So, I've made a big stink with the flooring guys about how they have installed the Hardiebacker cement board for our tile. They skipped putting down the thin set under the cement board even though the installation instruction specify that it is required.

from instructions:

Apply a supporting bed of mortar or modified thinset to subfloor using a 1/4” square-notched trowel.

The flooring guy called me and said that JamesHardie company tech support said that if he used screws instead of nails they could forego the thin set under the cement board.

So I called JamesHardie tech support and their offices are currently closed... So I can't verify this claim. This seems quite shady.

Am I wrong to push the issue or should I just let him use the screws and go about the job? They have umpteen years experience doing this stuff.

Also, will this cause problems down the road, and should I continue pushing the issue?

  • Tell them that you can't currently confirm, so they can go ahead, but if you later find from JamesHardie that they're wrong they'll have a big rework project on their hands. Jan 6, 2016 at 17:14
  • @Daniel Griscom Yeah that's what I was thinking too. Jan 6, 2016 at 17:22
  • Most contractors dont use thinset just screws. but the floor will be much stronger (less chance of cracking a tile if something is dropped on it) if thinset and screws are used.
    – Ed Beal
    Jan 6, 2016 at 17:22
  • It's not so much a matter as what's required as it's about what makes a good finished job. Will dry-setting the cementboard result in creaks or other noise? That would be my main concern.
    – isherwood
    Jan 6, 2016 at 17:23
  • @isherwood I agree but the only argument I have for them doing more work is the requirements specified by the manufacturer. My question is just what you asked, will this cause problems down the road from a professional's point of view and should I continue pushing the issue? I'll add that to the post. Jan 6, 2016 at 17:28

4 Answers 4


This is a subjective question. Here are the factors as I see them:

  1. Hardi apparently doesn't require mortar under its cement board if it's screwed down properly. The guide referenced above seems to be more suggestion than warranty requirement. If the fastener schedule provided is followed closely, problems are not likely to occur. However, a mortared system will be more rigid, potentially reducing tile and grout cracks between floor joists.

  2. Your subfloor is OSB, so it's questionable whether a mortar bond is wise at all. OSB is widely considered to be unsuitable as a tile substrate due to its tendency to swell and delaminate from moisture, but this is a bit different. It may have worked out with no ill effects. The mortar would serve to prevent any gaps between the layers of the substrate system, improving stability.

  3. Your contract apparently doesn't stipulate mortar in this case. With Hardi's blessing, you probably have no legal standing to insist on mortar.

In my opinion, you'll probably be satisfied with the results if only screws are used. It's difficult to predict how much floor movement will occur with the information provided. That said, a mortar bond would generally be considered a higher-quality installation.

In the end, the decision comes down to your willingness to settle for 95% quality or to deal with the dispute between yourself and your contractor.

  • 2
    I've agreed to go with just the screws, which is a step up from the roofing nails. But I agreed in order to keep it civil. So essentially I settled. Jan 7, 2016 at 21:51
  • It's definitely your house and job. If you do not like something or have concerns, you definitely were right in piping up and saying something to them. As long as they will stand behind their work and have been in business more than a few years, you should have some peace of mind in assuming they do not want to come back and fix it for free and completed the job in such a way to avoid just that. Reputability and integrity are important when choosing a contractor for these reasons.
    – Damon
    Jan 8, 2016 at 1:57

You cannot put tile on OSB because it is not rigid enough. If you do so, eventually it will crack.

You have to remove the OSB and put down high-rigidity plywood, then a membrane then a cement backer board OR use a wood subfloor with a full mortar bed.

  • 2
    I think you've misunderstood the question.
    – isherwood
    Jan 7, 2016 at 21:01

Tyler answered your question nothing really bonds to osb. Even screws eventually get loose on foot traffic. Putting mortar on it causes it to flake. take it up and install plywood 3/4” or install a floating floor.

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    – Community Bot
    Nov 15, 2021 at 19:18
  • Plenty of things bond to OSB. It's just not a good idea to use water-based products. Solvent-based construction adhesive, for example, bonds extremely well.
    – isherwood
    Feb 22 at 2:50

Nothing better than glue AND screw. Have them re-do it regardless. They can't say it's worse & they should be there to make you as happy as they can. Especially for being someone actually informed, they should want to impress.

  • 1
    This answer is better as a comment. It's useful information but doesn't answer the question.
    – Matthew
    Jun 14, 2018 at 2:44

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