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Would it be correct to tack HardieBacker to the floor joists directly or is it necessary to lay exterior plywood when renovating/installing a new bath floor? my home is 140 years old and the floor joists are massive. My fear is that the plywood will attract termites as I have had issues with them attracted and devouring new wood used in the house and they have cost me thousands in damages. The old wood is generally left untouched by the termites since it is old growth cypress.

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    I've updated your product name and added a link. Hardie Board is siding. Please revise again if I missed the mark on anything. – isherwood Feb 20 '20 at 19:30
  • No. For literally the tenth time. minimum 5/8ths exterior grade plywood or OSB. – Mazura Feb 21 '20 at 0:48
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No, Hardie cement board is not meant to be structural, and it will not work in place of a subfloor. The board is very strong in compression and very stable so that the tiles applied to it do not crack, but it has very little shear strength.

There are treatments you can put on wood to make it less attractive to insects. I would recommend approaching the problem from that direction and use traditional, proven materials for the subfloor.

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  • Thank you, that was exemplary advice. Much appreciated. I assume boracare or other termite proofing chemicals? – tom Feb 20 '20 at 17:48
  • I have heard of boracare being used, but I don't have any direct experience using it or other alternatives. – JPhi1618 Feb 20 '20 at 17:50
  • Correct in general, but tensile strength is a more relevant concern, no? Shear and bending strength are only distantly related. – isherwood Feb 20 '20 at 19:31
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    @isherwood, that might be a better term - I was thinking about the bending forces if you stepped in the middle of two joists. – JPhi1618 Feb 20 '20 at 19:40

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