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Unread 10-19-2016, 10:37 AM   #1
Flyingscot
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1912 Bungalow Complete Bathroom Renovation

Hi folks. I've been lurking for months as I plan a bathroom reno in my 1912 bungalow. Heaps of excellent info here and I will likely be asking at least a thousand questions because I want to do this right. I've already suffered through a DIY shower failure (not my work) and do NOT want to repeat it. I have currently demoed down to the studs and floor joists. With everything open I can now see what I'm dealing with (Termites! Yay). I removed 1x8" diagonal sub floor. From what I've read here I think I need to put down 3/4" A-C grade T&G exterior grade plywood nailed/screwed and glued(?) to the joists. My questions is MUST I put an additional layer of plywood down over top as well? If so, what would the minimum size be that I could use? I'm using 2" Marble hex tile. The joists are 2x10, 16" OC with only a 6 foot span. I'm well within range according to the deflector scale. I plan on using 1/2" Hardiboard. I'm sure you can see the issue. If I do all of that (ply+ply+thinset+HB+thinset+radiant heat mat+thinset+tile) there is going to be an unacceptable height difference between this floor and the adjoining one. So to distill this down. I have 1.5" to play with in flooring depth. I think a 1/4-1/2" difference would probably be okay to transition. I think my tiles are too small for Ditra. What would you suggest I do to properly floor this room?
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Unread 10-19-2016, 12:42 PM   #2
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Welcome to the forum, Lynne.

A six-foot span for joist is rare. There must be a room underneath, right?

You don't mention the spacing of the joists, but since you're installing marble, it's a given that you'll have two layers of plywood over the joists. You must have that with natural stone. There are ways to reduce the buildup, but the best way to build up that floor is with 3/4" exterior glue plywood with a face grade of A, B, or C.

Another layer of plywood on top would need to be a minimum of 3/8" thick. We typically recommend 1/2", since 3/8" is a little tough to work with, but 3/8" is the minimum.

If your tile are 2" in both directions, my recommendation for the heating system would be to use Ditra Heat. The whole system would add just a 1/4" of height to the floor.

But even if that won't work, you should use the thinnest CBU you can find. Instead of using 1/2", look for the 1/4" version. I think no matter what you do, you're not going to get it all within that 1 1/2" you have. Best to do it correctly, though, and deal with the transition.
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Unread 10-19-2016, 03:35 PM   #3
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There is a way to make this stackup thinner IF the top half or so of your joists are free of wires and plumbing...

You'd install cleats on the sides of the joists, then attach a layer of ply (perpendicular to them, so 4' strips) to the cleats making one layer to make the whole floor flat and level...then, install a second layer of ply on top of that. You do want to use ring shank nails or deck screws to attach the first layer along with construction adhesive.
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Unread 10-19-2016, 04:21 PM   #4
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Thank you both for your replies. This space is literally a 6x6ft square that used to be the delivery pantry off of a service porch hence the short span. It is over a crawl space with masonry walls and a cement floor.

1. As far as the Ditra Heat. I was under the impression that my 2x2" hex tiles were too small to use it. How about Noble Seal and the system from Warmly Yours if DH is out of the question? I'm assuming these are applied directly to the plywood so no Hardibacker needed.

2. The idea of plywood between the joists is intriguing and entirely doable. They are 16" OC. The one worry is I'm not sure it would pass inspection. The building department in my town is notorious for being uber difficult with homeowners. I've had some mildly unpleasant experiences with them myself. It's so bad that most people don't even permit their projects. Seriously, you have to get a $40 permit to change a $6 outlet. I'm doing it the legal way because if they get wind of it they will seriously fine me AND make me tear it completely out. Right now I'm trying to work with them to find a solution to the HVAC installer who unfortunately also owns a Sawzall and knows how to use it. Would this plan really meet the building code of the average municipality?

I know to do this right I'm going to have to deal with some type of transition and I'm fine with it. I just want to minimize it as much as possible while still doing a good job.
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Unread 10-19-2016, 04:31 PM   #5
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If you're that worried about the permit, install the first layer between the joists as suggested and then add the second layer on top. Then have it inspected. They don't need to see 100% how the sausage is made, it's your house. I mean what's going to be their complaint? It's overbuilt?
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Unread 10-19-2016, 05:48 PM   #6
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If you're that worried about the permit, install the first layer between the joists as suggested and then add the second layer on top. Then have it inspected. They don't need to see 100% how the sausage is made, it's your house. I mean what's going to be their complaint? It's overbuilt?
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Unread 10-19-2016, 07:12 PM   #7
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The minimum size tile that Schluter will warrantee is 2x2". If yours area actually at least that size, you should be fine using them on DitraHeat. I wouldn't sweat a millimeter or two, but more than that off, probably...safest to call them.

The whole goal on two layers of ply for natural stone is that with only one, deflection between the joists tends to cause the edge on the joist to act like a lever and push up along the seam. Natural stone is not all that homogenous, and thus, not as strong as a manufactured ceramic tile. The second layer helps prevent that jacking effect.
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Unread 10-19-2016, 07:37 PM   #8
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The minimum size tile that Schluter will warrantee is 2x2". If yours area actually at least that size, you should be fine using them on DitraHeat. I wouldn't sweat a millimeter or two, but more than that off, probably...safest to call them.

The whole goal on two layers of ply for natural stone is that with only one, deflection between the joists tends to cause the edge on the joist to act like a lever and push up along the seam. Natural stone is not all that homogenous, and thus, not as strong as a manufactured ceramic tile. The second layer helps prevent that jacking effect.
Thanks Jim! They are definitely 2". The Ditra Heat sounds like my best option and if I can apply it directly to the second layer of ply then the height difference between floors will be fine. I can deal with transitioning but didn't want the "ski slope" effect I read about in another post here. I think their difference was 2.5". Yikes!
As far as two layers of ply...I totally get what you're saying and suspected all along that I was going to do it this way because it makes sense. Then I read a few other "how tos" and the waters got a little muddy. I should just stick to this forum and I think I will!
I probably should change the title of this thread because I have a feeling ya'll will be hearing from me a lot. But for now I need to go and draw up the required "architect quality" floor plans with wall cross sections and electrical/plumbing schematics that my city is requiring before they will grant my permits. Hoping graph paper and Sharpie make the cut. Kinda doubt it.
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Unread 10-19-2016, 08:02 PM   #9
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Hi, Lynne.

I don't know any building inspector that would have a problem with the subfloor set-up that Jim described.

If you want the title changed, just holler. Any mod that sees your request will change it to whatever you'd like.
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Unread 10-19-2016, 10:38 PM   #10
Flyingscot
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[quote]
Quote:
I don't know any building inspector that would have a problem with the subfloor set-up that Jim described.
I'm with you on thinking that but after arguing with one that my 8x12ft shed was less than the 100 sq feet that would require a permit I'm a bit leery. It took a calculator to prove it to him. I have to believe that BIs are overall good guys and very knowledgable but my experience with this department has been less than stellar no matter how polite I've been. The last one I talked to was at least civil and he loosened up when I explained to him that I was just trying to follow their rules and do things the way they wanted them done. It will be interesting to see how things go with this reno. But good God I'm being permitted to death and I need 8 different inspections!
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Unread 10-19-2016, 11:32 PM   #11
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Just so you know this going in, Schluter requires that their heat system be on an electrical circuit by itself. You might want to look through the installation handbook to familiarize yourself with the product.
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Unread 10-20-2016, 04:29 AM   #12
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Quote:
Just so you know this going in, Schluter requires that their heat system be on an electrical circuit by itself. You might want to look through the installation handbook to familiarize yourself with the product.
Thanks K Man!! I actually read the installation booklet as soon as it was first mentioned. Fortunately I have room in my panel to add a dedicated circuit. I had a new one installed a couple of years ago in anticipation of this reno.
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Unread 10-20-2016, 12:45 PM   #13
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Jim,
I'm seriously looking at the sub between the joists idea and I was wondering what dimension cleat would you feel is adequate for these short joist runs assuming adequate gluing and screwing. I have two joists that have been notched more than they should have (which is being handled)leaving me with less surface than I would like.
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Unread 10-20-2016, 03:42 PM   #14
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A 2x4 or a 2x3 would seem to be adequate. See what the others think before jumping.
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Unread 10-20-2016, 08:05 PM   #15
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Yep, you need something adequate to screw the new plywood into. I wouldn't use anything less than a 2x3.
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