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4just1don 01-06-2009 01:02 PM

Ceramic tile subfloor and joists.
Was looking at my joist situation again under my proposed bath ceramic tile.

2X8's 20 inches on center( the span is 10 ' from outside rim joist to center support wall),,,plumbing stack wall is getting a double 2X6 sistered joist since the plumber 50 years ago decided to whack off joist twice in 6 feet,once for stack once, for tub install.

anyway there are plenty of other obstacles but the easist way to shore this up would be 2X6 with another 2 bye underneath for blocking on both ends. I am proposing to cut that span in half. 2 X 8's would be TOO hard to get in there. the 6's will turn,,,8's wont and have a cold air return under it to compound probs!!

It has 1 bye sub floor boards run on diagonal, non tongue and groove, and over that I am proposing 5/8" plywood(does it NEED to be treated??),,,then either 1/2 concret backer or ditra!! THEN ceramic tile.

There are three joist spaces that need shoring up and stregthening. Floors are solid now but bounce should be 'better' . all total the floor for tile is like 4'6" wide and 8 ' long because the tub is across entire outside wall

angus242 01-06-2009 01:37 PM

Just based off the 2x8's with 20"OC and 10' unsupported span, that should be sufficient joist specs to tile over. However, the issues you mention obviously change things. I'd definitely add support where you can.
As for the subfloor, instead of using 5/8" and then 1/2" backer, why not use 3/4" and then Ditra? Deflection is two-fold. Of course, there's the joists but then there's also the area between the joists. With 20"OC, I'd like to see as much ply as possible. That's what I'd recommend. And yes, spec's state exterior grade plywood although that doesn't mean treated. I believe (and I may be mistaken), exterior grade has to do with the glue that binds the layers, not treating the wood itself.

JazMan 01-06-2009 06:29 PM

I didn't know they placed joists at 20" o.c. 50 years ago. These days 19.2" is a recognized method. Do you know the species and grade of those joists? Probably not I'd guess. It makes a difference though.

I checked my span chart entering SYP, (usually the best) grade #2 at 19.2" and got an OK for tile up to 10' 5" span. This is the bare minimum standard if the joists are in excellent condition. So, if your unsupported span is 10', you're close to the limit of what I'd recommend. How does the floor feel when you walk and jump? While you're under there doing that other work, can you screw or bolt some lumber to the sides of the joists?


4just1don 01-07-2009 12:01 AM

It MAY have been 19.2". Its HARD to tell down there,,,light is poor,eyes are poor,,,bifocals dont help looking up,,,not alot of space to sit,,have to lay mostly. next time I will take my 'reading glasses' down to help see. I just thought 20" would be the worst it can be,,,didnt see WHY it was so odd tho.

figuring the 6" on outside and 3 " on inside support,,free span would be like 9' 3"

would it be 'better to sister 2 X 8's with ends trimmed enough to get them to 'turn'??? Just thought the 6's would be so much easier to get in and take alot of abuse if centered to cut spaces to like 10" or less. I still have the issue of the nails thru the floor bothering,,but they do break off rather well.(just another sight issue,looking up)

Thanks for the post!!

JazMan 01-07-2009 11:25 AM

The new span measurement of 9'3" improves the deflection to a comfortable level. All the joists that are in good condition do not need attention. Would still like to know the species and grade. However, when sistering, you do not have to run the new lumber the entire length to match the other joist. Sistering the mid 2/3 will do wonders. You can also use smaller dimensional lumber such as 2x6 or even 2x4's. Heck you could even use plywood.


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