What subfloor on joists and under tiles?

You don’t need to put cement board on floor. Use 18mm osb board or plywood( water resistant)

Definitely board with 18mm plywood or chipboard flooring before the hardie backer board

Hi, On all of our bathroom jobs, we would board with an 18mm ply first and then depending on your choice of flooring either an anti-cracking membrane (Ditra mat or similar) or Hardie Backer. Hope this helps

Ask yourself, are the backer cement boards flexible. The joists will flex during the seasons so any standard boarding is advised in place first. Keep to floor strengthening plywood and flexible tile adhesives as per manufacturers instructions

We recommend 1/4 in HardieBacker® board for floor applications, unless 1/2 in thickness is needed for transition. 1 | Ensure subfloor is structurally sound On existing structures: • Ensure subfloor is not damaged. Replace any loose, warped, uneven or damaged sections of floor. • Make certain subfloor is a clean and flat surface. For all floors: • Use minimum 5/8 exterior grade plywood or 23/32 OSB with Exposure 1 classification or better, complying with local building codes and ANSI A108.11. • Joist spacing not to exceed 24 in on center. • The floor must be engineered not to exceed the L/360 deflection criteria (L/720 for natural stone), including live and dead design loads, for the specific joist spacing used. 2 | Determine layout of HardieBacker cement board • Stagger all HardieBacker cement board joints. Do not align with subfloor joints. • Never allow all four corners of boards to meet at one point. • We recommend an 1/8 in gap between board edges. • Keep sheet edges 1/8 in back from walls and cabinet bases. • Score and snap boards to required sizes and make necessary cutouts. 3 | Attach HardieBacker cement board to subfloor • Apply a supporting bed of mortar or modified thinset to subfloor using a 1/4 in square-notched trowel. • Embed HardieBacker cement board firmly and evenly in the wet mortar. • Use the fastener pattern as a guide. Fasten HardieBacker cement board with specified nails or screws (as listed in “Materials Required”) every 8 in over the entire surface. Keep fasteners between 3/8 in and 3/4 in from board edges and 2 in from board corners. • Set fastener heads flush with the surface without overdriving. 4 | Tape joints prior to tiling • Prior to setting the tile, fill all joints with the same mortar used to set the tiles. • Embed 2 in wide high-strength alkali-resistant glass fiber tape in the mortar and level.