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Unread 09-22-2013, 10:24 PM   #1
ProTech Tile Restoration
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2nd Floor Wood Joist Deflection issue for Stone Installation

Hi,

I'm a stone restoration specialist and have general knowledge of tile systems, however do not address substrate or structural issues. The following question is regarding my personal residence. I have reviewed many postings here and am in search of a practical answer that will alleviate the majority of concerns with regard to potential failure of the floor tile to be installed. I appreciate any thoughts that you may have.

My master bathroom is upstairs, 1963 tract home, approx 12 x 14.5'. It is currently gutted and was not previously the bathroom space. The original joists are 2x10's, Douglas fir on 16" centers, spanning the 14.5' with a single 3/4 plywood subfloor. I have sistered each joist with 2x10 spruce at 12.5' in length, liquid nail bond and nailed alternately top and bottom at 8" centers. One end of the sistered joists does rest on the sill for full support but the other end does not, falling short about 2' due to framing restrictions. The joist cavities are blocked down the middle.

Using the deflection calculator accurately is difficult due to the fact the the sistered joist do not go completely support to support.

The only reason I became concerned is when walking through the bathroom space a ladder that leans against the wall will rattle just a little bit indicating some deflection / vibration of the floor (The entire second level does react similarly).

I have completely monitored the 1st floor ceiling height under the bathroom floor while placing more than 300 lbs within a 15" diameter dead weight in the middle of the span. I can detect no measurable change.

When a 200 lb person walks the floor "heavy footed" there is a detectable deflection when a measuring stick is placed floor to ceiling but would measure at 1/32" total deflection or movement at most.

Am I missing something with regard to real life deflection vs. calculated deflection? The calculated allowable deflection at 1/720 is nearly 1/4". If this means the floor would dip that low in the center of the span - I can't see that happening in any capacity, especially in a master bathroom that will get no traffic heavier than my foot step. Can I lay another sheet of 3/4" to control deflection of the subfloor if any and run Ditra to be safe or can I try for a sheet of 1/2" CBU instead of the 3/4" plywood? I am trying not to bring the total substrate up by more than another 3/4"

Thank you for your thoughts.
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Unread 09-22-2013, 10:59 PM   #2
dhagin
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The original joists were marginal, which we expect with tract homes. The sisters you added sound fine if they're attached well - you only need sister the middle 2/3 span minimum, which you have. Those joists now are OK for ceramic/porcelain, but NOT for natural stone.

You should have noticed a difference once you doubled the joists, yes? If the ladder still shakes a little, it musta really shook before. Sounds like you have nothing attached to the bottom of the joists? And are any other floor or walls opened up, meaning bare studs or joists? All these allow for more vibrations to be passed through the house, and installing drywall will help here. That said, tract homes are notorious for being built to the bone. Like your original joists, most of the rest of the structure is probably similarly built - this also contributes to shaking.

I think based on what you've told us so far, I'd add a 1/2 layer of AC, BC, or CC exterior glue plywood over the 3/4, installed like this -

http://www.johnbridge.com/images/mik...-0604.pdf..pdf

The plywood will do more structurally for you than backer board. Over that, you'll install some backer board, 1/4 is fine, or some other tile underlayment. NobleSeal, Ditra, etc...

What you might do, is look for any obvious broken/cracked studs or joists while things are open, and repair as necessary. You may have already done this. Check any other framing you can see too. Look for any missing nailing, straps, etc... Post some photos of any specific areas of concern.

edit; max deflection is measured with a uniform load over the whole floor. For bedrooms, this is 40 psf over every sqft of that floor. Don't think you had this. Those joists sound fine to me.
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Last edited by dhagin; 09-22-2013 at 11:05 PM.
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Unread 09-23-2013, 05:30 AM   #3
TonE
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You mentioned blocking.Is it solid blocking or the cross method.solid is better. a 2x10 will want to walk sideways or kick out from the bottom to bend more so then actually just dipping in the middle.
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Unread 09-23-2013, 10:57 AM   #4
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Hi, Thanks for the replies. The ceiling underneath is already closed up and the blocking was full 2x10 installed in the same orientation as the floor joist - no criss crossing 1x's accross the middle of the span.

The room is a shell at the moment but mostly drywalled and clean, ready for tile, cabinets, fixtures, etc.

I guess we are down to my confusion with regadr to real life measurements. As it sits I cannot get the floor to deflect in the middle more than 1/32" with several adults walking and bouncing. I have been reading on various frequency and vibrations within a structure as well. Is it possible this floor stands much stronger than the calculation implies?

Oh, of course I am aiming for a stone installation. Its tough to be in my business and do anything else.

I'd be willing to remove the subfloor, cut the joists down and mud float the floor at 2" if it would help but I think most will say that I'd be weaker than ever with total weight and probably way out of spec based on current build?
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Unread 09-23-2013, 12:54 PM   #5
dhagin
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Quote:
Originally Posted by Tony
I guess we are down to my confusion with regadr to real life measurements. As it sits I cannot get the floor to deflect in the middle more than 1/32" with several adults walking and bouncing.
Quote:
Originally Posted by me
max deflection is measured with a uniform load over the whole floor. For bedrooms, this is 40 psf over every sqft of that floor. Don't think you had this. Those joists sound fine to me.
To get anywhere near the maximum deflection number, you'll need a lot more weight up there. You need 40 lbs on every square foot of it, not just a few folks or a concentrated load.

Quote:
Originally Posted by Tony
Is it possible this floor stands much stronger than the calculation implies?
If the joists were actually doug fir, AND were supported properly at the ends, AND weren't cracked, broken, or had large knots, AND you sister job was done well, then the floor is as indicated above. If any of these were missed, then the floor is likely weaker than I've indicated above. Plug your floor construction numbers into the deflecto and see for yourself.

http://www.johnbridge.com/vbulletin/deflecto.pl

This is your house, and there are no tile police gonna come and arrest ya, so you can do as you like. We're here to try and point you in the right direction so your floor install holds up over time. The info above, and in the deflecto are based on industry standards and building codes, and are generally minimum requirements. Best practices are to go beyond minimums often times so we'd never recommend you try to get away with less.

Quote:
Originally Posted by Tony
I'd be willing to remove the subfloor, cut the joists down and mud float the floor at 2" if it would help but I think most will say that I'd be weaker than ever with total weight and probably way out of spec based on current build?
Correct. Cutting the joists will weaken them substantially more than just the loss of the wood - this is a big no-no, don't do this. Mudding the existing floor may help, but the added weight may cancel any advantage as the floor is not designed to hold that extra weight. To get closer to what you need to support stone tile, consider adding a beam under center of the joist span. Another option is a raised beam within the joist structure. Of course, you'd need a properly sized and supported beam to do any good here.
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Unread 09-23-2013, 06:10 PM   #6
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Quote:
If the joists were actually doug fir, AND were supported properly at the ends, AND weren't cracked, broken, or had large knots, AND you sister job was done well, then the floor is as indicated above. If any of these were missed, then the floor is likely weaker than I've indicated above. Plug your floor construction numbers into the deflecto and see for yourself.

http://www.johnbridge.com/vbulletin/deflecto.pl
Hi dhagin, again appreciate the comments. I have visited this calculator and others but have difficulty believing that I can use 3" for joist width due to the sister joists not spanning the entire distance support to support. I figure some width between 1.5 and 3 is in order for purpose of calculation. Would you agree?
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Unread 09-23-2013, 06:19 PM   #7
dhagin
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You can use the 3" width for your sistered joists as the sister needs to only span the middle 2/3 of the joist span. The tricky bit is that doug fir #2 joists are stiffer than spruce #2 joists, so you may be right. I gave you the benefit of the doubt.

Regardless, my recommendation is still that you need more support to handle a stone floor. You have a few options here, one of which is to pick a porcelain that looks like stone. Today I just picked up some porcelain that looks like wood planks, and they do a pretty good job of matching these days. Have a look around at a reputable tile store, not box stores.
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