Floor Thickness & Joist Spacing [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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02-12-2010, 05:35 AM
Folks, need some assistance, got 3/4 OSB T&G over 10" I-Joints 24" OC in my kitchen. I want to lay tile but am concerned about potential flex in floor cracking my tiles. I plan to screw existing OSB as a back up to the nails as I have a little movement over the joints/joists currently. I have 1/2" Hardi, is that too much, can I go with 1/4", or do I need to add another laying of exterior grade plywood perpendicular to OSB, then lay down the 1/4" Hardi? Thanks in advance!


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02-12-2010, 05:55 AM
STOP! Don't do anything yet.

The only underlayment I heard of that will do 24oc is ditra..However I believe it is only for plywood not OSB. Im not the guy to ask though.

wait until someone more familiar with ditra specs shows up who can advise you better then I.

02-12-2010, 05:58 AM
If i am correct, Ditra not cement board or hardi is the only product that can be used over a single layer of 3/4" osb subfloor over I - joists.
over any even and structurally sound OSB or
plywood subfloor with 24" (610 mm) o.c.
joist spacing

I'm not sure if cement board can be used IF you install another layer of plywood over the existing subfloor. someone else should be along soon to verify.

Jim wood
02-12-2010, 08:39 AM
How far do those 10" I-joist span?

Dave Gobis
02-12-2010, 10:53 AM
Any method is going to require an underlayment on 24" centers. How much depends on the product you select.

02-12-2010, 11:22 AM
Welcome, Joe. :)

I trust you really meant joist spacing in your title? If so, we'll fix that for ya.

I wouldn't use any tiling substrate of any kind without another layer of plywood on that floor. Kinda like Mr. Gobis is pointing out.

Single-layer plywood, even 3/4" plywood, over 16" centers is too close to marginal even to suit my taste. Same over 24" centers is just asking for trouble.

My opinion; worth price charged.

02-12-2010, 11:39 AM
and remember, cement board adds no (significant) structural integrity, neither does any plywood thinner than 1/2".

02-13-2010, 06:22 AM
Gentlemen, thanks the quick feedback- greatly appreciated! CX, yes, meant joists not joints-thanks. So it sounds like I need to beef up the floor thickness by at least half inch (strengthen/stiffen floor) prior to putting any system (Hardi, Schluter's, etc) on top followed by tiles. Question, anyone familiar with Duraceramic tiles, are they a little more forgiving, or should I handle the same as a ceramic/porcelain tile regarding floor thickness prior to laying tiles.

02-15-2010, 04:08 PM
TEC Specialty also has their Single Layer Floor System that appears to be appropriate for your situation. Please see the link below.