How to make a floor screed with your own hands
Concrete and cement floors serve for decades, do not creak and generally do not cause any problems to residents. Of course, if all the work is done technically correctly. Therefore, we offer you a comprehensive manual for pouring 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 final 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 the surface to be used as a subfloor. Cast ceilings can be seriously filled up in the general plane, especially in new buildings. In such cases, the screed is performed by the standard method.
Another thing is if the floor level needs to be raised by 15–20 cm, when pouring concrete is extremely unprofitable financially. A classic example is ground flooring on the first floor. In this case, the screed is poured over the bedding of crushed stone or expanded clay. This is called a screed on the bulk 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 the desired type of coating to be laid, a so-called preparatory screed is poured on top. The most common example is the floors in the bathrooms of wooden houses.
Keep in mind: the purpose of the screed is to both correct the general plane of the floor and level out local irregularities when covering the entire floor 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-setting or self-leveling floor.
What compounds to use
Traditionally, for screeding in residential premises, sand concrete is used in the proportion of 3.5 parts of sand to one part of grade 300 cement. In technical premises, the binder should be replaced with Portland cement of grade 400. With a screed layer up to 50 mm, this composition is optimal.
Thicker layers may require larger filler. It is allowed to use granite screenings and crumbs, expanded clay and crushed stone of a fine fraction. 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 make it easier to level, you can add about a tablespoon of dish detergent to 20-25 liters of water.
It is conditionally possible to call 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 difference, and after 2–3 days fill the self-leveling floor with the minimum possible layer.
Do I need reinforcement and insulation
Operating conditions of the floor can be completely different. If the layer thickness exceeds 40-50 mm, the coating may not tolerate thermal expansion and seasonal fluctuations of the building. With a 70–80 mm screed, cracking is practically guaranteed. If for linoleum and type-setting coatings this is quite tolerable, then bulk compositions will reflect all screed defects on themselves.
To strengthen the screed, use nylon or steel mesh with a mesh of 30-60 mm and reinforced (welded) crossings. The synthetic mesh is stretched on self-tapping screws screwed into the preparatory floor, or on thin knitting needles made of knitting wire, fixed in the bedding. It is also possible to lay the mesh in a freshly poured mixture. Due to their own high rigidity, steel reinforcing meshes 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 underfloor heating. Thermal insulation is carried out with materials resistant to uniform compression: expanded polystyrene and polyurethane boards. The thickness of the screed must be at least 30 mm with mandatory synthetic reinforcement. When laying insulation on the bedding, preliminary leveling with washed sand is required with a layer of 50–70 mm.
Work order
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 power grid laid inside the voids is possible.
Waterproofing needs to be done even by a private developer: the rapid outflow of water from the mass does not allow the hydration of the cement in the mixture to complete, which is why the floor will not gain the necessary strength. The controversial question about the screed on the bulk 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 bedding, although more and more often it is first covered with geotextiles so that the water does not entrain the cement. The upper layer will then have time to form normally, and the residual laitance flow will strengthen the underlying masses. The second version is also suitable for a wooden floor: the cavity to be filled is lined with a polyethylene film hermetically joined at the seams.
After the floor is sealed and waterproofed, we lay out the reinforcing mesh on remote supports. Next, we give a zero mark on the walls and install beacons. Pure alabaster should not be used here, it shrinks. Beacons can be quickly installed by mixing building gypsum into a solution of a freshly prepared batch. After applying small tubercles to the floor, we lay the first strip of the lighthouse 10-15 cm from the wall and align it with the lacing. The second and subsequent strips are installed according to the rack or laser level, after laying every third beacon, the common plane is checked by the rule.
Usually, pouring is carried out by two workers: one prepares the batch, and the other sets the next 2-3 beacons on the remnants of the previously mixed mass.
Subfloor post-processing
Speaking 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 aims to get rid of the upper layer formed by a fine fraction of sand and rare milk, which will wipe unevenly, creak and dust. Grinding is performed after two weeks of drying of the screed. Ironing, on the contrary, is carried out immediately after the sand concrete has set, and its purpose is exactly the opposite - to strengthen the top layer.
If the surface of the screed is not exploited, it is allowed not to perform such processing. For laminate, parquet and other type-setting coatings, it is better to impregnate the screed with a primer, and then wash it thoroughly. The difference here is linoleum - under it the screed is impregnated with diluted 1: 1 polyvinyl glue.
In order to lay the tiles qualitatively and evenly, grouting is performed on top of the primed screed with an adhesive composition that will be used when tiling. This helps to smooth out the remaining irregularities, reduce the screed layer above the insulation to 20 mm using a fiberglass facade mesh, and improve adhesion. After drying, the surface is treated with a grinding wheel to remove the “glaze” that prevents deep absorption of the tile adhesive.