Question: What is the suggested means of insallation of a travertine 1.25" thick over a 400/ sqft outdoor area with no roof?
Answer: If you already have concrete in place, you can lay the travertine pavers on top of sand by creating a mud set border in order to keep the deck in place. If not, a gravel and sand base installation would be best.
You can also choose to mud-set (wet-set) the whole area. For directions on installation, please refer to: http://www.travertinemart.com/travertine-installation
Please note: In freeze/thaw climates, sand-set is the only recommended installation method. If you need to mortar the travertine, use a flexible mortar such as Flex-Bond.