Floating foundation – building step by step

The foundation is designed for uniform distribution of the load created by the weight of the building, and its transfer to the ground. When constructing it, it should be borne in mind that it must lie on naturally compacted soil slightly below the freezing depth.

Depending on the type of soil, the size of buildings and a number of other factors, foundations are divided into floating, tape, pile and columnar. In this article, we will consider in detail the technology of building a floating foundation.


  • Floating foundation – what is it
  • Marking a plot for a floating type of base
  • Pit preparation
  • Preparatory layers
  • Formwork
  • Floating foundation device – reinforcement
  • Concrete floating foundation
  • The main mistakes in the construction

Floating foundation – what is it

On heaving soils, characterized by increased instability, as well as with high boggy areas, it is best to use a floating foundation. It is a very common misconception that this type is characterized by the “swim” property. In fact, the foundation will float with the house only if the work is done incorrectly. On the condition of the house, located on such a basis, the movement of the soil is not reflected in any way, since there is no skew of one corner of the house relative to others. And it is called floating because, moving with the house, it reliably protects it from destruction during any subsidence of the soil.

Another name that has such a foundation is a floating plate. It is the presence of a monolithic reinforced concrete slab that is a characteristic feature of a floating base. Therefore, it is very resistant to loads of varying severity arising from freezing or thawing of the soil, as well as during subsidence. Moreover, such a foundation serves as reliable protection against the harmful effects of groundwater..

Floating foundation structure

Floating foundation structure

The main disadvantage of a floating foundation is its high cost. To create a rigid structure requires a large amount of concrete and special equipment. That is why a monolithic slab is most often poured under small buildings, although it is it that gives a large degree of reliability. It is very difficult to build a floating foundation with your own hands due to the complexity of the work, so it is best to invite a team of specialists or at least enlist the support of relatives.

Marking a plot for a floating type of base

The construction of a floating foundation should begin with the marking of the territory intended for this purpose..

We will need:

  • cord or fishing line;
  • level;
  • roulette;
  • right-angled triangle (can be made independently of thin rails, and the legs should be equal to 30 and 40 cm).

Important: according to state standards, the house should be at least three meters from the neighboring plot and at least five meters from the frontal border of its own plot.

We mark on the site one of the corners of the structure and drive a peg into this place.

Tip: Pegs painted in red are best seen..

We put on a triangle on a peg and, continuing one of its sides, measure the length of the house, and on the other side – the width of the house. We drive the same pegs into the end points – the second and third.

Do-it-yourself site marking for a floating foundation

Marking accuracy is a guarantee of the correct geometric parameters of a house under construction

Now we determine the centerlines and hammer in the fourth peg. The correctness of its installation is checked using a triangle and by comparing the diagonals. If the boundary stakes are hammered correctly, then the diagonals will have a difference of no more than 2 cm. In case of divergence of the sizes of the diagonals, this condition must be achieved by moving the corners. After this, we connect the stakes in pairs with fishing line or cord and we get a clearly defined border of the future foundation.

Pit preparation

First you need to remove the entire layer of vegetation from the planned area of ​​the house. In this case, you need to add 1 meter around the perimeter, it will be needed under the blind area.

The blind area is a strip of asphalt or concrete that protects the finished foundation from the effects of water (rain and flood) and gives an attractive appearance to the finished structure. Located at an obtuse angle to the structure.

The entire removed layer can be transferred to the place where it is planned to break the lawns or plant trees. After that, you can start digging the pit.

Preparing a foundation pit for a floating foundation

The foundation pit can be digged like that, but it’s better to hire an excavator

Most often, an excavator is used to excavate the soil. They dig a pit under the design mark, and sometimes the bucket picks up slightly below it. With insufficient experience the superintendent digging just fill the machine with sand and carefully level. This is the most common mistake in the construction of the foundation..

Attention: a building put on such a foundation will surely burst at the place of a dig and in very rare cases it can be saved.

When sampling soil should take into account the coefficient of loosening. Rock formations will double their volume, sand and vegetation layer – three times, and crushed stone – four. Some of the excavated land should be left on the site, it can be useful in subsequent work and when breaking down the garden landscape, and the rest will need to be removed by special transport.

Tip: the foundation pit should be dug with a slight slope of the walls. The best option is a slope of 1: 0.25. This avoids shedding of the edges of the pit..

Preparatory layers

Before pouring a floating foundation, a number of preparatory work is necessary. To begin with, a layer of crushed stone about ten centimeters high should be poured onto a well-aligned bottom of the base (with the help of a level). Laying should be carried out in several stages, carefully tamping each layer with a special device (you can use a log with a handle). After this, a layer of sand is poured, wedging crushed stone and excluding its subsequent “walk”. In addition, high frost resistance of sand will help prevent crushing of the finished foundation. The sandy layer is recommended to be condensed by means of the vibrating tool. To achieve the best effect, sand can be slightly moistened with water..

Next, a waterproofing layer is laid. As it is used dense plastic film or any other material with similar properties. This will help to avoid not only subsequent leakage of moisture into the finished base, but also the flow of concrete “milk” when pouring the mortar. Extruded polystyrene can be laid for thermal protection, but this is not necessary..

After that, a thin layer of cheaper concrete grade M-100 is poured – footing.


After the pit is completely ready, we proceed to the formwork device. In that case, if the foundation is immersed in the ground, you can do without it, the pit walls will play this role. A significant disadvantage of this method is the inability to create high-quality waterproofing of the outer wall of the base.

Tip: it is better not to save on the formwork and fix it as carefully as possible to avoid bursting with a concrete layer.

Floating foundation formwork

Pouring concrete into the formwork for a monolithic slab

For formwork, boards with a thickness of not more than 25 mm and not less than 20 mm will be required, from which the boards are prepared. The width of each shield should be 50-100 mm greater than the height of the planned structure. Shields are attached to the vertical walls of the pit and sealed with roofing paper, paper or cardboard.

A control cord is laid along the upper edge of the formwork, and for convenience only one side is fixed, and the second with the attached load hangs freely on the other side and is removed at the time of pouring the solution.

Floating foundation device – reinforcement

Now we start laying reinforcement. Floating foundation reinforcement – a very important and significant stage. At the same time, it is necessary that in the places of erection of walls metal rods stick out from the foundation. Thus, an inextricable connection of the base and walls is achieved..

Reinforcement by soft wire

Soft wire tie rods

Important: when connecting parts of metal reinforcement to each other, in no case should you use a welding machine. When welding, any metal changes all its consumer properties. Fasten the reinforcement bars with a soft knitting wire.

When laying reinforcement, it is necessary to ensure that the distance between adjacent rods is not more than thirty centimeters (ideally 20-25 cm). The entire reinforcing structure should become a single indissoluble whole and at the same time the ends should go into the place of the planned blind area. In the corners, the metal rods should be closed in a semicircle, and their ends should be brought into the side walls of the foundation. The result is a flexible one-piece design.

Crushed stone and reinforcement layers

Now you can fill the concrete layer

Concrete floating foundation

Now proceed to concreting the base. Laying the solution is carried out in layers of not more than fifteen centimeters, carefully leveling each and tamping with bayonet. Concrete is poured in one step, so it is best to order a machine of the finished solution. If, nevertheless, you decide to fill the foundation yourself, then for it one part of cement is taken for one part of water and sand.

Important: the plate should be filled in no more than one hour. This is the only way to achieve the best result..

Concrete tamping

Concrete ramming to wet shine

Concrete is rammed until a shiny wet layer appears on the surface.

The top layer of the foundation can be leveled with a wooden lath..

How to dry a floating foundation

In order for the floating foundation to dry properly, it must be moistened in the early days

On the first day, the base should be moistened with water every 4-5 hours or covered with a film, otherwise cracks may appear, especially in hot weather. On the second day, the foundation is only wetted 3 times, and on the next only in the morning and evening. After drying, the formwork can be removed, but the foundation will not be completely ready until a month later..

Ideally, it is recommended to start the next stage of construction work only next year, that is, the finished structure should defend the winter, and only after complete drying from melt water can walling be started. But if the deadlines are completely running out, then you need to wait at least a month. Otherwise, you can achieve disastrous results, such as skewing, subsidence or destruction of the structure.

The main mistakes in the construction

Sometimes there is a violation of the building structure as a result of the destruction of the fundamental layer due to errors made during its construction. Only subject to a responsible approach and compliance with all recommendations will you be able to build a house that delights residents for more than a dozen years.

So, the most common errors:

  1. Placing the foundation above the freezing depth. What this can lead to is easy to guess;
  2. Construction of the structure on frozen soil. Frozen soil increases in size, swells and, after thawing, necessarily sags. Naturally, if a foundation lays on such soil, then it will sag;
  3. Lack of waterproofing. If vertical insulation is neglected, then in spring and autumn, with soil saturated with moisture, the foundation body begins to actively draw water into itself. In winter, this moisture freezes and tears the base. In the absence of horizontal waterproofing, moisture from the foundation actively moves to the walls, which leads to the same result.

Having chosen a floating foundation, you will never regret it. This type of foundation rarely requires repair. Of course, this is a very expensive, but at the same time the most reliable way..


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