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Unread 03-19-2021, 09:19 PM   #120
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Join Date: Jul 2001
Location: Boerne, Texas
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Originally Posted by Karen
Would the required stiffness be less for such a mosaic? I ask because I wonder if that is why the original tile for the bathroom put in back in 1954 was a 1X1 mosaic.
No. In fact, I've always thought the stiffness requirement should be much higher for small mosaics as it is for natural stone. A 1x1" mosaic floor is little more than a full spread of thinset mortar and grout with some little stones pressed into it. No real dimensional strength of its own to speak of. But once again, the industry did not invite me to vote.
Originally Posted by Karen
The house was originally built in the early-mid 50's as a very small 2 bedroom ranch. If the 1X1 mosaic is the original floor, does that mean it likely was installed on a mud base?
Almost certainly. You've not tried to find out what's really under any of your tile installations? Such restraint!

We got away with a lot back in the days when there was no substrate for ceramic tile installations other than mortar beds. That was for floors and walls and counter tops and anywhere else a fella wanted to set some ceramic or stone tiles.

While you can certainly remove your tile installation and mud bed (I'm presuming) and install plywood over your board subflooring, install a "modern" tile substrate of your choice and take your chances, you cannot expect the structure to provide you the same margins as it did with the mortar bed. Yes, some of the modern substrates are excellent, but you'll note they all require the same subflooring structural requirements be met. And the industry standards even require the same L/360 deflection requirement for new mortar bed installations. That might simply be because of today's building codes requiring a maximum floor joist deflection of L/360 regardless the intended floor covering.

And I'm told that requirement originated with the advent of drywall ceilings. Prior to that "improvement," I believe the requirement was only L/240 and that was based upon plaster ceilings not cracking. I've never seen an old enough code book to verify that, but maybe one of our more advanced researchers can either verify or debunk that bit of information.

But enough of that. For sistering your joists to improve deflection, you must sister an absolute minimum of 2/3rds of the center of the joist span. So, theoretically, a 10-foot joist would work. A 12-footer would be better. But if you have the capability, a support wall or support beam to reduce the unsupported span of the joists frequently provides much more bang for the buck and the man-hour. You only need to reduce that span a couple feet. Can't see your basement from over here, but the infamous TYW basement closet at one end of the joists might be just what you need and provide always needed storage space to boot, eh?

My opinion; worth price charged.

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