Why use a screed for a parquet installation?

If you are going to buy a wooden floor, you have to make the screed, i.e. the support layer on which you are going to lay or glue the floor. In order to be able to carry out this task, you will need to know the essential characteristics related to this action. Parquet, laminate flooring and PVC are among the most popular coverings that are attractive because of their advantages. The laying will have to be carefully prepared and carried out in compliance with the norms and standards in force.

What are the different types of screeds?

Today unused, the screed used for flooring the house consists of walnut wood sleepers on which will then be nailed the wooden planks or planks.  This screed is defined as a screed with drowned magatelli and the glued laying of the parquet will be called "laying on magatelli". Magatelli are wooden elements with a trapezoidal section with a wider base at the bottom. They have a thickness of 2 to 4 centimeters and must be prepared in the screed in such a way that the narrowest side is flush without protruding too much from the surface. The total thickness of the cement plaster will be about 5 centimetres and will have the characteristics of homogeneity, consistency, absence of systems, presence of vapour barriers. In order to determine the level of the finished floor, and therefore for the realization of the cement plaster, it is necessary to know the thickness of the parquet that will be nailed to it. Magatelli are usually arranged diagonally to the surface, in relation to the room, or at least in rows at an angle to the future direction of the floor and must have a magatelli frame arranged linearly along the perimeter of the room. The distance between the magatelli will vary, depending on the size of the wood floor that will be screwed or nailed down. But it is usually about 20 / 25 cm. The traditional mortar for gluing solid wood is the most used. This consists of laying a layer of concrete about 5 cm thick on a vapour barrier facing the walls with a mineral aggregate consisting of river sand of suitable grain size. Normally, 300 kg of cement is used for each cubic meter of sand. It is important that the mortar is well laid, compacted and smoothed with a trowel, even if the final smoothing does not have to be carried out, which could lead to an extension of the drying time: the surface must remain porous. This is also to allow the laminate flooring to hold optimally. The time needed to lay a tiled floor on a mortar like this is about 20 days from the end, just the time needed for curing. As already mentioned, the sand and cement screed needs a longer time to allow the floating wooden floor to be laid. To speed up drying, you can use hot air generators and dehumidifiers, but the results are modest. There are, however, preventive solutions that allow significantly faster drying times. The use of additives or special cements reduces the waiting times compared to those required for traditionally conditioned screeds. These times may be too long and may not be compatible with the organization of the building site or the need to move quickly to a new home, for example for a wedding or the birth of a child. In order to achieve relatively fast start-up times, it is possible to use binders which, while allowing the mortar to be worked and applied in a similar way to a normal cement mixture, allow a solid wood floor to be laid in a shorter time.

The screed: how to make it?

In order to avoid problems with parquet, there are a few precautions to be taken regarding the ideal characteristics of a traditional covering. It will also be necessary to see if they are suitable for the floating installation of a parquet floor. A screed suitable for laying a wooden floor must be homogeneous, have a uniform thickness, it must not contain pipes or other systems inside. It must be separated from the substrate by a vapor barrier and it must be separated from the walls by a mat. Be aware that the flooring is a cementitious element, made by mixing binders, aggregates and water, to which a floor must adhere. The issue of moisture in the cement coating and the potential ability of the screed to transfer moisture to the floor is therefore absolutely critical. With regard to the vapour barrier on the flooring, it is essential to provide a system of separation between the impermeable layer and the other elements by means of a vapour barrier to be erected on the walls to create a watertight "pool" in which to lay the screed. The insulation of the building cladding protects against unwanted moisture transfer to the wood floor. The choice of this "barrier" is not insignificant because this mat also fulfills the very important function of reducing the transmission of noise to the rooms below. There are different types depending on the budget and the needs to be met. To ensure ideal mortar behavior, foam strips on all edges must be installed for perimeter joints and appropriate contraction joints must be provided for thresholds, doors, etc. between the various rooms. Contrary to what has to be done in the case of ceramic floors, in the case of wooden floors, it is not mandatory to bring the contraction joints to the surface because the floor is more elastic and can resist the expansion and stresses to which the joint is subjected. In this case, the movement of the joint is important and the cut on the screed must match an identical cut on the wood floor.

Regarding the drying time of the screed

The main problem with a screed laying parquet that does not incorporate a radiant system is the drying times and methods. In fact, to be able to lay acoustic parquet, the percentage of water that the screed must contain must be less than 2%.  In essence, the covering, which is a mixture of sand, cement and water, must get rid of the water until it becomes a sufficiently dry layer so as not to compromise the parquet. The literature is replete with cases where there have been misunderstandings between screed manufacturers and customers as to how long it takes for the screed to "dry". But what are the causes of slow drying? Sometimes it is the mixtures that are made using a water content. For inert materials with a better thickness and too fine grain size, very fine sand takes longer than coarser sand. Another mistake is to over-smooth the surface by closing the "pores" that facilitate the evaporation of water and creating an impermeable film/layer instead. A problem could also be the lack of a vapor barrier that would allow moisture from deep layers or plaster to seep into the mortar. It is clear that the season of the year in which it is done and therefore the outside temperatures, and especially the percentage of moisture in the room are also affected. If it is done at a time of year when the climate is dry, provided that the room is ventilated. Cement plaster dries more quickly. Conversely, a period of fog or continuous rain does not promote drying. Then, the most important reason: thickness. An impermeable layer of normal thickness, i.e. 4 or 5 centimeters, dries much faster than an impermeable layer of 8 or 10 centimeters. With the flooring system, these problems are solved. All that is needed is to perform the thermal shock correctly to expel the water from the screed. Therefore, one has to wait until the screed has reached maturity and then the radiant system has to be switched on and gradually brought to the maximum operating temperature for about ten days. Then the system stops and you proceed, after a few days, to the installation. If you would like more information about this procedure, how to implement it and the risks you run if you do not follow it.

Tests to verify the characteristics of the screed before gluing the wood

The following checks must be performed in the first row, before installing a wood floor. Concerning flatness, with a 2 m long pylon, by placing it on the envelope in all directions, check that there are no hollows or bumps. The permitted tolerance is 3 mm. For compactness, check that the envelope does not crumble by tapping forcefully on a solid 750 gram package on the surface. There should be no obvious indentations. For rigidity, the mantle must not show any deformation or sagging as a result of the load. For surface hardness, vigorously scrape the surface with a fingernail by drawing perpendicular lines. No deep incisions or chipping. Stable capillary cracks are allowed. If the cracks jump over, "blowing" the dust out is not good. They must be healed with appropriate products. The humidity of the screed laying parquet must be checked with suitable instruments. Both the carbide hygrometer and the electric hygrometer should be used. The first test consists of destroying a small part of the floor which must be reacted in a cylindrical tube containing carbide. The amount of moisture is deduced from the pressure measured on the pressure gauge of the hygrometer. The electric ball hygrometer is equipped with a sensor capable of determining the electrical conduction characteristics of the screed which are strongly influenced by the presence of moisture at low temperatures. The electric hygrometer is easier to use and therefore allows more tests. Use both tools for this purpose. The level of the top floor must be checked by superimposing a sample of the wood on the mantle made in correspondence of the thresholds, armored doors, French windows, etc. The final level of the flooring is the one determined by the construction management, which must be followed. The covering must not be dusty, it must not contain oils, waxes, grit, various substances, stains of color or paint, plaster or other inconsistent substances. The risk of the screed laying parquet is that of imperfect adhesion of the parquet.
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