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I am installing hardiebacker concrete board with thinset to a plywood subfloor.

I am putting down the new subfloor and want to be sure the screws that I use to secure the plywood to the joists will not corrode from the layer of thinset over the subfloor.

All the floor screws I could find in the box stores are black phosphate screws. Are these black phosphate screws alkali resistant and will they be OK in contact with a layer of thinset and cement board on top of the subfloor?

  • My box store had "Backer-On" and "Rock-On" brand screws that were designed for concrete backer boards of various types. Look near where the backer boards are stocked. – DaveM Jul 6 at 2:28
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Phosphate offers no more protection than galvanized or paint coating ; The good news is once the thinset cures ,they will always be dry so corrosion won't be a problem.

  • Not worried so much about moisture as alkali exposure to mortar and concrete. My understanding is the alkaline characteristic of mortar and concrete can cause corrosion but I don't know whether phosphate screws offer protection or not. – Chrysippus Jul 6 at 1:59
  • The thinset will hold the board down after it dries, the screws in my opinion simply hold the board in place until it sets up and dries. If there were no thinset, then I would be concerned about the longevity of the screws. – Jack Jul 6 at 5:30

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