Need advice sub floor & underlay

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Old 07-21-07, 01:35 PM
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Need advice sub floor & underlay

I bought 5/8 inch underlay (poplar?) to go over 3/4 ply subfloor nailed on 2x10 SPF #2 joists, spanning 13' 4". Going to block joists underneath as joists have bounce, then install underlay. I've been told don't use the poplar ply under ceramic tile?? Can I make this work, and how? I can use the extra height to match adjoining floors. Thanks.
 
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Old 07-21-07, 10:38 PM
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What do you mean by "under ceramic"? Are you wanting to glue the tile to this plywood or use the plywood to strengthen the floor before starting on the tile install? If your joists have "bounce", this must be dealt with before tile is even an option. Tile is not flex friendly and any movement in your subfloor will result in a failed tile job. How far apart are the joists? Sixteen on center or wider?
 
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Old 07-21-07, 11:43 PM
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Red face Need advice sub floor & underlay

Hi, Yes I need to get rid of the deflection, was intending the 5/8 to strengthen the floor as well as be a nice flat tile base. The 5/8 is unmarked and I'm told since I bought it, that it is not "underlay" but probably paint-grade cabinet material. I'm going to block the joists underneath. How will I know if I've made the floor rigid enough? The joist spacing is 16 inches and when the washer is spinning, all the dishes clank. The existing plywood is 3/4 inch, not t&g., nailed not screwed to joists. I am new to this, obviously.
 
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Old 07-22-07, 06:56 AM
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Return that plywood. You need an exterior grade plywood. The glues used in exterior plywood will be unaffected by moisture. The plywood you have will suck the moisture out of the thinset and may delaminate. Buy exterior grade plywood bc grade or better. Its expensive stuff but it'll hold up well for what you want to do. Now, you need to be concerned with 2 areas when you talk about deflection. One is between the joists, and yes the added plywood will help that. The second though is deflection along the joists, and neither the added blocking, nor the added plywood will help with that. If your joist system is too bouncy you have a few options. You can cut the span with a support wall below or you may be able to sister the joists. If the floor is bouncy, its good that you recognized this in advance so you can correct it.

Tell us about the floor system. What size are the joists, what is their on center spacing and what is the unsupported span from below (distant between supports below the joists). I see you already told us the oc spacing is 16" but we still need the rest.

While you may be new to this, I see that you are doing your homework ahead of time. Too many people do that after they screw up. Keep the questions coming.
 
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Old 07-22-07, 07:41 AM
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Need advice subfloor & underlay

I've read about (and it's hard to imagine!), deflection between the joists with 3/4" plywood!! The span is 13'4", 2x10 #2 SPF on 16" centres. One end is resting on the block foundation wall and nailed to the band joist, other end butts against the center beam, in joist hangers. the 3/4 ply is nailed, not screwed. It's over an unfinished basement and I was hoping not to divide the space with a mid-span beam, but that looks like the best (or only?) real solution. Thanks for the input.
 
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Old 07-22-07, 07:58 AM
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The 13'4" span is borderline for L360 deflection for ceramic tile. You indicated that the floor is bouncy though, and tile wont like bouncy. You could sister those joists since you say its unfinished space below provided theres not lots of plumbing and other stuff in the way. You could sister with 2" x 8"'s about 10' long attached in the center of the span. The sisters dont have to be the entire length of the span. Line them up with the bottom edge of the existing joists. Glue and screw the sister joists to the originals. Use a glue like titebond II and lots and lots of screws.

Screw the existing subfloor to the existing joists where needed. The second layer of plywood should be exterior grade bc or better and needs to be a minimum of 3/8" but thicker is always better if you can afford the height. Screw the plywood to the subfloor with deck screws every 8" in the field and every 6" at the edges. No glue is needed. Try to screw only to the plywood, not the joists. Use screws only slighty longer than the thickness of the two layers of plywood. Leave 1/8" gaps between the sheets of plywood and then fill them with a cheap pliable caulk to allow for expansion and keep the thinset out of the joints. Some dont do the caulk and it may not be necessary but its good cheap insurance.

Then install your backerboard or uncoupling membrane over the second layer of ply according to manufacturers instructions.
 
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Old 07-22-07, 08:47 AM
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Need advice subfloor & underlay

HJ, should the second layer of plywood go the same direction lengthwise as the first, or the opposite way?
 

Last edited by inamess; 07-22-07 at 08:59 AM. Reason: a further question
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Old 07-22-07, 09:07 PM
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Your joists should be sturdi enough for ceramic or porcelain tiles. I believe your deflection is just over L400. Your present 3/4" subfloor may need fastening, and so why all the shaken going on? Normally one layer of 3/4 plywood is strong enough with 16" o.c. But you said it is not t&g, so you need blocking under every end joint, and I also recommend another layer of underlayment 3/8" or thicker. The thicker the better. Were you planning a cement backer board or membrane over the underlayment?

The second layer of ply goes in the same direction. (across the joists) It must be off-set from the subfloor though. Do not try to line up the ends over a joists, try to have an end joint about one-quarter of the distance between the span, in your case it should be roughly 4" from a joists if possible. It should not be nailed to the joists, but only to the subfloor. Use 1 1/4" screws in most cases. Be sure to leave the proper spacing between sheets and at perimeter.

Jaz
 
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Old 07-23-07, 06:40 AM
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Need advice sub floor & underlay.

Hi Jaz, This place has been unfinished for a very long time. Floor deck build under duress (working alone, big storm threatening). House built on top, never got back to finish floor deck. Probably weak in the joist-hanger end, not fully nailed. The 'floor' rush is now because I have found a great set of used kitchen cabinets, want flooring to go wall-to-wall under cabinets, and having lived with plywood for so long, thinking of ceramic/porcelain rather than vinyl. Trying not to let this get beyond my means, but want a good job. If I use cement board, do I still need the extra plywood layer? (I can use the extra height to match abutting wood flooring). Thanks
 
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Old 07-23-07, 07:35 AM
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The obvious, if the joist hangers arent nailed properly thats the first thing you need to fix. But if the floor is still bounce after that then you need to do something else to make it stronger. If you block the plywood seems underneath you can dont need the extra layer of plywood. The cement board doesnt add any structural strength to the floor. Its purpose is a good substrate for the thinset to bond to. If height is an issue you could also use an uncoupling membrane like ditra.
 
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Old 07-23-07, 11:39 AM
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Need advice sub floor & underlay

Thanks HJ. I've been moving stuff to get all the weight off that end, next down to cellar to finish underneath stuff. Think I've got it 'all nailed down" now, at least, in my head!! Thanks for all the wonderful advice what a great forum!
 
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Old 07-29-07, 06:01 AM
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Need advice subfloor & underlay

one more question. Is my choice cement board OR Ditra membrane? If the cement board doesn't add strength to the floor, what determines whether the cement board or Ditra is the best choice? Thanks.
 
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Old 07-29-07, 06:31 PM
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Cement backer boards or membranes such as Ditra are not designed to add strength to the subfloor. Their purpose is to supply a suitable surface to bond tiles to. Membranes also give many other benefits.

If your subfloor meets the standards for tile, I recommend Ditra over cement backer board. The only drawback is that it's not always easy to find, you may have to buy in quantities much larger than you need, and it costs about double as much as most CBU's.

Jaz
 
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