- The essence of leveling the floor with a screed
- What formulations to use
- Do you need reinforcement and insulation
- Work order
- Subfloor post-processing
The concrete and cement floor serves for decades, does not creak and, in general, does not cause any problems for residents. Of course, if all the work is technically correct. Therefore, we offer you comprehensive instructions for pouring a floor screed with sand concrete mixtures for different types of coatings with your own hands..
The essence of leveling the floor with a screed
There are three typical circumstances when the screed becomes almost the only way to arrange an even and reliable base for the subsequent laying of the finished floor..
The first option is concrete floors and ceilings with significant irregularities and defects. First of all, this is typical for apartments in panel houses, where the gaps between the “voids” and casting defects do not allow using the surface as a subfloor. Cast ceilings can be seriously overwhelmed in the general plane, especially in new buildings. In such cases, the screed is performed using the standard method..
It is another matter if the floor level needs to be raised by 15–20 cm, when concrete pouring is extremely unprofitable financially. A classic example is ground flooring on the first floor. In this case, the screed is poured over a bed of crushed stone or expanded clay. This is called a screed on a fill layer, the technology of work has significant differences.
The third option is the most exotic. If the mechanical characteristics of the subfloor do not allow laying the desired type of coating, a so-called preparatory screed is poured over. The most common example is bathroom floors in wooden houses..
Keep in mind: the screed has its goals both to correct the general floor plane and to level out local irregularities when the entire floor is covered with a common layer of small thickness. On the practical side, sand concrete screed is the most acceptable and affordable way to prepare almost any floor for popular types of coatings: linoleum, laminate, vinyl type or self-leveling floor.
What formulations to use
Traditionally, sand concrete is used for screed in residential premises in a ratio of 3.5 parts of sand to one part of grade 300 cement. In technical areas, the binder should be replaced with Portland cement grade 400. With a screed layer of up to 50 mm, this composition is optimal..
Thicker layers may require larger filler. It is allowed to use granite screenings and chips, expanded clay and fine crushed stone. Filler larger than 15 mm is not recommended.
To improve some characteristics, frost-resistant additives, plasticizers and modifiers can be added to the mixture. To increase the fluidity of the mixture and for easier leveling, you can add about a tablespoon of dish detergent for 20-25 liters of water.
Conventionally, it can be called a screed and a self-leveling floor that does not require alignment with beacons. With a minimum layer of 10 mm, such a screed can cost a pretty penny, especially if the difference between the lowest and highest points exceeds 35-50 mm. It will be easier if you level the subfloor with ordinary sand concrete to eliminate the general drop, and after 2-3 days fill the self-leveling floor with the minimum possible layer.
Do you need reinforcement and insulation
The operating conditions of the floor can be completely different. If the layer thickness exceeds 40-50 mm, the coating may not withstand thermal expansion and seasonal fluctuations in the building. Cracking is almost guaranteed at 70–80 mm screed. If this is quite tolerable for linoleum and type-setting coatings, then the bulk formulations will reflect all the defects of the screed on themselves.
To reinforce the screed, use a 30-60mm nylon or steel mesh with reinforced (welded) intersections. The synthetic mesh is stretched on self-tapping screws screwed into the prep floor, or on thin knitting wire needles fixed in the bedding. It is also possible to lay the mesh in a freshly poured mixture. Steel reinforcing nets due to their own high rigidity can be placed on remote “chairs”.
Cement floor insulation is also widely practiced. First of all, when the screed is used as an accumulation layer for a warm floor. Thermal insulation is carried out with materials resistant to uniform compression: foamed polystyrene and polyurethane boards. In this case, the thickness of the screed must be at least 30 mm with obligatory synthetic reinforcement. When laying the insulation on the backfill, preliminary leveling with washed sand is required at a layer of 50–70 mm.
The first step before pouring is to eliminate absolutely all gaps and crevices through which water can seep. Hollow floor slabs promise another danger: water can flow into them and not flow out from below. A damp ceiling and a swelling floor are guaranteed in the next six months, damage to the electrical network laid inside the voids is possible.
Waterproofing needs to be done even by a private developer: the rapid outflow of water from the mass prevents the hydration of the cement in the mixture from completing, which is why the floor will not gain the required strength. The controversial issue of screed over the fill layer: how, in this case, to prevent seepage? Here it is necessary to fill in two layers with a time interval of at least a day. The first layer is poured directly onto the backfill, although more and more often it is first covered with geotextiles so that the water does not carry the cement along with it. The upper layer will then have time to form normally, and the residual flow of cement laitance will strengthen the underlying masses. The second version is also suitable for a wooden floor: the cavity to be poured is covered with a polyethylene film, tightly joined at the seams.
After the floor is sealed and waterproofed, we lay out the reinforcing mesh on the spacer props. Next, we give a zero mark on the walls and install beacons. You should not use pure alabaster here, it will shrink. The installation of beacons can be carried out promptly by mixing plaster of paris into the solution of the just prepared batch. After applying small bumps on the floor, we lay the first strip of the lighthouse 10-15 cm from the wall and align with the lacing. The second and subsequent strips are installed at a rack or laser level, after laying every third beacon, the general plane is checked by the rule.
Typically, pouring is carried out by two workers: one prepares the batch, and the other sets the next 2-3 beacons on the remains of the previously mixed mass.
When talking about the thickness of the screed, we always mean the final value after surface treatment. Depending on the floor covering, the floor can be processed in different ways, while the thickness can either decrease or increase within 0.5 mm.
The two most well-known processing methods are grinding and ironing. The first is to get rid of the top layer formed by the fine fraction of sand and rare milk, which will wipe unevenly, creak and dust. Sanding is carried out after the screed has dried for two weeks. Ironing, on the contrary, is carried out immediately after the setting of sand concrete and its purpose is exactly the opposite – to strengthen the top layer.
If the surface of the screed will not be exploited, it is allowed not to perform such processing. For laminate, parquet and other type-setting coatings, it is better to saturate the screed with a primer and then wash thoroughly. The difference here is linoleum – under it the screed is impregnated with diluted 1: 1 polyvinyl glue.
In order to lay the tiles efficiently and evenly, grouting with an adhesive composition is performed over the primed screed, which will be used for tiling. This helps to smooth out the remaining irregularities, reduce the screed layer above the insulation to 20 mm using fiberglass facade mesh, and improve adhesion. After drying, the surface is treated with a grinding wheel to remove the “glaze”, which prevents the deep absorption of the tile adhesive.