What decides the type of foundation?
The ground, the groundwater, the frost, the weight of the house and the structural system decide the foundation type. The engineering-geological survey supplies the first two. The Geological Works Act defines it as a survey of the engineering-geological and hydrogeological conditions of an area. Its purposes include documentation and the founding of structures (569/2007 Z. z., § 3 písm. g). The choice of structural system is covered in the comparison of masonry, timber frame and CLT. The concrete, its exposure classes and the cover to the reinforcement are covered below.
- Ground and water. The design starts from the survey, not from how the neighbour built. Standards cover geotechnical design (STN EN 1997-1) and the investigation and testing of the ground (STN EN 1997-2).
- Frost. The designer sets the frost depth for the founding plane. The decree requires that the effect of temperature change on the soil is taken into account for cohesive soils (§ 23 ods. 2).
- Weight of the house and structural system. The structural engineer calculates the load on the foundations. For a timber house, ask the manufacturer for the flatness requirements for the slab.
- Basement. A house with a basement faces waterproofing against pressure water and a deep excavation. This article deals with a house without a basement.
What does decree 532/2002 say about foundations?
The decree sets the general technical requirements for buildings. It stays in force until the new technical decrees apply, at the latest on 31 March 2027 (Building Act 25/2025, § 84 ods. 23). Section 23 covers foundations:
- Foundations ensure the stability of the building, and any uneven settlement must meet the Slovak technical standards (paragraph 1).
- For foundations exposed to temperature change, the effect on the properties of the soil is taken into account for cohesive soils (paragraph 2).
- An underground structure that separates the interior from the soil is insulated against ground moisture, groundwater and heat spreading into the subsoil. Where needed, it is also insulated against radon (paragraph 3).
- Foundations are protected as needed against aggressive water and substances that damage it (paragraph 4).
The excavation must be secured against collapse (§ 22 ods. 2). A water or sewer connection laid in the ground must lie below the frost line or be protected against freezing (§ 35 ods. 2, § 36 ods. 2). The decree gives no numerical depth for foundations. The ÚNMS catalogue lists STN 73 1001 "Geotechnical structures. Foundation of buildings" (issued 1 Apr 2010). The text is paid.
Strips, slab, foam glass or piles: what is the difference?
The foundation types differ mainly in how they pass the load into the ground. The table gives the meaning of the names and the questions to ask. Prices are left out because none can be given without a dated source.
| Type | What it is | Question for the designer |
|---|---|---|
| Strip foundations | Concrete strips under the load-bearing walls | What is the floor between the strips, and how is it insulated? |
| Foundation slab | A continuous slab under the whole footprint | How is the slab reinforced and protected from frost? |
| Slab on foam glass | A slab on a layer of foam glass | Which manufacturer data set the thickness and compaction? |
| Piles | Deep foundations that carry the load through unsuitable layers | Why does the survey call for them? |
When you compare quotes, compare the whole build-up from the soil to the finished floor. A quote for strips may leave out the floor slab, the fill and the insulation that another quote already includes.
What goes into the build-up under and around the slab?
After the pour, all layers are covered, so check them on the drawing. The list below names typical layers. The project sets the exact build-up.
- Blinding concrete under the insulation and the reinforcement.
- Compacted fill or a foam-glass layer, as the structural engineer designs.
- A radon barrier, where the project requires one.
- Thermal insulation under the slab, with a compressive strength set by the structural design.
- Reinforcement, for example rebar mesh, with the cover secured by spacers.
- Perimeter insulation that connects to the wall insulation.
What concrete goes into foundations and the slab?
The project sets the strength class and the exposure classes for each concrete element separately, not a phone call to the batching plant. The catalogue of the Slovak Office of Standards, Metrology and Testing (ÚNMS) lists STN EN 206+A2 "Concrete. Specification, performance, production and conformity" (issued 1 Oct 2021) and its national annex (issued 1 Oct 2023). The text is paid. This article relies on a free technical report on Eurocode 2 by the Joint Research Centre of the European Commission (JRC).
The JRC report uses concrete strength as an indirect measure of durability: higher strength means less porous concrete. The national annexes to EN 206 set the maximum water-cement ratio and the minimum cement content. Several classes in one house make sense only when the elements are cast separately, for example foundations and walls. In the worked example house, the report chose class C25/30 and exposure class XC1 for internal slabs, beams and foundations. It is an example of one design, not a Slovak rule.
Fibre-reinforced concrete contains dispersed fibres. For steel fibres, the ÚNMS catalogue lists STN EN 14889-1. Whether the fibres replace the mesh in a load-bearing slab is a matter for the structural engineer's calculation.
What do exposure classes XC, XD, XF and XA mean?
An exposure class is written with the letter X and the initial letter of the deterioration mechanism (JRC, section 1.4.1.2).
- XC: corrosion of reinforcement by carbonation.
- XD: corrosion of reinforcement by chlorides from de-icing agents, industrial wastes and pools.
- XS: corrosion of reinforcement by chlorides from sea water.
- XF: damage to the concrete by freeze and thaw.
- XA: chemical attack.
The designer decides which classes apply to which element. Freeze-thaw resistance is a property that must be specified in advance, not added later.
| Element | What loads the concrete | Question for the designer |
|---|---|---|
| Interior slab and staircase | Corrosion of reinforcement by carbonation (XC) | What is the cover to the reinforcement? |
| Foundations and slab in the ground | The soil and possibly chemically aggressive water (XA) | Did the water analysis show aggressivity? |
| Terrace, driveway and steps | Frost and de-icing salt (XF, XD) | Which XF and XD classes are specified, and what is the fall? |
| Retaining wall | Soil and water behind the wall, frost in front of it | Who designed it, and how is it drained? |
How thick is the cover to the reinforcement?
The cover protects the reinforcement from corrosion and secures the bond between steel and concrete. According to the JRC report, Eurocode 2 derives the minimum cover from the exposure class and the structural class. The drawings show the nominal value: the minimum cover plus an execution tolerance of 0 to 10 mm, and at least 20 mm. It also takes fire resistance into account. For earth retaining walls and foundations, the report gives 40 mm as common, because deterioration there is hard to detect by eye. National tables may differ, so the value on the structural engineer's drawing governs.
The cover is created by spacers under the reinforcement. Check it before the pour, not after.
What does radon mean for foundations?
The designer of a residential building with an underground floor or a ground floor for long-term occupation must take preventive measures. They keep radon indoors below the reference level of 300 Bq/m³ as an annual average (Act 87/2018 Z. z., § 130 ods. 1). The owner must arrange a measurement within one year of the start of use (§ 130 ods. 3).
Decree 57/2024 Z. z. names these preventive measures (§ 6 ods. 1): determining the radon index of the plot, a measure against radon entry from the ground, and ventilation of soil radon. It also names more airtight ground-contact structures with under-pressure below the building. The method for the radon index is in Annex 6 (§ 5). Draw the pipe penetrations through the slab on the foundation drawing, so that nobody drills the finished slab later.
Do I need a permit to pump water out of the excavation?
Possibly. The Water Act requires a permit for special use of water. This covers pumping groundwater to lower its level. It also covers pumping and discharging groundwater while founding structures. General use of water is an exception (364/2004 Z. z., § 21 ods. 1 písm. b bod 3 and písm. g). The state water authority issues the permit (§ 21 ods. 2). Ask it before you dig.
Why take over the founding plane before the pour?
A survey takes samples at single points. The excavation is the first moment when the whole area under the house is exposed. So agree a formal handover of the founding plane. Record it in the site diary, which is the documentary record of the building work (25/2025 Z. z., § 10 ods. 2).
- Does the soil match the survey?
- Is the plane soaked, frozen or disturbed by the excavation?
- Is there standing water in the excavation?
- Are all penetrations and sleeves in place?
- Does the reinforcement match the drawing, including the cover?
How do I supervise the pour and keep the site diary?
The execution of concrete structures is covered by STN EN 13670 (ÚNMS catalogue, issued 1 Aug 2010). Five steps help with the pour.
- Order the concrete from the project, with all classes and exposure classes.
- Compare the delivery note with the drawing before the concrete is unloaded.
- Check the reinforcement and the cover spacers against the drawing before the pour.
- Do not add water on site, because the national annex to EN 206 sets the maximum water-cement ratio.
- Ask the contractor for a plan for the curing of the concrete.
The site manager keeps the site diary. It is available to the manager from the first day the site is set up until the handover of the building (25/2025 Z. z., § 39 ods. 1). Among others, the general designer, the person appointed for construction supervision, the geologist and the geotechnician may write in it (§ 39 ods. 2). The designer records any deviations found and not removed (§ 35 ods. 7 písm. b). When the builder builds the house themselves, the builder keeps the diary (§ 30 ods. 6 písm. g). Entries on the handover of the founding plane, the reinforcement and the pour belong in the site diary.
A retaining wall by the house: is it a simple structure?
A structural engineer designs a retaining wall on the basis of the survey. The survey is also to assess the stability of the slope (§ 3 písm. g). A distinction is made between a gravity wall and a cantilever wall. The term is explained in the entry on the retaining wall, and a separate article covers a house on a slope.
The Building Act 25/2025 Z. z. lists retaining walls up to 3 m high above the lowest level of the adjoining ground among simple structures (§ 2 ods. 6 písm. e). Retaining and revetment walls over 10 m high are reserved structures (§ 2 ods. 8 písm. i). Act 243/2026 Z. z. also amends § 2 from 1 January 2027, so check the wording before you build. A simple structure as a rule needs a decision on the building intention (§ 18 ods. 2). A notification is enough for a minor structure or a building alteration (§ 18 ods. 3), and a retaining wall is not among the examples of minor structures in § 2 ods. 4. The building office decides whether a small garden wall counts as a minor structure.
How do I decide?
Start with the survey. The builder arranges the engineering-geological and hydrogeological survey works for the designer, unless they agree otherwise (25/2025 Z. z., § 30 ods. 2 písm. b). Then ask the designer to propose the foundations and compare the whole build-ups from the quotes. The contractor builds to the verified project. Deviations from the verified project are handled by an extraordinary inspection. The designer marks them in the project and the building authority approves the inspection record (25/2025 Z. z., § 43 para. 2 and 3).