I have a slightly different twist on the recent thread that answers the minimum thickness of sub floor underneath tile floors. In fact searching the topic lead me to this forum. I know what the hardie board instructions say versus the general consensus among pros.

My situation is a bit different: 1967 house...old tile floor in foyer and first floor 1/2 bath on 1/2 inch plywood, then the 5/8 subfloor. The whole job is about 175 SF. Ripped up the tile and 1/2 underlayment. When to Home Depot bought 1/4 hardie board and put half of it down with unmodified mortar and a 1/4 by1/4 trowel. Green cement board screws in every mark. Now the next day I notice squeaks in that section of the floor.

What are my options for reinforcement of the floor...hopefully without starting over? I can get to this floor from underneath although it is in a crawl space and not the most fun thing to do. Any advice is appreciated. Thanks