How to do the work correctly, what materials to use and what mistakes should be avoided. The answers to these and other questions are in our guide to liquid floor screed.
Building codes allow a level difference at the joints of floor slabs up to 12 mm and a deviation from the horizontal within a span of 4 m – up to 10 mm. In practice, these values are often exceeded, and even more noticeable ledges and slopes are formed in the process of house shrinkage . The universal and most reliable way to level the floor in an apartment is to pour a wet screed, the technology of which is constantly being improved.
1. Why is it necessary to prepare a project before work?
It should be borne in mind that a permit is required for dismantling and construction works that change the structure of the subfloor. And for this, it is necessary to prepare a project that provides for the protection of the downstream premises from leaks and impact noise.
2. What materials are used in the work?
For the device of the screed, cement-sand mortar, light and cellular concrete, ready-made dry mixes for self-leveling floors, leveling mixes are used. The material is selected depending on the required surface quality. This, in turn, is determined by the type of floor covering. The surface of a cement-sand or concrete screed is often (for example, when laying parquet) leveled with a layer of a special mixture. A leveling layer is not required under the ceramic tile cladding.
- Dry mixes. Usually, for the device of the screed, they buy a ready-made dry cement-sand mixture, packaged in bags of 50 kg. You can buy bags of 25 and 30 kg, but then the material will cost more. You should buy a mixture specially designed for the screed device and produced by a reputable company, since low-quality products of this kind come across quite often. You can also use masonry or all-purpose dry mix. When buying, pay attention to the expiration date and color of the composition. The dry cement-sand mixture should be gray, without a brownish-red tint, which indicates an excessive sand content or the presence of clay. A high-quality mixture is ready for use, and to obtain a solution, only a strictly defined amount of water needs to be added to it.
- Cement-sand mortar. If the finished product cannot be purchased, the cement-sand mortar for the screed can be prepared from cement grade no lower than 400 and pure quartz sand, taken in a ratio of 1: 2.8 or 1: 3 by weight. Water is added, taking the ratio of water to cement (also by weight, but water can be counted in liters) – 0.45-0.55: 1, that is, 0.45 or 0.55 liters of water is taken for 1 kg of cement. To obtain a homogeneous solution, cement and sand are first mixed dry, and then water is added. The solution prepared by this method must have a grade of at least 150-200. More water – lower mark.
- Concrete. For the installation of screeds, foam concrete of class B 3.5 with an average density of 600-1000 kg / m3, both monolithic and in the form of slabs, is also used; lightweight concrete (expanded clay concrete or perlite concrete) of a class not lower than 5.0 with an average density of up to 1300 kg / m3, etc. Screeds made of cellular and lightweight concrete have a porous structure and have heat-insulating properties. But the surface of such a screed in any case requires additional leveling. The use of foam concrete gives better results: they have both an average density and a thermal conductivity (0.18-0.25 W / mS) lower, and the surface is smoother. However, due to the high fracturing of this material, it is recommended to additionally lay a layer of cement-sand mortar on it.
3. What are the main parameters of the screed?
This indicator is determined for each individual case and depends on specific conditions. Is the screed placed on the floor slab or on an insulating layer? What material is it made of and what kind of floor covering is it intended for? After all, the screed can be laid on a concrete floor or on a heat and sound insulating layer made of solid heat insulating products or bulk materials. In the last two cases, its thickness should be at least 4 cm. Moreover, the screed is reinforced with a metal mesh or “Fibrin” (made of polypropylene fibers). The same is done if it is laid on a waterproofing layer in a bathroom or kitchen.
A cement-sand screed is used when it is necessary to compensate for the unevenness of the concrete floor surface by more than 20 mm. But if the cement-sand mortar contains a plasticizer (in the overwhelming majority of cases this is exactly the case), the minimum thickness of the screed must be no less than 30 mm. If it is thinner, cracks will appear in it, and the developer has a question: what to do? In other words, it is better that anywhere the thickness of the cement-sand mortar with plasticizer is at least 30 mm thick. If the slabs were laid somehow and the level differences exceed 60 mm, it is more expedient to make a screed from sand concrete (it does not include ordinary sand, but coarse-grained sand). Its thickness can reach 100-150 mm. When level differences and slopes reach almost prohibitive values of 150-170 mm.
In case of slight differences and roughness (less than 20 mm), they resort to using leveling compounds. Even the term “screed” in this case is often replaced with the words “interlayer” or “preparation”. The manufacturer determines the permissible minimum and maximum layer thickness for each specific composition.
The time for hardening and drying the screed under normal conditions is determined mainly based on its material and thickness, as well as the type of floor covering. For a cement-sand mortar under parquet, this is usually at least 25-30 days (this period is a week for each centimeter of the screed up to 4 cm and 1.5-2 weeks for each subsequent centimeter). Ceramic tiles can be laid on a cement-sand screed after 7-10 days. Leveling and bulk mixtures harden in different ways – from 1 day to 3-4 weeks, depending on the type of polymer additives used for their manufacture. Thus, when choosing a material for a screed, one must take into account not only its possible thickness, but also the holding time before laying the coating.
Screeds made of concrete and cement-based mortars are arranged at a temperature at the floor level not lower than +5 C.
The level of the screed surface common for all rooms (or for one room, if it is the only one) is set using the so-called zero level. The zero level must be set very accurately, since this ultimately determines how smooth the surface of the future floor will turn out. The zero level is set using a device – a laser or hydraulic (builders usually call it water) level.
Then the position of the screed surface is determined. To do this, on each wall of each room, measure at 2-4 points (but the more measurements, the better) the distance from the zero level to the bottom surface of the screed. At each measuring point, the result is recorded directly on the wall. The smallest value will indicate that this is where the highest protrusion is on the floor. Conversely, the highest value is obtained where the floor is lower. Now, having assigned the minimum thickness of the screed, you can determine the position of its top level.
Almost always, apartments have floors with different coatings: parquet, tiles, linoleum. Different coatings have different thicknesses, and the floor surface must be level. Therefore, it is necessary to provide for different levels of the screed surface for different coatings.
4. What does the screed consist of?
When determining the zero level, one proceeds from the minimum permissible (in terms of strength) local screed thickness – 25–30 mm. A laser level and beacons, for example, made of steel guiding profiles for drywall, help to “hit the zero”. The lighthouses are securely fixed so that they do not move during concrete work.
If the overlap is even and the average thickness of the screed does not exceed 40 mm, the base layer in most cases can be made of cement-sand mortar of grade not lower than M200. It is desirable to add plasticizing, sealing and water-repelling additives.
With an estimated screed thickness of more than 40 mm, lightweight concrete is used – expanded clay concrete, foam concrete, polystyrene concrete, etc. The advantage of expanded clay concrete is low cost, availability of components and the ability to prepare a solution on site (using a compact concrete mixer or manually).
The density of the material is 800–1000 kg / m3, that is, it is 1.5–1.7 times lighter than sand concrete. Monoliths made of ready-made mixtures with special fillers (for example, foam glass) have approximately the same characteristics, but their cost is 2–2.5 times higher.
The density of foam concrete is even less (500–600 kg / m3). However, it is difficult to prepare it yourself: specific precisely metered components are required, which will have to be mixed for a long time. Some companies have equipment that allows the ready-made mortar to be supplied to a height of 40-50 m, but the cost of the subfloor at the same time doubles, besides, firms using concrete pumps undertake only large volumes of work (from 100 m2). An alternative to commercial aerated concrete is polystyrene concrete from ready-made mixes. By the way, this material is more ductile and gives less shrinkage.
When using light mixtures, even grades 400 and 500, the minimum base layer thickness should be 45-50 mm, otherwise there is a high risk of cracking.
It is impossible to make the base layer perfectly even: the filler fraction is too large, and besides, the solution gives uneven shrinkage (depending on the layer thickness). To “bring out” the surface, use special mixtures. They are applied in a thin layer (3-5 mm), when the base has gained about 70% strength, that is, after 1-2 weeks; some polymer compositions are allowed to be laid only on completely dried concrete, treated with a contact primer.
So that the quality of concrete does not deteriorate as a result of rapid evaporation of moisture, the base layer is covered with a polyethylene film; another option is to moisturize it regularly. It is impossible to open wide window sashes, only slot or window ventilation is allowed.
Floor levelers are subdivided into filler and self-leveling. The former (a wide range of cement, acrylic and epoxy finishing putties) have a pasty consistency; they are applied with a long spatula. From the latter a liquid solution is prepared, which is capable of spreading itself over the surface. Self-leveling floors are optimal for leveling large areas, but working with them requires skill and responsibility: you must carefully follow the instructions for preparing the mortar and very quickly distribute it over the surface. Another nuance is the presence on the market of fakes and expired mixtures (their shelf life does not exceed six months). A mortar prepared from low-quality raw materials does not have the required compressive strength and can flake off from the base screed.
In the process of pouring a liquid solution, moisture can seep into the cavity of the floor and into the apartment on the ground floor through the joints of the slabs. In addition, dry slabs are able to quickly “pull” water from the lower layer of the solution – the concrete will dry out and will not gain the required strength. To avoid these troubles, before starting concrete work, you need to create a waterproof “trough”, using for this purpose plastering or roll materials (we will return to them later). The created water seal will be useful in the future – in the event of small leaks, it will prevent a flood from the neighbors below.
The sound insulating ability of the floor is characterized by the reduced impact noise index, measured according to a special method. Moreover, in residential buildings, the maximum permissible value is 58 dB. However, tests show that this parameter is, as a rule, higher (the worst results, up to 65 dB, were obtained when examining panel buildings built in the 70s – 80s of the last century). Damping underlays under the floor screed and / or floor covering help to achieve an acceptable level of sound insulation. At the same time, some materials with a thickness of only 3-5 mm allow you to reduce by 20-25 dB and provide peace of mind to your neighbors, and in addition, protect against structure-borne noise that occurs in multi-storey buildings.
A thin damping substrate is quite enough so that the neighbors below do not hear your steps (speech and other airborne noises are successfully insulated by a massive floor slab). But if you plan to install a powerful speaker system with a bass amplifier, you need a more serious barrier, for example, two layers of high density mineral wool boards with a total thickness of at least 80 mm. A fairly effective way is to build a vibration-isolating podium under the speakers and a subwoofer. However, without wall and ceiling sound insulation, the probability of airborne noise “leakage” to neighbors remains.
What universal screed solutions are there?
Until recently , different materials were used for waterproofing and sound insulation – for example, first, soft fiberboard was laid, and then the surface was covered with plastic wrap. Today, there are universal substrates on sale – both waterproof and vibration damping (that is, damping shock vibrations). Some of them are produced in the form of plates glued to the base. Others are mineral fiber mats with bitumen or rubber-bitumen coating. In addition, substrates are made from extruded cork, polyethylene foam or foam rubber.
Before laying insulating materials, the joints of the slabs are smoothed out with cement putty, and in wet areas it is recommended to then apply a layer of cement-polymer or rubber-bitumen mastic to the concrete (as an additional insurance against leaks). Waterproofing and soundproofing mats (slabs) must be installed on the walls to a height equal to the estimated thickness of the floor “pie”. This eliminates the transmission of structure-borne noise from the screed to the walls and vice versa. The joints of roll materials are glued with special tape or mastic.
Subfloor requirements largely depend on the type of flooring. In particular, the tiles can be laid directly on a roughly leveled cement-sand screed: it is not difficult for a good craftsman to eliminate small irregularities in the course of work. Linoleum with a thickness of 4 mm or more is also allowed to be laid directly on the screed, but the quality of its surface should be higher. For carpet, you need to level the base with a bulk mixture. The most “finicky” is block parquet and solid board. For them, it is necessary to equip a base of plywood, which is glued to a carefully dried screed. At the same time, the compressive strength of the cement-sand screed should be at least 15 MPa, the peel strength of the top layer of putty – from 3.5 MPa, and the thickness of the underlying plywood layer – at least ¾ of the coating thickness.
Another nuance concerns the residual moisture content of the base, measured by a special device – a hygrometer. When laying wooden flooring, it should reach no more than 3%, linoleum – 7%, ceramic tiles – 9%.
When installing a screed, it is important to achieve a high strength of the base layer and a perfectly flat surface (the limiting level difference is 4 mm per 2 m). In addition, you cannot create an excessive additional load on the floor: deformation of the supporting structures as a result of reworking floors is not so uncommon.
5. Do you need to reinforce the screed?
Most experts recommend reinforcing the screed (regardless of its thickness and the type of mortar used) with a mesh of ribbed rods with a diameter of 4 mm or more with cells no more than 80-100 mm. The mesh is placed on low supports, and then poured with a solution; another option – first, the first layer of the solution is applied, the mesh is laid on it, and then the second layer is poured. As an alternative to steel reinforcement, you can use a cement-sand mixture with polypropylene fiber. At the same time, it is possible to achieve high concrete strength and speed up work.
Cables can be embedded in the base layer of the screed, as well as steel and polymer pipes with non-separable joints and an estimated service life of 40 years. The wires in the double insulation is permissible to stack without additional protection, but still reasonable to put them in corrugated PVC pipes.
6. What are the typical mistakes when installing a floor screed?
- Pouring thick (40 mm) from the heavy concrete ties.
- Pouring the solution directly onto the floor slab (without installing a waterproofing layer): leaks to the lower floor are inevitable, there is a high risk of damage to the hidden electrical wiring.
- Fast and uneven drying of concrete, causing deformation of the screed, a decrease in its strength and delamination.
- Refusal of reinforcement or improper reinforcement and, as a result, cracking of the screed (especially likely when using lightweight concrete and a small layer thickness).
- Ignoring the thickness of the flooring is fraught with the appearance of differences in the level of the finished floor.
7. How to level the floor using special mixtures?
First of all, the base layer is formed.
The mixture intended for this purpose includes special cements, lime and coarse sand. After 15 hours, you can proceed to the final leveling with a liquid solution.
A needle roller on a long handle helps to remove air bubbles.