What is a screed and what is it intended for? Types of screeds

According to SNiP 2.03.13-88 “Floors”, a screed (base for a covering) is a floor layer that serves to level the surface of the underlying floor layer or floor, give the floor covering on the floor a given slope, cover various pipelines, distribute loads across the non-rigid underlying floor layers on overlap

In accordance with the definition, screeds should be used in cases where it is necessary:

• leveling the surface of the underlying layer;
• covering pipelines;
• distribution of loads across heat and sound insulating layers;
• creating a slope in layers on floors.

They can be directly the finishing surface or can serve as an intermediate layer for laying the finishing coating on them.

According to their design, screeds are divided into solid (single-layer and multi-layer) and prefabricated. A single-layer screed is a screed that is laid in one layer and immediately over its entire thickness. A multilayer screed is a screed consisting of several sequential layers, and each layer must be adhered to its neighbors. Prefabricated screeds consist of prefabricated elements that are completely ready for installation.

Previously, solid screeds were most often made from cement-sand mortar. The main disadvantage of such screeds was the need for a long period of time for the mortar to gain strength and dry before laying the finishing coating, which extended the work time, and shortening this time period led to defects. Now quick-setting mixtures have appeared. If, after pouring a 40 mm thick cement-sand screed, for example, laying ceramic tiles used to take at least a month to achieve the recommended humidity, but now when using mixtures with polymer additives it only takes a few days.

Modern screeds are, as a rule, two-layer structures consisting of: the first - a rough, leveling layer, and the second - a thin, finishing layer. When choosing a material for constructing two-layer screeds, you must first find out what binder they are made of. Modern dry mixes are made using cement binder, cement-polymer, gypsum and gypsum-polymer. You need to remember about the compatibility of materials. If, for example, the rough leveling layer of the screed is made from dry mixes with a cement binder, and the finishing leveling layer is made of dry mixes with a gypsum binder, then a primer layer must be made between them. Because cement binders give a mild alkaline reaction, while gypsum binders give an almost neutral reaction. Two layers of the same screed made of dissimilar materials lead to gypsum leaching. A gypsum screed layer can be made over a cement layer through a layer of primer, but a cement screed layer cannot be made over a gypsum screed - hardened gypsum is afraid of water, and a damp cement layer will turn it into mush. When choosing a screed material, one more point needs to be taken into account. Screeds based on cement binders are shrinkage, that is, they can crack during the setting process. To reduce cracking, polymer additives are added to dry mixtures. Screeds based on gypsum binder are non-shrinking and more environmentally friendly, they are vapor permeable. However, when very wet, gypsum, unlike cement, turns back into dough. Gypsum screeds are suitable only for dry ones, and cement screeds are suitable for all types of premises.

According to the method of adhesion to the ceiling, screeds are divided into those connected to the base, on the separating layer, and “floating” screeds on the insulating layer (screeds connected to the base on the separating layer).

Bonded ties are ties that are interlocked with the base. With this arrangement, there are no separating layers between the screed and the base. Bound ties can withstand significant loads. However, they are characterized by uneven shrinkage, often leading to the formation of cracks. In addition, the moisture content in such screeds is highly dependent on the moisture content of the floor.

To reduce dependence on base moisture, screeds are used on a separating layer , which prevents adhesion of the screed to the ceiling. Bituminous paper, oiled paper (materials such as glassine and roofing felt) or polyethylene films can be used as a separating layer. To ensure the required strength, the thickness of such ties must be at least 30 mm. Laying the screed on the separating layer is justified in cases where special waterproofing measures are required.

“Floating” screeds on the insulating layer are not connected to the base, but represent an independent building structure. The underlying layer between the concrete floor and the “floating” screed consists of heat and sound insulating materials: stone or mineral wool, foamed polystyrene, cork boards, fiber boards. The minimum thickness of “floating” screeds is 50 mm. The arrangement of the screed on the insulating layer makes it possible to increase the heat and sound insulation of the floor; in addition, the moisture content in the screed does not depend on the concrete floor. However, this design scheme is characterized by a relatively low compressive strength, as well as an increased thickness of the structure. This type of screed often requires additional reinforcement of the top layer.

Prefabricated screeds consist of elements that are completely ready for installation, designed to be connected by seams and transfer loads. They are mounted from large-sized sheets and slabs - plywood, chipboard and fiberboard, gypsum fiber (GVL) sheets. The mass of the elements of prefabricated screeds is small, which allows one person to cope with the installation. The use of prefabricated screeds eliminates “wet” processes, so you can begin laying the face covering almost immediately. However, the use of prefabricated ties is not possible for all types of face coverings.

​When calculating the strength of the base for the coating, pay attention to the following values. According to SNiP 2.03.13-88 “Floors”, the strength of a screed based on a cement binder for all types of floor coverings must be at least 150 kg/cm², and if the screed will be leveled with liquid polymer mixtures - at least 200 kg/cm². The strength of screeds based on gypsum binder for all types of floor coverings must be at least 100 kg/cm², for self-leveling polymer mixtures - at least 200 kg/cm². It is also necessary to take into account that when completely replacing or installing a new screed, the weight of the screed will put pressure on the reinforced concrete floor slabs. Reinforced concrete floors in old houses have a load-bearing capacity limitation of approximately 400 kg/m² - permanent load and plus 150 kg/m² - temporary load. For example, every centimeter of height of a cement-sand screed with a volumetric weight of 1800 kg/m³ will put pressure on a floor weighing 18 kg/m², and a screed 5 cm thick will transfer a load of 90 kg to each square meter. From these figures it follows that such a screed will take on almost 25% of the load-bearing capacity of the floor.