Cement board thickness question [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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08-06-2019, 07:09 PM

Please advise whether I might use a thinner (1/4" HardieBacker) vs. nominal 1/2" (actual 7/16") cement board on my job.

This remodel needed to consider matching floor height with adjacent existing hardwood. For that reason I have purposely kept kitchen floor elements as thin and low as possible, and therefore put only 5/8" plywood subfloor rather than 3/4".

HardieBacker installation Guide advises "We recommend 1/4 HardieBacker® board for floor applications, unless 1/2 thickness is needed for transition."

The HardieBacker installation guide also says 'ensure subfloor is structurally sound'.

But the floor wasn't structurally sound deflection-wise. It was near or under L/360.

Kitchen had 12' span, 2" x 8" joists. I have fully sistered all 7 joists, supported at ends. Mid-span, I've put tight solid blocking with Simpson Strongtie 6" L-brackets, which *seems* massively stout once bolted. I also put a strut from the foundation wall over to 2nd joist at mid-span for additional support.

After the improvement, the 'Deflectolator' says I'm at L/603 and says I'm OK for ceramic tile.

I'd like to use the 1/4" HardieBacker rather than the thicker cement board so adjacent floor heights will match, with no step up or down.

If I must use the thicker cement board, I'll still only have a 1/4" step up/down, which is not so bad, but not ideal.

Tile is 0.35" x 12" x 12" porcelain.

What I'm looking for is if someone has done something similar and had grout failure or NOT, and any other useful comments/advice.

Thanks very much.


P.S. In case you couldn't tell, I'm doing this on MY OWN house, not someone elses.

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08-06-2019, 07:33 PM
Although I prefer an actual cement based board (like Durock) you should be good with that. Be sure to embed the board with l rather loose coat of thinset just before you screw it down. "Walk" the Hardy down thoroughly as you screw it down.

08-06-2019, 08:49 PM
Welcome, Lee. :)

There is no advantage at all to the use of thicker CBU for floor applications as evidenced by James Hardie's recommendation of their thinner panels.

The use of the thinner plywood, by comparison, was not a good choice at all. I would not tile over a single layer of nominal 5/8ths" plywood using any tile installation substrate, despite manufacturers' recommendations of their minimum requirements. If your subflooring is attached to your sisters as well as your original joists, your subfloor is improved, but I still wouldn't have used that thinner plywood.

A thicker subfloor and a thinner tiling substrate would have been my choice there.

The between-joist blocking in your application, by the way, bought you nothing at all as far as the design deflection is concerned. Doesn't hurt anything to have it there, of course.

My opinion; worth price charged.

08-06-2019, 09:10 PM
Blocking between joists helps with a point load, but not a distributed load. IT also will change the resonant frequency of the floor, which can make it appear stronger. Essentially, blocking helps to prevent the joists from twisting, especially prior to the installation of the subflooring which, when installed with construction adhesive and good fasteners, locks things in place pretty well all by itself. But, as said, blocking does not change the deflection rating of the overall floor. It also increases the dead load of the floor, which could be a detriment, depending on what else is going on.