mouths the wall defining an opening adapted for placing the first chamber and the second chamber in communication application_number US 201213598322 A description in particular the invention discloses a spacing levelling device for laying slab formed products for cladding surfaces that comprises a base positionable below the laying surface of at least two adjacent and flanked slab products with respect to a flanking direction from which at least a separating element projects suitable for contacting at least a portion of the facing flanks of the two slabs and presser means associated to the separator element suitable for pressing the in view surfaces of the slab products such as to level them characterised in that it comprises at least a corner spacer which projects from the base with respect to the separating element and is suitable for coming into contact with the flanks that are perpendicular to the facing flanks of the slab products for aligning thereof along a perpendicular direction to the flanking direction the corner spacer being mobile between a raised position in which it projects superiorly of the base and a non interfering position with the perpendicular flanks of the products or a lowered position for example in the non interfering position the area of the corner spacer is at least partially contained within the area of the base thanks to this solution the spacing levelling element can be used both at the lateral edges of two slab products to be flanked and at the corners of the products to.