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Old 02-19-2013, 10:53 AM   #1
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5/8" or 3/4" subfloor.


Hi,
I am remodeling my bathroom and haven't chosen the floor tile yet but most likely it will be ceramic. FYI, I'm in Ontario, Canada.

I've found out from an online deflection calculator that my current joists are capable of greater than L/720 rating. Here's the specs of my joists, 2" x 10", 12" OC, 12' span....that's for the majority of the bathroom. One section near one wall has the joists running perpendicular to the rest of the room for 2' and these are on 16" centers....much more solid than the rest of the floor. So from what I have been able to tell, I can go ceramic or stone tile with the joist configuration that I have.

I currently have 5/8" OSB subfloor that I'd like to upgrade to plywood, not be cause it is damaged at all but I'm there and I might as well upgrade it so that I start with a solid foundation. I have to pull up at least a third of the floor anyways to modify drain plumbing, changeout the PVC water supply lines (as a preventive measure as some of the houses in my neighbourhood have had major water damage from these lines due to failure...from what I can tell it's the plastic fittings and not the pipe).

My question is, do I just replace the OSB with 5/8" plywood or go the 3/4" plywood route? I would also like to use Ditra above the floor heat and try to keep the total height to a minimum given that my adjoining bedroom floor is 5/8" OSB with 3/4" hardwood on top.

Also, given my joist deflection criteria, can I just use 3/4" plywood and ditra or should another layer be added?

Secondly, what is the best way to get the whole floor up and cut right up against the wall? Some of the walls are bare to the studs and some have drywall right down to the floor pretty well.

If i stay with the 5/8" plywood option I could leave a trim of about 1 - 1.5" (whatever my circular saw allows) along the walls then add the 5/8" plywood and maybe use a 2 x 4 joining piece extending on both sides of the old and new floor so that the deflection at those spots are minimized as there won't be a tongue / groove connection there.

If I go with the 3/4" new plywood, how do I screw down the floor at the edges? Just into the joists? Will that be sufficient to prevent deflection of the subfloor between the joists?

Sorry for all the questions and long post but I have done as much research as I can but just need some help here?

thanks,

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Old 02-19-2013, 06:44 PM   #2
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5/8" or 3/4" subfloor.


Well I always prefer to use more then 3/4" ply even tell the home owner thats its better when I do installs just for peace of mind. You can get away with 3/4" over 16 oc with ditra but not over wider joists. If you have the room to put 5/8" then 1/4 or 1/2" then I would suggest that.

You also can leave osb and lay 1/2" bc ply over then ditra.

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Last edited by JetSwet; 02-19-2013 at 06:47 PM.
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Old 02-19-2013, 07:13 PM   #3
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5/8" or 3/4" subfloor.


3/4" bc exposure 1 would be good for your installation----this ply has no voids and an exterior glue----

cutting out the old subfloor can be done with a Saws All with a long blade---installed upside down---

Skim cut is possible with the blade mounted this way------
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Old 02-19-2013, 07:24 PM   #4
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5/8" or 3/4" subfloor.


Looks like both of you are saying 3/4" is the way to go ideally. Should I also add another small layer on top of that as JetSwet has mentioned with the 5/8"?

oh'mike...Long blade bent so that it cuts vertical but allows the Saws All to be out from the wall a bit?

thx,
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Old 02-19-2013, 07:37 PM   #5
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5/8" or 3/4" subfloor.


I would let Mike say to add a second layer or not, all I can say is that you want less deflection...thats up and down movement, yes the deflection app is handy but wont replace or fix repairs god forbid. So I say its always better to beat what the deflection shows unless you have hight room for more ply.

Mike stated the BC exposure 1 very good ply ness or no voids the better you will also be.
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Old 02-19-2013, 08:02 PM   #6
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5/8" or 3/4" subfloor.


3/4" should do--if natural stone is used---an additional 3/8 or 1/2 would be ideal---but 12" on center? I think you will be okay with 3/4"

1/4" is not recommended in a tile installation---to much pucker----so 3/8 is minimum if you do add another layer---
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Old 02-19-2013, 08:09 PM   #7
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5/8" or 3/4" subfloor.


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3/4" should do--if natural stone is used---an additional 3/8 or 1/2 would be ideal---but 12" on center? I think you will be okay with 3/4"

1/4" is not recommended in a tile installation---to much pucker----so 3/8 is minimum if you do add another layer---
Your absolutely correct on 1/4" thats my bad
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Old 02-19-2013, 09:16 PM   #8
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5/8" or 3/4" subfloor.


Thanks Guys. So I've got a couple of options depending if I go 5/8" or 3/4".

Now in either case if I replace the whole floor and cut right to the walls, how do I go about securing the plywood at a wall that has no joists to attach to? For instance, I will have about a 10" gap between the last joist and one wall with nothing to attach the flopping end of the plywood to.

Any suggestions?
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Old 02-20-2013, 06:53 AM   #9
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5/8" or 3/4" subfloor.


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Originally Posted by Gail45 View Post
Thanks Guys. So I've got a couple of options depending if I go 5/8" or 3/4".

Now in either case if I replace the whole floor and cut right to the walls, how do I go about securing the plywood at a wall that has no joists to attach to? For instance, I will have about a 10" gap between the last joist and one wall with nothing to attach the flopping end of the plywood to.

Any suggestions?
So the wall is sitting between the joists. So you will either have to add a joist at the wall or you can run some 2x6 the other way attached beween the joists if you can get to to joist that is passed the wall.
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Old 02-20-2013, 09:36 AM   #10
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5/8" or 3/4" subfloor.


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Originally Posted by JetSwet View Post
So the wall is sitting between the joists. So you will either have to add a joist at the wall or you can run some 2x6 the other way attached beween the joists if you can get to to joist that is passed the wall.
If I add some joists perpendicular to the wall, what minimum distance between would be ok so that I don't have to run any blocking between these new joists? Or do I have to run blocking regardless as there will be too much potential deflection of the subfloor between the joists?
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Old 02-20-2013, 03:37 PM   #11
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5/8" or 3/4" subfloor.


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If I add some joists perpendicular to the wall, what minimum distance between would be ok so that I don't have to run any blocking between these new joists? Or do I have to run blocking regardless as there will be too much potential deflection of the subfloor between the joists?
Add the new joist were the plywood will end at the wall the new ply and old ply "were you made your cut will share that new joist Or..... you can put what builders call cats in, its were you run 2x6 or 8 or even 10" if you wanted, evenly spaced apart between joists perpendicular to joists.
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Old 02-20-2013, 03:52 PM   #12
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5/8" or 3/4" subfloor.


In all cases I'll need a joist to screw down the outside edge of the new subfloor (and old subfloor if I have left it overhanging).

So for the plywood that ends at the wall that already has joists running perpendicular to the wall, I'll have to run blocking in between these perpedicualar joists.

For the plywood that ends at the wall that has only joists running parallel to the wall, I'll have to either run a joist parallel to the wall to attach the subfloor to, OR, run short joists from the last joist to the joist in under / past the wall and then blocking in between the joists as well.

Do I have this correct?
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Old 02-20-2013, 04:09 PM   #13
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5/8" or 3/4" subfloor.


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In all cases I'll need a joist to screw down the outside edge of the new subfloor (and old subfloor if I have left it overhanging).

So for the plywood that ends at the wall that already has joists running perpendicular to the wall, I'll have to run blocking in between these perpedicualar joists.

For the plywood that ends at the wall that has only joists running parallel to the wall, I'll have to either run a joist parallel to the wall to attach the subfloor to, OR, run short joists from the last joist to the joist in under / past the wall and then blocking in between the joists as well.

Do I have this correct?
Can you take a pic and post it up? Do you have the joists exposed now or it this what you are looking from under the floor.

To help you quicker with out any of us over thinking about it post pic because I thought your wall was running same direction as the joists between the joists.
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Old 02-20-2013, 04:12 PM   #14
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5/8" or 3/4" subfloor.


If I go 5/8" instead of 3/4" I don't have to go right to the wall with the current floor removal making it much quicker to remove. I could then add 1/2 plywood on top then heat, Ditra and Tile.

If I go 3/4", I have to cut right to he wall making it a much longer demo. I would then add 3/8" plywood on top then heat, Ditra and Tile.

5/8" option looks better to me and I think in terms of strength, both are about the same.
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Old 02-20-2013, 04:15 PM   #15
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5/8" or 3/4" subfloor.


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Originally Posted by JetSwet View Post
Can you take a pic and post it up? Do you have the joists exposed now or it this what you are looking from under the floor.

To help you quicker with out any of us over thinking about it post pic because I thought your wall was running same direction as the joists between the joists.
I have 3 walls that have joists running perpendicular to the wall and one wall where the joist is running parallel to the wall. I'd supply pics but I've only cut out small sections of the for exploratory investigation of joist size, direction, spacing, etc..

I'll get some pics once I pull out the floor.

Let me try to elaborate:

If you can imagine this as a top view of the bathroom,

Joists run full length, left to right in the room.
The wall at the bottom of the room is in between joists running parallel to the joists.
At the top of the room, the wall has joists that run perpendicular to the wall for about 24" until it hits the first left to right joist.

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Last edited by Gail45; 02-20-2013 at 04:21 PM.
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