1. Whats a floating slab?
Which is better - a strip foundation or a foundation slab? Its important to make a distinction between a foundation slab and a reinforced concrete floor slab constructed over the strip foundation and usually 10 to 15 cm thick. The thickness of a foundation slab depends on the expected load, including self-weight of the building, weight of the furniture, household items, and occupants, but also weather-related loads, like snow, wind, and seismic loads. The usual thickness of a foundation slab for a residential building ranges between 30 and 40 cm. According to the latest standards, thermal insulation is installed underneath the slab, making it a floating slab. This kind of foundation slab can serve as the basis for both a prefab and an on-site build.
2. How to lay a foundation slab
2.1 Foundation pit excavation and laying installations
You want to start with excavating a construction pit. The depth will depend on the terrain and the soils bearing capacity. When you get to a hard, bearing substrate, fill the pit with compacted gravel up to the point where you want your future floor slab. This is the part where you also lay conduits for services. A 10 cm thick concrete base is installed over the substrate prepared as described.
2.2 Foundation slab waterproofing and thermal insulation
This layer of concrete will serve as the basis for installing a double-layer waterproofing material - self-adhesive bitumen tapes. The next is a 20 cm thick thermal insulation layer, i.e. extruded polystyrene with overlapped edges, which is what makes this foundation a floating slab. The first layer of thermal insulation must cover the entire surface of the foundation slab, and be approximately 1 m away from the edge of the slab.
2.3 Laying a concrete slab
The next step involves setting up formwork for the foundation slab. Reinforced concrete and conduits are installed at this stage. Concreting comes next. The thickness of the foundation slab is calculated through a structural analysis, but it usually ranges between 30 and 40 cm. Thermal insulation and waterproofing products are also installed along the edges of the foundation slab. Horizontal and vertical insulation cant join at a right angle as this may lead to cracking. Special supports ensuring a 45° angle need to be used.
3. Advantages of a foundation slab include:
less digging,
less covers,
simple reinforcement,
faster construction,
uniform settlement,
a foundation slab is a better solution for harsh terrains, and
optimal thermal protection of the building.
4. What to pay attention to when laying a foundation slab?
Thermal insulation underneath a foundation slab has to have the right designed hardness and cant change its designed technical properties even after long-term exposure to load.
Its of utmost importance that the lowest edge of the foundation slab be deep enough for drainage purposes. The lower edge of a foundation slab has to be placed below the frost line, at least 60 cm deep. Alternatively, extra thermal insulation can be installed at least 1 m away from the edge of the slab, which produces a similar effect.
A joint between vertical and horizontal waterproofing product has to be secured with supports. A joint at an acute angle may jeopardize the waterproofing function. Before the waterproofing material is secured, the supports and thermal insulation should be protected with a thin layer of plaster to prevent the materials from fusing.
5. Price of a foundation slab
To estimate the costs of laying a foundation slab, youll need a design covering the type of terrain, the depth of the pit, accessibility, the thickness of the slab and insulation layers, etc. Well use a family house without a basement on a flat terrain as an example. A 100 sqm foundation slab is placed above a 20 cm thermal insulation layer. The cost of excavation accounts for 15–20% of the total investment value.
The cost of thermal insulation and waterproofing, along with the execution of all works, may account for as much as 30% of the total costs. The price of a foundation slab dramatically increases with the increase in the thermal insulation thickness. The cost of shuttering, reinforcing, and concreting accounts for 40–50% of the total investment.