Wood and outside is not a good combo. In fact its sooooo not recommended, there is nothing relating to it in british standards. Therefore none of the adhesive and accessory manufacturers (including Bal, Schluter, Ardex) will back you if you even attempt it. When i was asked before, even when i enquired about laying marmox, hardie and ditra, it was still an overwhelming No from all of them for the fact its still all screwed/glued to wood which is affected in changes in weather/tempetature.