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pictures: https://i.stack.imgur.com/W4j22.jpg

(the holes you see go are connected to the crawlspace)

I had a badly built vessel sink here that was leaking from many areas. As I tore down to repair, it ended up needing to be a complete overhaul. I found out there was nothing underneath the sink.

I want to add the correct subfloor material to bring it to level where there is enough spacing left that I can install penny tile.

The existing flooring looks like there was some rooftop felt laid down then stapled metal lath laid on top of the joists. Then they poured concrete, put a cement board on top and then installed penny file flooring? Is this correct?

Would following the same method be wise?

If so, what kind of concrete would I use on top of the felt/lath? How do I attach the cement board to the concrete?

Otherwise, is stacking plywood a wise idea?

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  • recommend backer boards
    – Traveler
    Dec 2, 2022 at 23:07
  • 2
    What you are describing is a standard tile base in older houses. It's still acceptable, and even has some advantages, but almost nobody does it that way in new work, as thick plywood + backerboard is a lot faster.
    – Ecnerwal
    Dec 2, 2022 at 23:12
  • @Ecnerwal, Is that an Answer?
    – keshlam
    Dec 3, 2022 at 14:48
  • No, this is: diy.stackexchange.com/a/40066/18078 TCNA F145-02
    – Ecnerwal
    Dec 3, 2022 at 15:14

3 Answers 3

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The modern best practice is 1 1/4" thick substrate. Two layers of 5/8 ply stagger the seams. You want to minimize deflection to avoid future grout join cracks. This depends on your joist spacing/size and span. You can do less but the no-think way is 1 1/4" thick subfloor.

TCNA - Tile Council North America - publishes a guide that goes into details for all tiling best practices.

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  • Thank you, we ended up doing exactly this. We added a joist to the left. 23/32" OSB on top, 1/2" plywood ontop of that, 1/2" backer board on top of that. Each layer got screwed down very well.
    – byte
    Dec 5, 2022 at 0:47
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We did it right the old way.

rooftop felt laid down then stapled metal lath laid on top of the joists. Then they poured concrete, put a cement board on top and then installed penny file flooring

The felt was for humidity barrier, the metal lath was for concrete adhesion to the felt.

The cement board, now called backboard is still used, but placed directly on the play wood without concrete layer and without the mesh and secured with screws to the sub floor.

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If it is all wooden then you will need very stable support for the top tile layer. General advise is to avoid tiling/cement on wooden floor and use more flexible materials (linoleum for humid place would be the simplest). So if going for e.g. linoleum, roughly there are few steps to create support:

  • recreate even mounting surface (I assume from the photo these would be top surface of the lies) using for example special floor polyurethane putty.
  • mount strong but flexible plate like thick OSB or plywood
  • do waterproof finishing so it won't decay with time ( I used primer + polyurethane putty)

If you still want tiling then there should be more effort to make the support monolithic, so the tiling won't crack. Another layer of plywood or steel lattice might do the job.

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