Expert opinion on the floor screed

Proper installation of wooden flooring requires prior preparation of the subfloor. During the free measurements, our technician will check, among other things, the moisture level of the floor, assess its strength and levelness, and using modern measuring devices will measure the conditions in the room (including temperature and air humidity). One of the key issues during the expertise of the screed is the precise checking of the hardness and cohesion of the screed, as insufficiently compacted or excessively dusty substrate may be one of the reasons for later detachment of the parquet from the screed.

To start the installation of wooden floors, make sure the screed is:

• adequately moist - maximum moisture for cement screeds without underfloor heating is 2% on the CM scale, and for anhydrite screeds without underfloor heating is 0.5%. The maximum moisture level for cement screeds with underfloor heating is 1.8% on the CM scale and 0.3% for anhydrite screeds with underfloor heating,
• uniform - without cracks and significant irregularities,
• compact - the screed should be characterized by maximum cohesion, without tendencies to delaminate,
• hard - the hardness of the screed should be adjusted to the type of planned flooring,
• level - the maximum tolerance for unevenness is 2mm per every 1m or 3mm per every 2m, but at the same time it must not exceed 5mm for the entire length/width of the room. If larger irregularities are observed, they should be ground down before laying the floor.

The proper parameters of the screed are of great importance for the comfort of using wooden flooring and its later durability. As an experienced contractor, we operate on several installation systems, which allows us to offer our Clients solutions tailored to their needs. We care about the highest durability of the installed wooden floors, therefore we always propose to our Clients the most optimal installation system based on the condition of their screed.

Accurate measurement of screed moisture is very important, which is why we pay special attention to it before installing floors. Based on the measurement result, we determine whether further work can proceed or if, for example, prior screed drying will be necessary.

Before starting installation work, our technician assesses the screed moisture using the CM method (carbide method), which is one of the most popular methods for assessing substrate moisture. CM measurements are made using a CM hygrometer, which operates based on a chemical process where carbide (calcium carbide) is decomposed by water present in the screed. This chemical reaction produces gas (acetylene), causing an increase in pressure in a steel bottle. The screed moisture level is determined based on the reading from a manometer attached to the bottle cover.

Moisture measurement begins with creating a small hole in the screed using a hammer or chisel. This action cannot be performed with a drill or other tool, as it may distort the measurement result. The hole is typically around 2.5-3 cm deep, while the total screed thickness is usually 5 cm. The hole is necessary to extract material for testing from the lower part of the screed. A sample is taken from inside the hole using a metal spoon, then placed on a tray and thoroughly crushed. All work should be done wearing rubber gloves to avoid contaminating the sample and disrupting test results.

After crushing the sample, 20 g of dust is measured and placed in a special steel bottle along with a sample of carbide in a glass ampoule and metal balls. The bottle should be tightly closed and vigorously shaken for about 2-3 minutes. The metal balls will break the ampoule with carbide upon shaking, initiating the chemical reaction. After approximately 2 minutes, shake the container again, then wait about 20 minutes for the chemical reaction to complete. The manometer reading is then taken. The higher the screed moisture, the more gas will be produced due to the chemical reaction, resulting in a higher pressure reading.

In addition to the CM method, moisture measurement in the floor can also be done using desiccators. This method is more precise but requires specialized equipment and skills. Parkiet Studio, with years of experience in the industry and proper equipment, provides screed expertise using both the CM method and desiccators and the GANN electronic meter, depending on requirements.

If the screed moisture measurement result is too high compared to accepted standards, it will be necessary to bring the substrate to the desired state before proceeding with further work. Effective methods such as moisture barrier with appropriate primer or using dehumidifiers in the room can be employed to expedite achieving the desired moisture level.

During expertise, it is also necessary to carefully check the shear strength. It is assumed that it should not be less than 2N/mm2. Shear strength can be verified using the PressoMess meter. This device allows determining the surface hardness of the substrate. The device is placed between two wooden blocks, glued with cyanoacrylate adhesive to the substrate. In the next step, a knob is used, which, when turned, increases the supporting force acting on the blocks. The measurement of shear strength ends when one of the blocks is sheared. At that point, the final measurement result, which is the force exerted at the moment of shearing, can be read on the analog clock.

Two types of blocks are used in the measurements - long and short. The sizes and spacing of the blocks for this type of measurement are precisely determined by the manufacturer, so each such expertise provides reliable results. Although current regulations do not specify clear parameters regarding the desired shear strength, the measurement results allow our technicians to select the appropriate type of soil and adhesive for installing wooden floors.