Proper subfloor thickness [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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04-12-2011, 08:19 AM
Hi All,
This is probably a very elementary question, but Ive asked it a few times and have gotten conflicting answers. Im planning on laying a mosaic tile floor; 1 x x 3/8 thick mesh-backed marble basket weave pattern. Ive got full 2 x 10 joists sistered with new nominal 2 x 6s that I installed to compensate for settlement that has occurred over the course of my homes 90 year lifespan. My tile guy told me that installing AC exterior ply would suffice as a subfloor. Ive painstakingly installed and leveled all the sistered joists with this dimension in mind so that the finished floor will match the plane of the floor in the adjoining room. Im now reading on some tile forums that I should have 1 worth of subfloor under my tile to prevent deflection. I dont want to pull out all the sistered 2x6s and cut meat out of the original 2 x 10 joists. That just doesnt seem like a logical direction. If deflection is probable with a subfloor, Id rather add cross bracing on 16 centers to stiffen things up and add more support under the ply. Any suggestions? Am I worried about nothing of substance?

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04-12-2011, 08:42 AM
No, you are rightfully worried. Natural stone has hidden flaws that make a stiff subfloor and supporting structure necessary. I don't have enough information to discuss your joists, but I do know that you will need at least 3/8" more subfloor than what you have now. You will also need an underlayment of some sort, too.

If you cannot tolerate an increase of about 1/2" of floor height, then we must look under the subfloor for ways to beef things up. I don't know that you could add enough cross blocking to do the job. I do think that you could install some plywood directly under your subfloor and support that on cleats glued and screwed to the sides of your joists. Use 3/4" plywood and cut it so the face grain runs perpendicular to the joists. 2x2 cleats would be fine. Once you have the new plywood in, drive screws in from the top to tie them together.

04-12-2011, 09:25 AM
Thanks for the comments. If I decide to add another 3/8 to the height of the floor, Id do that by gluing & screwing a sheet of 3/8 AC ply with grain perpendicular to the ply? For glue, are we talking yellow woodworking glue or something like a PL 200 construction adhesive troweled on for uniform thickness?

04-12-2011, 10:21 AM
You don't need to glue the new plywood on top. Screws alone are sufficient. However, if you did want to glue, use a yellow waterproof carpenter's glue and spread it thin and uniformly over the area.

04-12-2011, 10:42 AM
Many thanks

04-12-2011, 12:53 PM
quote.. " I’d do that by gluing & screwing a sheet of 3/8” AC ply with grain perpendicular to the ” ply? "

The additional wood should be placed peropindicular to the joists, not to perpindicular to the plywood that is already there. Both layers should be running the same way. Do not use drywall screws if screwing. see the links.