Points to remember when laying Cotto outdoors: 

GRADIENTS

Gradients must be incorporated beneath the waterproofing layer and can be as steep or shallow as appropriate, but no less than 1.5 – 2 %. If gradients are not properly incorporated, water will collect underneath the tiles and cause problems in freezing temperatures. The formation of ice could even cause the tiles or the screed beneath to lift. 

WATERPROOFING

When laying floors on a slab or deck, the surface must be rendered waterproof before putting down a substrate. The waterproof membrane can be applied using whatever proprietary product may suit the application, but whatever system is adopted, it is essential that the water running off the membrane should be channelled into a drain. To ensure that water drains properly from the substrate, it may help to add an additional layer of material over the waterproofing layer, consisting of a sheet with a dovetail-channelled sectional profile especially designed for the purpose.

SEPARATION AND SLIPPAGE LAYER

This is needed for any floor laid over a waterproof membrane, and can be provided by an ordinary sheet of polythene. The function of this interfacing layer is to prevent direct contact between the waterproof membrane on the one hand, applied normally to the substrate or slab, and the screed on the other. Without it, the opposing forces of shrinkage may cause the membrane to tear and this inevitably will mean that water cannot be kept out. 

EXPANSION JOINTS

Expansion joints serve to accommodate any background movement in the fabric of the building; they must be incorporated both into the main structure and into the screed and floor covering. A flexible joint must also be included around the entire perimeter of the tiled area, where the floor meets the walls.