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Unread 03-14-2012, 08:14 PM   #1
Dusty Nebulus
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Nebulus Tile/Joist/Subfloor Issues in Saint Louis

Hello all,

I have been browsing these forums for a while, and I think they may be preventing me from making several mistakes on my kitchen/bathroom remodel. This is a good thing, but things are sure getting more complicated. Here are my questions for which I could use some help.

The house is a ranch with a basement, built in 1967. It has had next to no improvements made since it was new, with the exception of a shower bumpout in the master bath (a story unto itself). The joists are SYP, 2 x 8, 16" OC, with a steel I-beam running down the center of the house supporting them. The span between the I beam and the foundation walls is 13'-6". I have removed the old flooring in the kitchen, which was vinyl tile on 1/2" plywood. Both bathrooms had ceramic tile on mortar bed with mesh (removed now). The subfloor is 1/2" plywood. I know many posts are from people surprised by 1/2" subfloor, but trust me, that is what it is. The floor joist running under the tub drain in the bathroom has been sistered.

I have plugged the numbers into the deflection calculator, and it indicates the floor is not strong enough for ceramic tile. However, the tile in both bathrooms was undamaged even after 40 years. They are small rooms with only one person in them at a time, so is it okay to tile them with ceramic even though the deflection is too much? Does the backer board behave differently than the mortar bed?

The trunk for the HVAC is under the kitchen in the basement and covered with drywall, so strengthening the joists from below could be quite a hassle. Any suggestions? Should I pull up the subfloor from above and then sister joists or place new ones 8" OC? All input is appreciated. Thanks.
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Unread 03-14-2012, 08:26 PM   #2
cx
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Welcome, Dusty.

That floor structure is borderline criminal.

Even if those joists are of good specie and good grade, that floor hasta feel like a trampoline.
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Originally Posted by Dusty
Does the backer board behave differently than the mortar bed?
Yes, it does, but your subflooring and joists aren't even suitable for that kind of installation under tile industry standards. Yeah, y'all got by with it, and folks have gotten by with a lot of other misconduct when using mud beds, but I still wouldn't recommend it and I wouldn't do a mud bed and tile over what you have even in my own house.

You're certainly free to do so if you like, of course.

You really need to reinforce the joist structure and you really need to get a suitable subfloor. Removing the existing half-inch material sounds like a good start to moi.

My opinion; worth price charged.
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Unread 03-14-2012, 08:59 PM   #3
Dusty Nebulus
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Thanks for the reply, CX.

To elaborate a little more, I was planning on installing an additional layer of 1/2" BC plywood over the original 1/2" plywood, which indeed bounces, but I would not belly flop on it like a trampoline though. Forgetting the joist problem for a moment, is 2 layers of 1/2" acceptable? I actually bought the plywood today, right before I found the Deflecto calculator.

The rest of the house has hardwood over the 1/2". Like I said, the kitchen was the 1/2" subfloor with 1/2" underlayment and vinyl tile. The tile was original. It all felt solid when the 2 layers of plywood was there. The more I have gotten into this, the more I realize the houses in my subdivision were built as cheaply and as fast as possible.
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Unread 03-14-2012, 09:05 PM   #4
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The only way I'd find that acceptable is if the existing half-inch layer is in excellent condition, with the surface in equal condition, and you glue-laminate the second layer over the first using a full spread of a suitable wood glue.

But a fella who's gotta sister his joist from the top can pretty much rule that out, anyway, eh?

My opinion; worth price charged.
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Unread 03-14-2012, 09:27 PM   #5
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Is there a chance the small bathrooms were mud set? Very possible in '67.

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Unread 03-14-2012, 09:29 PM   #6
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B'lieve he pointed that out in his opening post, Hammy.
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Unread 03-15-2012, 06:43 AM   #7
Dusty Nebulus
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So at this point, it sounds like I need to make a decision one way or another. I will either have to rip the subfloor down to the joists and then make repairs, or the only other possible option is to use a floor other than tile (sheet vinyl, etc). Is sheet vinyl even a good idea with the 1/2" subfloor with 1/2" underlayment? Should the extra 1/2" layer be still be glued in that case?

If I take the floor down to the joists, what should I use to replace the subfloor? Is 3/4" tongue and groove plywood appropriate? Then, would I need another layer of wood for tile on top of the 3/4" before I lay down the backer board?

Thanks again.
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Unread 03-15-2012, 07:20 AM   #8
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Hi Dusty,

Seems to me the easiest thing to do is go back with mud beds. I probably missed something here, though.

It's very easy at this point to shore up the joists whether they need it or not. And they do. Then go back in with a couple layers, say, 5/8 and 1/2. No need for glue; just screw the two layers together. Avoid the joists with the second layer. We have info in the Liberry about that.
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Unread 03-15-2012, 07:56 AM   #9
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Dusty,
I just had a similar situation. My house was built in '62 and I had to rip out (2) 1/2" layers of subfloor and sister joisted all the joists. My span was the same as yours but I had 2x12's instead of your 8's. When I bought the house a year ago, I had ab soo lute lee no idea that I would have to deal with all this BS. After the joists were sistered up, I layed down 3/4 T&G plywood, screwed down with deckscrews and also liquidnailed the T&G's and joist tops. After that, 1/4" hardiebacker and then the 13" pocelin tile. I just finished the grout on Monday and it looks sick! This site is great and gave me the knowledge and courage to pull it off.

-Ray
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Unread 03-16-2012, 09:16 PM   #10
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Quote:
B'lieve he pointed that out in his opening post, Hammy.
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I guess I did miss it.

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Unread 03-17-2012, 08:55 AM   #11
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Here is a question I have about Deflecto. I notice that it does not take specific grade of the joists into account. For example, the joists in my house are No 2 grade. If I put this information in the AWC span length calculator, the approved span for L/360 with 50 psf total load is 12'-10". If I input the information in deflecto, I find that 11' is the maximum allowable span for the same conditions. It is a moot point in my case as I am over the limit of both calculators. However, I was just wondering about this. Does Deflecto assume the worst case scenerio when it comes to grade, just to be on the safe side? I am an engineer by trade (so you know I know nothing! )--I am just curious to know how Deflecto handles this.

I searched the forums thinking an answer may already been given, but I could not find one. If it is out there, let me know.
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Unread 03-17-2012, 09:18 AM   #12
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The Deflectolator uses a full 50psf load for its calculations. Your AWC calculator or span table is using only the live load for its calculations. If you modify your specs to increase your dead load to 20psf instead of the standard 10psf, you'll lose about a foot of available span, but will still be a little above the Deflecto allowance.

When Injineer Bob designed the Deflectolator, he intentionally made it somewhat conservative because it's always being used for tile installations and almost always dealing with used and possibly damaged material in remodel applications.

I can always find a span table that will yield a longer span while maintaining L/360 deflection, but it's not always the better calculation for the application at hand.
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