If you want to install your slate floor tiles to your outdoor patio or deck, then you will need to follow a couple of rules. Slate tile can be installed outside, but it can only be properly installed on a durable surface such as mounted plywood or concrete. Slate can not be installed on dirt, grass or sand. The plywood should be at least ½-inch thick and be exterior grade.
Installing a slate floor outdoors means that the mortar will need to be able to handle temperature changes. Use an acrylic or latex mortar to install your outdoor slate floor. The best guideline to use for exterior mortar is one that complies with American National Standards Institute code A118.4.