The fact that ceramic pieces remain highly rigid when movements and deformations occur makes it necessary to leave spaces between pieces so that they are better able to absorb these stresses. Therefore, from a technical standpoint it is necessary to render installation joints between tiles, leaving a minimum width of 5 mm (open joint). Tiles should never be installed end-on-end (butt-jointed). The product that should be used to grout these joints is colorstuk, from the professional line of cement-based coloured mortars for grouting installation joints (type CG2, as per EN 13888). The water in the mix should be totally replaced with cl-stuk, a synthetic resin-based latex. This additive helps to increase the imperviousness of joints, while enhancing the grout's flexibility and bonding strength.
These joints are of fundamental importance in conserving the ceramic-tiled covering against the building's own structural movements as well as those brought on by external factors which may affect the façade. They help to reduce the stresses that may be transmitted to tiles and cause breakage or detachments. These joints should be rendered in those areas where the nature of the substrate changes, as in the case of windows, corners and shifts in plane along the façade. They should also be used in the presence of other structural elements, as with the union between floor slabs or fixed elements. Expansion joints should form sections no larger than 12-16 m2, with these being as square as possible; recommended joint width is 8-10 mm. To seal these joints, use s-502 acetic silicone-based sealant, which offers a high degree of elasticity and weather resistance.