> 40 m² in one piece), plan movement joints. They limit cracking by creating controlled break points.

  • Maximum spacing: 6 metres in each direction
  • Depth: 1/3 of slab thickness
  • Method: PVC profiles cast in or sawing after pouring

Step 6: Pouring

Characteristic Value
Slab thickness 12 to 15 cm (on insulation)
Concrete class C25/30
Consistency S3 or S4
Cement content 300 to 350 kg/m³

Procedure:

  1. Set the level guides (screed rails or spots) to the final slab height
  2. Pour concrete starting from the far end of the room
  3. Spread with a shovel and rake
  4. Vibrate with a screed bar, pulling the concrete along the guides
  5. Smooth with a power float (helicopter) if a smooth finish is required
  6. Place all service openings (ducts, pipes, floor drains) before the concrete sets

💡 Tip — For a 100 m² house, expect approximately 12 to 15 m³ of concrete. The pour must be completed in a single day. Plan to receive mixer trucks at regular intervals (every 30 to 45 minutes).

Building a Suspended Floor over a Crawl Space

A suspended floor rests on prestressed concrete beams placed on foundation walls, with infill blocks (entrevous) between the beams.

Components

  1. Beams: prestressed concrete load-bearing elements, spaced at 60 cm
  2. Infill blocks: lightweight blocks (polystyrene, concrete, clay) placed between beams
  3. Structural topping: reinforced concrete screed of 4 to 5 cm poured on top
  4. Welded mesh: placed over the infill blocks before pouring the topping

Method

  1. Place the beams on the foundation walls (minimum 5 cm bearing)
  2. Prop the beams at mid-span with adjustable props
  3. Place the infill blocks between the beams
  4. Lay welded mesh over the infill blocks
  5. Install perimeter ring beam reinforcement
  6. Pour the structural topping (C25/30 concrete, 4 to 5 cm thick)
  7. Do not remove props until 28 days of curing

⚠️ Warning — The crawl space must be ventilated through openings (air bricks) in the foundation walls. Provide a total ventilation area of at least 1/150th of the crawl space surface area.

Common Mistakes

  1. Forgetting the DPM: capillary moisture will damage the insulation and floor finish
  2. Poor sub-base compaction: settlement will appear after pouring
  3. Placing mesh at the bottom of the slab: it no longer serves its anti-crack purpose
  4. Pouring in hot weather without curing: the concrete surface will crack
  5. Forgetting service openings: drilling through a slab after the fact is costly and weakening

✅ Checklist before pouring

  • Sub-base compacted and levelled
  • DPM laid with overlaps
  • Insulation placed on a flat surface
  • Welded mesh in upper position with spacers
  • Service openings (drains, ducts) in place
  • Movement joints planned if area > 40 m²
  • Level guides set
  • Concrete ordered in sufficient quantity (+ 10% margin)