Hello, I am trying to decide if the kitchen floor is sound enough to be tiled. A joist deflection calculator gives some numbers but how should they be interpreted if only a fraction of the area over the joist system is considered for tiling, not the entire area? Would appreciate any suggestion, h.

How big is the area you wish to tile? Underneath access? Planning on backer board?

Deflection should be no more than 1/360 of the span between joists?
The question is what is doing the deflecting? A typical building code is 40 lbs/sq ft under live load for residential construction. That could be a crowded kitchen but not packed in like sardines.
You could use self leveling compound if your floor doesn't deflect too much. Even if it hardens nice and flat, it still can crack.
Bigger tiles are more problematic than small ones.
I've never done any of this, just read about it.

How big is the area you wish to tile? Underneath access? Planning on backer board?

I thought backer board is for uncoupling mostly, not for deflection reduction. The area is about 100 sq. ft. Yes, the area has a crawl space underneath, just enough to crawl around for joist measurements. According to the deflection calculator, the joists are just a bit too flexible for tile. I thouht that, considering that the calculator is conservative and the area for tiling is a fraction of the joist length, tiles could still work.

Harum,

You're correct that CBU's do not add any structural strength, just a tile-friendly substrate.

It doesn't matter that the tiles are partially over a beam. The deflection in 11-12 ft. span can be the same as the span in 4-5 ft. It's a ratio, not a total amount of deflection.

How about you give some numbers? How far off are your joists? How about the subfloor?

Jaz

Harum,

You're correct that CBU's do not add any structural strength, just a tile-friendly substrate.

It doesn't matter that the tiles are partially over a beam. The deflection in 11-12 ft. span can be the same as the span in 4-5 ft. It's a ratio, not a total amount of deflection.

How about you give some numbers? How far off are your joists? How about the subfloor?

Jaz

Well, wondering why you are asking about the sub-floor? I thought that if the joists are too weak then the quality of sub-floors is totally irrelevant.

If you insist, it is, according to actual measurements, 1.5"-by-5.5" joists, 10' long and 16" apart on center. The material is probably SYP.

Harum,

The deflection in 11-12 ft. span can be the same as the span in 4-5 ft.

Jaz

This is a wrong statement according to material science.

This is a wrong statement according to material science.

OH? What material science are you talking about?

If a span has a deflection of let's say I-270, any portion of the same span will be 270. So let's say the actual deflection is .5" for the 10' span, then we can say that the deflection will be .25" for a 5' section of that span.

It's actually simple. Using your info of 2x6 joists, spanning 10' 0" @ 16" o.c. (2x6 is very unusual). Most of these joists would be grade #2. So according to my chart you would be at I-360 at 9' 9" max. You are over spanned if it measures 10' 0". Since I-360 is the max for tiles. Will is work at I-250? It might, but the industry standard does not allow it.

You can sister those joists, at least the mid 2/3 will do wonders. Build a supporting wall or a beam to shorten the span. Or you can take a chance and pray.

Another choice is to find a thin set that says will work even at I-240. Hydroment Reflex. http://www.bostik-us.com/sites/default/files/Reflex.pdf It's a great company and you'll want to consider this if you can not do the other remedies. I said can not, not don't want to.

Jaz

OH? What material science are you talking about?

It's actually simple. Using your info of 2x6 joists, spanning 10' 0" @ 16" o.c. (2x6 is very unusual). Most of these joists would be grade #2. So according to my chart you would be at I-360 at 9' 9" max. You are over spanned if it measures 10' 0". Since I-360 is the max for tiles. Will is work at I-250? It might, but the industry standard does not allow it.

You can sister those joists, at least the mid 2/3 will do wonders. Build a supporting wall or a beam to shorten the span. Or you can take a chance and pray.

Another choice is to find a thin set that says will work even at I-240. Hydroment Reflex. http://www.bostik-us.com/sites/default/files/Reflex.pdf It's a great company and you'll want to consider this if you can not do the other remedies. I said can not, not don't want to.

Jaz

Thanks. What does the chart say for:

1.5"-by-5.5" joists, 10' long and 16" apart on center
?
I am afraid the numbers will be worse than for 2"x6". Or does the chart take into account that the actual dimensions of 2"x6" are 1.5"x5.5"?

Well, the max deflection is proportional roughly to the squared length, isn't it? Then the same weight can't get the same deflection for joists of different lengths. Even more so if the tile is off the centers of joists -- the points of max deflection -- the deflections will be different for different lengths and positions. On the other hand, this is all just a silly theory, only capable of very rough estimates, so rough that they become useless in real cases.

2x6 lumber measures 1.5" x 5.5" as you already know. If yours are old, they might measure the full stated size.

The charts don't tell me what the deflection is for any specific span. Charts give a max span based on the species, grade, spacing and load input for the assembly.

The deflection charts are offered as a guide. One could always take the time to do actual field calculations if there's unknown factors.

Jaz