I'm in the process of renovating my kitchen. I bought enough tile to cover my kicthen floor and the staircase landing for basement stairs/side door. I'm concerned about the strength of the landing. It was contructed with 2x4 framing, 16 OC, with 3/4" oak flooring. There is no subfloor, just finshed tongue and grove flooring. There is about 3/4" gap from the floor to the bottom of the door. The landing is 36x36. My plan is to rip out the existing hardwood floor and add sister joists from 2x4 studs, making them 3-4" OC. Then, I will glue and screw 1/2 plywood on top. I plan to use 1/4 hardi backer for tile base. Do you think this will be strong enough? I know everyone on this forum says minimum subfloor must be 1-1/4 thick but I can't do it in the case as the bottom of the door will hit. I think 2x4 joists, 3-4" OC will add a lot more strength than another layer of plywood. Please let me know what you think. Thanks.

Ok so here is the story. I.m from England and now live in Virginia. I worked for a local building contractor back home and did my fair share of tiling. back home we used "flooring" adhesive for floors, bathroom adhesive for bathrooms, you get the picture. Over here in the states i get thinset of various kinds. so my question is what do i use to tile a shower on backer board or any other board for that matter? The guy at Lowes told me, regular thin set mixed with the fortifier at $30 a go.....seems a bit expensive to me or the premixed stuff. don't want to get it wrong....the wife will kill me if they all drop off............Please help, this project has gone on too long and according to the wife a oneway ticket back to England is beckoning................haha