The first step is to prepare screed, a horizontal element made from water, cement and inert minerals. The flatter and more high quality the screed, the greater the aesthetic result and the durability of the floor.
How do you prepare screed?
Bonding screed
Elements that bond to the supporting surface, ideal if there is only a thin surface on which to place the screed (less than 35mm). It is advisable to place an adhesive grout on the surface (synthetic rubber latex and the binding agent used to make the screed). The fresh screed mix will be placed on the fresh grout (this is called "fresh on fresh").
Isolating screed
Screed that lies above an isolating layer, attached to the supporting structure; has a thickness of 35/40mm. Isolating screed is a preferred solution to adhesive screed, provided it is thick enough, as it is unaffected by bending and deformation of the supporting structure. First, the isolating layer must be laid; then one layer of the compressible material along the walls (such as expanded polystyrene). In order to create floating isolating screed, an electro-welded mesh will have to be inserted through the centre due to the low mechanical strength and the high compressibility of the isolating material. To make heated isolating screed, add super-plasticising admixtures to the screed (or special cement binders/premixed mortar), that are designed to incorporate heating coils.
How to spread the screed?
Properly laying the screed is essential for achieving a level surface on which to install the new tiles. If the screed contains pipes, ensure a thickness of at least 2.5 cm; place a metal mesh over the pipes to prevent cracking. After creating leveling strips, lay the screed and tamp it down while compressing and leveling the mixture. Finish the surface with specialized tools, taking care not to over-trowel to avoid issues such as bleeding.
Before laying the new tiles, check that the supporting surface is:
- resistant to mechanical stress, guaranteed by solidity across the whole depth of the screed.
- cleaning: it will be necessary to remove any elements that could compromise the bonding of the new flooring (grease, oil, plaster, paint, etc.).
- no cracks, it is imperative to seal any cracks/splits that may be present in the screed before laying.
- left to dry, i.e. the screed must have completed its hygrometric shrinking (period of 7-10 days per cm thick) before laying, to avoid creating cracks after laying which could cause it to detach or break.
