For just a ¼ inch Hardiebacker use 1 ¼ inch screws. Now tape the seams working with fiber mesh tape. Use a flat trowel and thinset to go more than it. Let it overcome overnight in advance of tiling. Placing your shower outside the house is a great concept. Use https://buildingregulations39505.blogdeazar.com/36831627/the-ultimate-guide-to-bathroom-tiling