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Concrete floor slab: construction and costs
Concrete floor slab: construction and costs

Video: Concrete floor slab: construction and costs

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Video: ICC SBS 2023, February
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The floor slab acts as a transition between the floor and the building. A distinction can be made between non-load-bearing basement slabs and foundation slabs, which are used in houses that do not have a basement. In terms of statics, the latter therefore plays a key role - in the truest sense of the word. If you want to concrete a floor slab yourself when building a house, you must have the necessary specialist knowledge in advance.

Table of contents Table of contents Concrete a floor slab: This is how the house base is created

  • Structural engineer draws up a foundation plan
  • Building a floor slab
  • Tip: Foam glass gravel as an insulation layer under the floor slab
  • Pour the floor slab: distribute the concrete evenly
  • The cost of a floor slab

Table of contents Table of contents Concrete a floor slab: This is how the house base is created

  • Structural engineer draws up a foundation plan
  • Building a floor slab
  • Tip: Foam glass gravel as an insulation layer under the floor slab
  • Pour the floor slab: distribute the concrete evenly
  • The cost of a floor slab

Anyone who dares to undertake the project "Concrete floor slab themselves" must know about the risks that the project entails. After all, this is anything but an ordinary do-it-yourself activity - such as wallpapering or laying laminate - into which you can somehow fool yourself. If the devil creeps in, this has immense consequences, and even a small faux pas is not so easy to iron out. Because: If the floor slab is faulty, it affects the entire structure that is to be built on the foundation. Changes in planning or re-concreting the floor slab not only cost time, but are also extremely expensive. In addition, the warranty obligations of the construction companies that you hire for successor buildings can be impaired. A high degree of experience is essential and precise work is essential.

Bear in mind that different requirements are made on a floor slab from case to case: a non-loadbearing floor slab rests on the foundations that bear the main load of the building. However, family houses are increasingly being built on so-called floor slabs without additional point or strip foundations. They have some advantages in terms of building physics, but they must be dimensioned much stronger and calculated exactly by the structural engineer. Guide values: In prefabricated houses, foundation slabs should be at least 30 centimeters thick, and in solid houses more 40 centimeters. One of the tasks for the load-bearing slab is to distribute the enormous load of the building evenly over the floor. In addition, a foundation plate counteracts natural movements of the floor - unlike classic foundations, sagging is not so easy, which can cause cracks in the outer walls of the building.

Structural engineer draws up a foundation plan

If you want to concrete a floor slab for a house yourself, you cannot avoid hiring a structural engineer to draw up a foundation plan - or reinforcement plan - because the floor slab has to be connected to the foundations. You also need to know about the extensive standards on which this is based. If you have sound prior knowledge, but have not dealt with the topic for some time, the structural engineer will be happy to bring you up to date so that no questions remain and your project is crowned with flawless success.

n einem Fundamentplan ist festgelegt, wie das Fundament ausgebildet sein muss
n einem Fundamentplan ist festgelegt, wie das Fundament ausgebildet sein muss

A foundation plan specifies how the foundation must be designed.

Photo: iStock / photo

But what does a foundation plan actually include? Well: It determines - depending on the construction plan and the conditions on site - how exactly the foundation must be designed. This affects, among other things, the nature of the concrete mix, where applicable the positioning of the steel and the location of all required connections. So you see, the work of the structural engineer is elementary and the foundation plan must be followed exactly in any case.

Building a floor slab

Use a batter board to mark out the area for the base plate, including a certain amount. In this area you have to remove the topsoil. The necessary depth depends on the intended structure. A professionally constructed concrete floor slab including the thermal insulation is around 70 centimeters high. In contrast to the foundations, the base plate itself does not have to be frost-free, but requires a so-called roller coating or cleanliness layer - if you use a bulk material such as gravel for this, this layer should be 15 centimeters high, 5 centimeters for lean concrete. The sewage pipes and empty pipes for all supply lines (electricity, gas, water, telecommunications) are laid in that layer or slightly below - in any case, the layer must be high enough to cover all the lines below. As a rule, you proceed by leveling and compacting the topsoil, then laying out a polypropylene filter fleece and installing the pipes at the correct height and with the correct slope. Then the cleanliness layer is applied, leveled and also compacted.

All house connections must be exactly at the locations provided in the construction plan and be routed vertically through the subsequent floor slab. The regulations of the still relatively new DIN 18533 must be observed for the construction and connection. Among other things, it stipulates that special casing pipes must be installed in the area of ​​the floor plate feedthrough, which can be used to provide a watertight seal to the ground. Also important: The empty pipes for supply lines must be laid in one piece under the base plate without connecting pieces such as sockets or real pieces and must be raised with a minimum radius of one meter. There are various manufacturers who offer standards-compliant solutions made from empty pipes and casing pipes. If you use them and install them correctly, you are on the safe side and there are no problems afterwards with the connection of the supply lines.

The thermal insulation is now placed on the leveled cleanliness layer - the insulation in the floor connection of the building is also known as perimeter insulation. Mostly, high-pressure-resistant extruded polystyrene sheets, so-called XPS sheets, with a rebate are used. For a good thermal insulation, a construction height of at least 30 centimeters is recommended, which corresponds to a two-layer construction. Important: If you lay XPS boards in two layers, the surfaces in the spaces between them must not be damp. In principle, thermal insulation is also possible on the top of the floor slab - however, this variant should always be preferred for new buildings because it is completely free of thermal bridges. Lay a double-layer, diffusion-proof film made of polyethylene as a moisture barrier on the thermal insulation. Glue the places where the pipes have to be passed with tissue tape.

By the way: You can achieve particularly good thermal insulation by using a so-called thermal floor panel. That has the great advantage that it not only keeps the heat inside the building, but also stores it temporarily and releases it again. By the way: thermal floor panels can also contain an integrated underfloor heating.

Es empfiehlt sich, die Bodenplatte mit einer Frostschürze zu umgeben
Es empfiehlt sich, die Bodenplatte mit einer Frostschürze zu umgeben

It is advisable to surround the base plate with a frost apron.

Photo: iStock / Max Labeille

Tip: Foam glass gravel as an insulation layer under the floor slab

Nowadays, so-called foam glass gravel is used more and more as floor slab insulation. It offers the following advantages:

  • The recycling material made from waste glass is highly heat-insulating because it has a very high pore volume.
  • After compacting with a vibrating plate, it is extremely stable. Therefore, in most cases, the installation of additional foundations can be dispensed with.
  • Foam glass ballast is 20 times lighter than ballast and is easy to process.
  • In contrast to polystyrene, it is a mineral and therefore sustainable building material.
  • It does not change chemically, is not reactive and is therefore not flammable.

Foam glass gravel should be installed in a layer thickness of 60 centimeters - so it offers extremely good thermal insulation and the overall structure of the floor slab reaches the required frost-free depth of at least 80 centimeters.

Pour the floor slab: distribute the concrete evenly

Before you start pouring the floor slab, a surveying engineer should measure and mark its position again exactly. Usually, it is also advisable to surround the base plate with a frost apron that is about 80 centimeters deep and about 40 centimeters wide. To do this, you will need to dig a trench into which you will lay the earthing strap for the power and water pipe earthing. The trench is filled with concrete.

Align the formwork panels of the surrounding formwork exactly using a spirit level and nail them together at the corner joints. The correct height of the formwork edge can also be checked over larger distances using a hose scale. Alternatively, you can also put formwork blocks. Now lay reinforcement mats on the surface on which the floor slab is to be created, which you will then tie together with construction wire at the connection points. Place the steel mats on reinforcement profiles or plastic spacers to ensure that the concrete encloses them at least 3.5 centimeters from all sides. Tip: Don't forget the foundation earth electrode!

Now the big moment has come: the delivery of the ready-mixed concrete. Distribute it evenly within the formwork and compact it with a concrete vibrator, also known as a bottle vibrator. The device is inserted into the still liquid concrete at short intervals - it ensures that air pockets disappear and the reinforcing steel is well enclosed by the concrete everywhere. Finally, smooth the surface with an aluminum slat, for example.

The base plate must then rest for the time being. This rest phase cannot be determined exactly to the day, since hardening depends primarily on the weather conditions, but a general guideline is based on 28 days. It is important to protect the concrete from frost with a film during the rest period. It is also important to prevent cracking on summer days by covering the base plate and watering it a little now and then. As with baking with a cake tin, remove the formwork as soon as the concrete has hardened. If the surface is not smooth enough, you can grind the concrete.

The cost of a floor slab

How deeply you have to dig into your wallet for the project “Concrete floor slab yourself” cannot be said with certainty, because there are too many individual factors that differ from project to project. The general rule is: building a house without a basement usually results in cost savings compared to a building including a basement - even if a thermal floor panel with integrated underfloor heating is used. The following can be calculated a little more precisely: For a cellar-free solid house that has a floor space of 100 square meters, you have to invest between 10, 000 and 12, 000 euros for a foundation slab - i.e. around 100 to 120 euros per square meter. A thermal floor plate costs between 15, 000 and 16, 000 euros. For comparison: the basement of such a house would cause additional costs of around 30, 000 euros.

Hendrik Jürgens

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