Leveling a low spot AFTER laying HardieBacker Board

I put down half of my HB boards before noticing that my floor drops 1/4 inch in a span of 17 inches. Do you think it will work if I build a 1/4 inch form along the wall on the low side and use thinset to build up the floor? I've attached a diagram of the bathroom in order for you to visualize what I'm facing.


I posted a similar thread on John Bridges site - my apologies if cross posting is frowned upon.

Thanks for any help!

Jim

1/4" is about the max you can use with thinset, but yes, it should work. I'd burn it into the surface to get good adhesion (it's always a good idea to take a sponge and wet Hardi before applying thinset - it sucks moisture quickly) then use a long straightedge to screed it flat. Thinset sticks to thinset fine, and will stick to the cbu, especially well, if you clean the dust off and add a little moisture prior to application of the thinset. For this app, not as important as when setting tile. When setting the tile, you need some time to get them installed. Here, you don't really care if it is still sticky since you are using it as a filler.

Shouldn't be a problem at all. The tile should cover the transition areas along the edge just fine.

Just another point...all cbu on the floor needs to be set in thinset. This is not to actually hold the cbu to the floor, but to ensure it is 100% supported with no voids. The screws or nails hold it. Actually, over the years, the cbu ends up getting crushed around the fastener holes as the plywood subflooring expands and contracts - it's just that the cbu (with the joints reinforced with the special tape) moves as a unit with the tile, sort of floating on the subfloor. If you tried to flatten the floor before installing the cbu, you'd possibly break up that thinset driving nails or screws through it...this would leave you with less than the 100% coverage, and potentially that disheartening crunch as you walk across that point.

Thanks for your replies

I set the remaining cbu in thinset and will tape the seams tonight. I'll post back after leveling the floor this weekend and let everyone know my results.

Jim