knots in subfloor

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Old 09-15-12, 11:30 PM
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knots in subfloor

I just took up my laminate and luan to lay down Ditra under travertine in my kitchen. My plywood subfloor has several knots from the pine which make the floor not completely smooth. Also there are a few spots where the plywood has some peel out and there are small spaces. My question is, Can I just put down my thinset underneath the Ditra and have it fill in some of these knots and small spaces, or do I need to fill it with another product before applying thinset?
 
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Old 09-15-12, 11:33 PM
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By the way... I notice in my basement the builder must have layed my subfloor incorrectly. I see printed "this side up" when I look up in my basement, and I have all the knots on the top. Not sure why they did that, but it's what I have to work with....
 
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Old 09-16-12, 03:39 AM
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They were standing in the basement when they installed the flooring??? Just kidding. You probably have OSB in the basement and plywood on the main floor. Generally you don't need instructions with plywood, just a good eye for the boogers. They most likely installed CDX, which isn't the best for subflooring, anyway. You can fill the voids with an epoxy, or run your thinset over it if they only go one ply deep, which they usually do.
 
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Old 09-16-12, 05:10 AM
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You cannot install natural stone like your travertine over a single layer of plywood. You need at least two layers of plywood totalling 1 1/4", then a proper tile underlayment like cement board or ditra. Additionally, the floor joist system must be twice as stiff as that for ceramic tile. Give us the details as to the floor joists and we can tell you if you meet that criteria.
 
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Old 09-16-12, 07:40 AM
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Not sure how to tell you the details on the joists. There are the main steel beams that run cross of the wood beams. Then there are several stronger looking beams that are layered solid wood but most of the beams consist of OSB in the middle with solid wood on the top/bottom.

What happens if I don't add the second layer of Plywood? And how the heck am I ever supposed to get out the dishwasher if I add that much thickness? (this is going into a kitchen)
 
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Old 09-16-12, 08:00 AM
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No matter what, you have to add something to what you have, you cannot set tile directly on your subfloor. You need a tile friendly underlayment like cement board or a sheet membrane. As for natural stone, if you dont add a second layer of plywood, its likely that over a reasonably shore period of time, the tile wil begin to crack. You could consider a porcelain tile that has the look of travertime. That way, you can get away with your subfloor and a 1/4" cement board or even thinner sheet membrane.

As for your joists, they sound like tji's. They should have manufacturers markings and size on them. From there, go to the manuracturers website for deflection information. For ceramic/porcelain tile you need L360 and for natural stone you need L720. You'll need to measure the unsupported span of the I-joists and the on center spacing as well as the markings on the joists to use the manufacturers info for deflection. Typically from my experience these joists are usually speced to L480 and not good for a natural stone installation at all. Additionally, they are typically installed at either 19.2 or 24" centers which makes the requirement for more plywood even more important.

No easy answer on the dish washer. They do have some adjustment capability and you will not know how much until you remove it. Most times, kitchen cabinets are installed on top of the underlayment or the tile itself for this very reason. I know too late for that.
 
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Old 09-16-12, 08:23 AM
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From what I can see, they are made by LP Building Products. They are "solid start engineered I joists" 11 7/8" deep and spaced 13 1/2" apart from each other. They have the number 476 stamped everywhere.

I already purchased all of my travertine (doing a french pattern, so my tiles all vary in size) and Ditra underlayment.... My dishwasher will be fine with just the Ditra, thinset and travertine, but another 3/4" subfloor is setting that thing in for good...
 
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Old 09-16-12, 08:34 AM
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Johnny knows his tile, you aren't likely to get a good long term outcome if you don't follow his advice.

Your house so it's your call.
 
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Old 09-16-12, 10:56 AM
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I missed the "travertine" thing. Thanks for the catch Johnny.
 
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Old 09-16-12, 11:27 AM
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Johnny is telling you exactly right. You can not install any natural stone over a single layer subfloor for one. Secondly, both the subfloor and joists system should meet the industry minimum deflection standard of L720. I bet my house yours does not.

As Johnny already mentioned, I don't know of an I-Joists system that is designed for anything over L480. You need to call the manufacturer and tell then which joists you have and the exact span, choose the longest span. Tell them of your intentions and ask them at what live/dead load they figured. In the likely event they give you a number less than L720 deflection and 50/20 live/dead, you may have to reconsider this project.

Now, having said the above, there are a few thinsets that claim to work fine with natural stone even at L480. Mind you this is against industry standards and you'd be on your own. This info (L480) is for the joists' deflection, NOT the subfloor deflection. So, you need the 2nd layer regardless.

Your dishwasher may not be as big a problem as you think. First, no one told you to add another 3/4" ply. You should determine exactly how much clearance you have, then remove the machine and remove the adjustable feet. You can even do a dry run by shimming the floor to simulate the finished job. Often a DW can be sorta tilted in place since the front is the tallest. Also note there is no need or requirement to tile under a dishwasher. If in the other hand the cabinets were not raised as they should be, you may have a problem. You then do what many others have done, 1. raise all the base cabinets. 2. look far and wide for a dishwasher that is shorter.

Jaz
 
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