Safe deflection for 24"x24" porcelain?

[Winghut]

Currently redoing the kitchen floor of my 1923 house and was thinking if putting some 24"x24" porcelain tiles. My question is: Should I consider a different deflection rating than the typical L360 given the huge size of the tiles?

My floor is made of: 3"X9" rough BC fir @ 22" on center with a 11' span, capped with structural t&g diagonal planking; for a theoretical L956. In practice, with the string and weight test, I get about 1mm (0.05") of deflection on center of span with a 260lb weight doing jumping jacks above (that would be me ).

I will re-screw the planking and cap with 5/8 t&g plywood screwed into the joists every 6" and then go with a Ditra membrane. Should this cover me for 24x24 tiles or should I compromise on the design and go with 18x18's? Would a mortar bed be better?

Thanks

[cx]

Welcome, Winghut. Give us a first name to use, eh?

The biggest consideration for those large-format tiles is the flatness of the floor. You appear to have plenty structure for whatever you want to lay on there.

But flat, flat, flat is the key. And the best and easiest way to achieve that is with a mud bed over your existing board floor. If you've got an inch or so of vertical room to spare, that's sure what I'd recommend.

Did I mention that your floor should be flat?

My opinion; worth price charged.

[Winghut]

Thanks for the reply, as a fellow member of the flat earth society I see that it makes perfect sense. I thought that deflection would dictate the installation but I see that flatness is the real issue in my case. While the floor is pretty straight considering the age of the house it does dip by about 3/8" away from the supporting beam towards the center of the room, which will have to be worked out somehow.

I have no problem with the cost of mudbed and it would be my first choice, however I only have a total of 1.5" to work with above the board floor, is this enough for a mud bed?
Thanks
Louis

[cx]

Aw, hell. I just wrote this big long post about how you could just mud over what you've got and then realized you've got that 22" joist spacing. That's the limiting factor here. Unless those boards are especially thick.

What I'm thinking is that I'd still want mud to make the thing flat, then maybe use Ditra or another membrane over that. I think I'd call Herr Schluter's people (800-472-4588) and describe exactly what you've got, tell'em you'd really like to mud the floor for flatness, and axe'em if they'd approve a Ditra installation over that. I'm thinkin' they'll go for it.

Or not.

My opinion; worth price charged.

[nforcer2]

going to very hard to get approval for a warrantied install with 22" on center. TCNA detail f149-05 is based on 24"on joists. you might want to give that a look see.

[jadnashua]

A big BUT here, did you notice that the joists are 3" wide? That puts the between joist distance at 19" or so, very near the 19.2 spacing...just my 2-cents in... I would think a mudbed should work, especially with the Ditra on top. Definately call Schluter.