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Old 03-14-2011, 04:13 PM   #1
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Replacing subfloor in bathroom


I've started a bathroom remodel and have gutted my 90 year old bathroom. I've removed the floor and the subfloor and i've secured the joists. The joists are solid as a rock!! Now, I'm getting ready to put the new subfloor in. I'm putting in 3/4" T&G plywood. On top of that I'll be putting 1/2 " plywood, followed by thinset-ditra-thinset, and finally mesh backed small porclein tile.

Ok, so here are my questions:
1. When I put the subfloor in do i need to leave a gap between the outside edges and the old subfloor, or do i butt up the two subfloors? (I don't know if I need to be concerned about expansion.)

2. With the second layer of plywood, do I need to leave any gaps for expansion? If so, where do i leave the gaps?

3. I'm planning on glueing and screwing the subfloor onto the joists. Do I need to screw the second layer of plywood to the joist or simply to the subfloor?

I think those are all my questions at this point in time. Any help would be appreciated.

Thanks,
Matthew

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Old 03-14-2011, 04:53 PM   #2
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Replacing subfloor in bathroom


Plywood is a dimensionally stable product, and should not expand even if it gets a little damp. Since you are putting ditra in, there is no reason for the plywood to get wet, so you could butt it directly to the wall. However, this is not necessary, you can leave a 1/8 inch gap with no issues. Make sure you use exterior rated plywood for the underlayment, interior rated is not approved by Schluter to the best of my recollection.

As for the subfloor, most of the "experts" who I have heard comment suggest that the upper (second) layer be screwed to the lower layer, but NOT into the joists. I am not really clear why this is recommended, I have not heard a satisfactory explanation, but I have heard this theory enough to believe there may be something to it. As for the underlayer, it is common practice to screw this layer to the joists approximately every 8 inches, but you may want to check the Schluter website for specific, up to date fastening guidelines.

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Old 03-14-2011, 06:17 PM   #3
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Replacing subfloor in bathroom


Ditra is not recommended for use under ' small mesh backed mosaics' ---I have never used their product---however I read the literature---

Be patient,please---we have several members that are expert in Ditras application--

They will come along soon,---Mike---
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Old 03-15-2011, 12:10 AM   #4
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Replacing subfloor in bathroom


Thanks for the input! Ok, I'm still planning on the small octagonal tiles. What are my options now? Does anyone know about other membranes that will prevent fractures and provide waterproofing? I've looked at Noble and Merkrete products. If those aren't recommended, do I need to change the plywood and go with 3/4" T&G followed by 1/4" ply and then 1/4 backer board? I'm trying to get the transitions level. I've got 1 3/4" to work with.
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Old 03-15-2011, 07:07 AM   #5
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Give me your floor joist size--(2x10?) spacing--(16"oc?) --and span--(number of feet support to support)----I'll calculate the deflrction--or someone else will---

The chances are that you are already good to go for mosaic---If so--use 1/2" Durrock set in thinset and then set the tiles---

Never use and plywood less than 3/8" as an under layment--The 1/4" gets wavy and puckered when you put the thinset on top---besides,it is to thin to add usable strength to the structure.--Mike--

I'm off to work soon----several folks here have deflection charts---Mike---
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Old 03-15-2011, 08:48 AM   #6
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My joists are 2x8's 24" oc. However, they have each been sistered, and I've reinforced with 2x4's between the joists. The bathroom is about 14' x 7'. I hope those dimensions help.

Thanks for your help, Mike.
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Old 03-15-2011, 12:23 PM   #7
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I'm looking at using Laticrete 125 instead of the ditra. That seems like it would solve the problem with the pressure points on the mosaic tile. Any ideas about this product?
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Old 03-16-2011, 07:33 AM   #8
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The Amazing John Bridge Forums Deflect-O-Lator :-)

You didn't tell me the joist length--

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