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, 2019 at 2:28

1 Answer 1


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, 2019 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, 2019 at 5:30
  • @Jack the thinset will hold the cement board to the plywood subfloor, but as I read it, the question is about holding the subfloor to the joists. Those screw heads will be exposed to the thinset, and the alkaline nature of the thinset in contact with the screw heads is the question. Without the thinset, there wouldn't be a question about the screw life.
    – FreeMan
    Nov 27, 2020 at 14:54
  • After I read it again, you are correct. It was regarding only what would happen to the screws below the thinset. blacksmiths answer is spot on. Thanks for the clarification.
    – Jack
    Nov 28, 2020 at 4:04

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