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Unread 03-28-2011, 07:34 PM   #1
Toph
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Subfloor Prep - Additional Subfloor Over Original 1x8's

First of all, I need to say this is a great forum. I'm glad I stumbled across it in my hunt to solve my problem.

I'm in the process of gutting my kitchen for a full remodel, and I'm trying to prep myself for the best subfloor solution. The existing structure is 2x8 joists on 16" center right around 9' span with 1x8 planking perpendicular to the joists. There was parquet on top of 1/2" MDF on top of this.

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My plan is to screw the existing 1x8's down to the joists since there are alot of loose nails. From there I plan to put plywood on top of the 1x8's perpendicular to the joists, then Ditra, then the tile. I'm considering sistering plywood to the joists, along with adding blocking since there is none. My first question is what thickness ply should go down? I'm trying to stay as thin as possible (but with a minimum of 1/2" ply) as I plan to level the floor as well. In the picture, the floor is sloping about 3/4" from the door (high) to the existing oak floor (low). The house is a small ranch and the planks in the picture run the length of the house. Underneath the oak is a steel I-beam in the basement that has settled a bit. I really wish I had explored this a bit more before I finished the basement, which happened in the last few months. I'd really like to avoid opening up everything down there that I just finished. My plan is to put ply on top of the 1x8's then use self-levelling on top of that to level the floor. Good idea? Bad idea? Alternatively, I'm considering pulling up the 1x8's and either gluing and screwing wedges on top of the existing joists to level it, or sistering 2x8's to the existing joists, then starting fresh with 3/4" T/G.

I'll be putting down 16"x24" porcelain.

Anyone have suggestions? Thanks in advance!
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Unread 03-28-2011, 07:54 PM   #2
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Welcome, Chris.
Quote:
Originally Posted by Chris
I'm considering pulling up the 1x8's and either gluing and screwing wedges on top of the existing joists to level it, or sistering 2x8's to the existing joists, then starting fresh with 3/4" T/G.
By far the best method of flattening and/or leveling a subfloor, so long as by "T/G" you mean plywood.

And some of us will still want your should add a second layer of plywood over that before your tiling substrate and tile.

But your existing subflooring looks to be in good condition. Keep in mind that your tile don't give a rat's patooti about level, only flat. You, on the other hand, might want level, but all those tiles care about is flat.

And tiles that size want really, really, really flat. That's where removing the existing floor and bringing your joist tops into near perfect plane would be a really good eye-dee.

You might even wanna consider dropping the new subflooring between the joists and using a reinforced mud bed over that to get yourownself a truly flat substrate for those tiles.

Did I mention that your floor is gonna need to be flat?

My opinion; worth price charged.
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Unread 03-28-2011, 08:12 PM   #3
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Wow, thanks for the quick reply and thanks for the welcome!

Yeah, when I said T/G I meant tongue and groove plywood. If I pulled up the planking I'd put 3/4" down, then 1/2" on top of that. I've been in the house for 5 years and I never really noticed the slope before, but I want to make sure I do this the right way. The floor doesn't slope as much on the far end of the kitchen as it does on the end in the photo, so I guess you'd say it has more of a "twist" to it. I was hoping to get away with using self levelling over some 1/2" or 5/8", but (from the lack of your commenting on that in your reply) I'd assume that's not a very good idea.

I'm starting to wonder if a little more drywall work in the basement would be easier than pulling all this up.
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Unread 03-28-2011, 10:25 PM   #4
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Couple problems with using self-leveling products over wood-framed subfloors, Chris. One is that they try to...........ready?...........level. That may or may not always be a good thing when trying to deal with adjacent flooring that may not be level, doorways that may not wanna have a higher floor, etc. If you're really prepared to have the floor leveled, that's fine, of course.

Second is that many/most/all? manufacturers of SLC require metal, or sometimes plastic, lath in the product when used over wood-framed floors and require a minimum thickness. That doesn't work well when all you wanna do is fill one area to flatten the existing floor.

There are some cementitious patching materials that are made for fixing small areas, but they're gonna have limitations, too.

Not as simple as it might sound before you look into it for a ceramic tile installation. Is why I always recommend wood-framed floors be leveled or flattened at the joist level whenever possible.

My opinion; worth price charged.
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Unread 03-28-2011, 11:36 PM   #5
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Yeah, I had thought about everything you've mentioned and it definitely all makes sense. There was just a part of me that hoped it'd be that easy. I'm spending what is (to me) a whole lot of money. I don't want to compromise the installation and regret it in a few years.

I did some closer looking this evening and while it hurts to open up the work I recently did in the basement, the best way to fix this as far as the big picture correct way goes is to jack up the I-Beam that the joists rest on in the center of the house. I'm kicking myself for not checking the floors upstairs before drywalling the basement, but I figure if I take my time I can minimize the repair work that will have to be done to the drywall.
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Unread 03-31-2011, 07:56 PM   #6
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So I spent the last two evenings jacking up the main beam of the house. The floor is dead-on level now, but after looking closer I've got a few spots where I can see 1/16"-3/32" between my level and the subfloor. I figure I'll take a closer look once the plywood is down on top of the planks.

A couple more quick questions....

I was thinking of putting 1/2" exterior grade ply down on the planks, screwed like was mentioned earlier in the thread. If I put thinker ply down, do the benefits outweigh the extra cost and additional height transition? And is there any benefits to putting rosin paper or something like that down between the ply and the planks? I understand that you don't want to glue because you want the two surfaces to move separately, but I always worry about two wood surfaces squeaking in a floor application.

Also, I've got two spots where I've had to replace the planking. True measured thickness of what is down already is 13/16" and the 3/4 ply I had put down leaves an obvious low spot. One area is about a 30" x 30" square. What is the best way to level this out? I'd be ok buying thicker planking and planing it down until it matched, replacing the plywood patch, but it seems like there'd be a better way.
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