Before laying the first tile or the first wooden plank, the concrete foundation of a terrace determines its longevity. The type of soil, the expected load, and urban planning constraints guide the choice towards very different solutions, and a mistake at this stage can be costly in terms of cracks or settling a few years later.
Permeable surfaces and concrete foundations: what changes with permeable surfaces
Most guides focus on solid slabs or pedestals. Few address the impact of drainage concrete on foundation design. Permeable systems (drainage concrete, cellular slabs) respond to a growing concern among communities: limiting soil sealing and better managing rainwater.
With this type of surface, the classic foundation (reinforced slab laid on a polyethylene film) becomes counterproductive. Water must be able to flow through the entire structure. The principle relies on an appropriate excavation, a layer of untreated gravel that acts as a buffer basin, and a contaminant-resistant geotextile to prevent fines from rising into the gravel.
Several communities now require a systematic check of the need for a permit whenever the terrace creates a significant impermeable surface or alters the soil’s relief. Before finalizing the type of foundation, it is essential to consult the Local Urban Planning Plan of the municipality. A project designed with an appropriate concrete terrace foundation guide allows for anticipating these constraints from the design phase.
Soil type and concrete foundation: a parameter that should not be overlooked
Clay soil behaves very differently from sandy or rocky soil. Clay swells with moisture and shrinks during dry periods, causing differential movements that crack a poorly sized slab. Sandy soil, on the other hand, offers good natural drainage but can compact under concentrated loads.
A geotechnical study remains the only reliable way to know the actual bearing capacity of the ground. Field feedback varies on the necessity of this study for a simple terrace, with some professionals deeming it unnecessary on soils considered stable. However, on filled or clayey land, forgoing this study is akin to betting on the durability of the structure.
Foundation depth and frost protection
The foundation depth directly depends on the climatic zone. In deep frost zones, the footings must extend below the frost line to prevent soil uplift from deforming the structure. This depth varies by region.
For a terrace attached to a house, the question becomes more complex. The foundations of the terrace and those of the house should not be interconnected if they do not rest at the same depth. A decoupling joint between the two structures absorbs differential movements and prevents the transmission of stresses.
Poured slab, strip footing, or concrete pedestals: concrete selection criteria
Three main types of foundations cover the majority of terrace projects. The choice depends on the area, the expected load, and the budget.
- The reinforced concrete slab poured in mass is suitable for large terraces or those intended to support heavy loads (spa, masonry planter). It requires careful formwork, appropriate reinforcement, and an unavoidable drying time before any loading.
- The peripheral strip footings are an alternative when the terrace is attached to the house. They bear the loads around the perimeter and allow for the installation of rows of concrete blocks to create a crawl space or adjust a level. This solution limits the volume of concrete needed compared to a solid slab.
- The concrete pedestals represent the quickest and least expensive solution. Suitable for lightweight terraces (wooden planks on joists), they do not require formwork and allow for natural drainage under the structure. However, the pedestals cannot support significant point loads and require sufficiently stable and level ground.
Formwork and reinforcement: two areas not to underestimate
The formwork defines the geometry of the foundation. Poorly positioned boards or formwork that gives way under the pressure of fresh concrete produce an irregular slab, which is difficult to correct later. For a terrace slab, formwork boards held by regularly spaced stakes are sufficient in most cases.
The reinforcement (welded mesh or rebar) prevents the slab from cracking due to internal stresses and soil movements. A non-reinforced slab on clay soil almost systematically cracks in the first few years. The mesh should be positioned in the lower third of the slab, elevated by spacers, to effectively fulfill its mechanical role.
Concrete drying and commissioning of the terrace
The drying (or more accurately, the setting and hardening) of the concrete is a phase often rushed. Walking on a slab too soon, laying a covering before the residual moisture has sufficiently decreased, or pouring in freezing weather compromises the final strength of the foundation.
The mechanical strength of concrete increases rapidly in the first few days, then more slowly over several weeks. No covering should be laid before a sufficient drying period, which varies according to the thickness of the slab and the climatic conditions. In hot and dry weather, lightly watering the surface during the first few days limits shrinkage cracking.
For a terrace attached to the house, the expansion joint between the terrace slab and the wall must be installed before pouring, not after. This detail, often overlooked, conditions the proper management of runoff water and the freedom of movement between the two structures.
The choice of a concrete foundation for a terrace is based on three concrete data: the nature of the soil, the load that the terrace will have to bear, and the local urban planning rules regarding impermeability. Neglecting any of these three variables means accepting a risk of costly repairs in the medium term.